JP5819226B2 - Excitation circuit of electromagnetic flow meter - Google Patents

Description

  The present invention relates to an excitation circuit for an electromagnetic flow meter that measures the flow rate of a fluid having conductivity in various process systems.

In general, in an electromagnetic flow meter that measures the flow rate of a fluid having electrical conductivity, an excitation current whose polarity is alternately switched to an excitation coil arranged so that the direction of magnetic field generation is perpendicular to the flow direction of the fluid flowing in the measurement tube Is detected, the electromotive force generated between a pair of electrodes arranged in the measuring tube orthogonal to the magnetic field generated from the exciting coil is detected, the electromotive force generated between the electrodes is amplified, and then sampled and signal-processed Thus, the flow rate of the fluid flowing in the measuring tube is measured.
FIG. 5 is a block diagram showing a configuration of a general electromagnetic magnetometer.
The electromagnetic flow meter 50 includes a detector 50A and a converter 50B.
The detector 50A is provided with a measuring tube 51, an electrode 52, and an exciting coil 53 as main components.
The measurement tube 51 is made of a cylindrical body made of a nonmagnetic metal such as stainless steel as a whole, and constitutes a flow path 51F through which a fluid to be measured flows.
The exciting coil 53 is composed of a pair of coils arranged facing the outside of the measuring tube 51, and is orthogonal to the flow direction of the fluid flowing through the flow path 51F in accordance with the exciting current Iex supplied from the converter 50B. The function of generating the magnetic field B in the direction of

  The electrode 52 is composed of a pair of electrodes arranged on the inner wall of the measurement tube 51 so as to be in contact with the fluid flowing in the flow path 51F, facing the direction orthogonal to the direction of the magnetic field B generated by the exciting coil 53. It has a function of detecting an electromotive force E generated in the fluid in response to excitation of the fluid by the magnetic field B and outputting it to the converter 50B.

The converter 50B is provided with a communication I / F unit 55, a signal processing unit 56, and an excitation circuit 57 as main circuit units.
The communication I / F unit 55 is connected to a host device (not shown) such as a controller via the signal line W, generates an operation power source from the power supplied from the host device via the signal line W, and performs each function. And the function of notifying the flow rate value of the fluid obtained by the signal processing unit 56 to the host device via the signal line W by data communication.

The signal processing unit 56 generates an excitation signal composed of a pulse signal having a constant excitation frequency and outputs the excitation signal to the excitation circuit 57. The signal processing unit 56 performs signal amplification and excitation frequency detection on the electromotive force E detected by the electrode 52. By performing signal processing such as sampling based on this, it has a function of calculating the flow rate value of the fluid and a function of outputting the obtained flow rate value to the communication I / F unit 55.
The excitation circuit 57 has a function of generating an alternating excitation current consisting of a rectangular wave for switching and controlling the excitation polarity based on the excitation signal from the signal processing unit 56 and supplying it to the excitation coil 53.

  In such an electromagnetic flow meter 50, various noises such as electrochemical noise, fluid noise, and slurry noise are superimposed on the electromotive force detected by the electrode 52. Therefore, in order to accurately calculate the flow rate value from the electromotive force, it is necessary to reduce these noises. Here, these noises have a so-called 1 / f characteristic whose level is higher in the lower frequency band. For this reason, if the excitation frequency is increased, the S / N ratio of the electromotive force is improved, so that the flow rate value can be calculated with high accuracy.

  On the other hand, when an AC excitation current consisting of such a rectangular wave is applied to the excitation coil 53, the excitation current rises gently due to the influence of the self-inductance of the excitation coil, resulting in a delay in the waveform. Therefore, if the excitation frequency is increased, the wavelength of the excitation signal is shortened, and the rate of rise delay with respect to the wavelength is increased, so the period during which a sufficient magnetic field is generated is shortened and the electromotive force detected from the electrode is reduced. Among them, the width of the steady region where the amplitude is flat becomes shorter. This makes it difficult to stably sample the electromotive force, and as a result, an error in the flow rate value increases. Therefore, it is important to accelerate the rise of the excitation current even at a high excitation frequency.

Conventionally, in such an electromagnetic flow meter, the back electromotive force generated by the excitation coil is charged to the capacitive element and reused as excitation power, thereby improving the rise of the excitation current when switching the excitation polarity. Techniques have been proposed (see, for example, Patent Documents 1 and 2).
FIG. 6 is a circuit showing a conventional excitation circuit.
The excitation circuit 60 includes a switching circuit 61, a constant current circuit 62, a diode D60, a diode bridge DB, and a capacitive element C.

  The constant current circuit 62 includes, for example, an emitter follower circuit including a transistor Q, an operational amplifier OP, and a resistance element R. The constant current circuit 62 is connected between the current output terminal Tout of the switching circuit 61 and the ground potential GND, and is set to the set voltage Vcnt. Based on this, it is a general constant current circuit that supplies an input current with a constant current from the power supply potential VP to the switching circuit 61.

  The switching circuit 61 is operated by the constant current circuit 62 with the power supply potential VP based on excitation signals SA and SB which are output from a signal processing unit (not shown) of the converter and are composed of pulse signals having complementary phase relationships. Has a function of generating an alternating excitation current Iex and supplying it to the excitation coil L by switching and controlling the polarity of the constant current supplied from the current input terminal Tin to the current input terminal Tin.

  The diode bridge DB has a function of rectifying the counter electromotive voltage generated between the terminals L1 and L2 (both ends) of the exciting coil L and charging the capacitive element C. The AC terminals of DB are connected to L1 and L2, respectively, the plus terminal is connected to one end of the capacitive element C, and the minus terminal is connected to the ground potential GND. If this DB is constituted by a Schottky diode, the voltage drop in the forward direction in each diode constituting the DB can be reduced.

The capacitive element C is connected between the current input terminal Tin and the ground potential GND, and has a function of charging the counter electromotive voltage rectified by DB.
The diode 60 is connected in series between the power supply potential VP and the current input terminal Tin, and has a function of preventing the charging voltage VC charged in the capacitor C from flowing backward to the power supply potential VP side.

  The switching circuit 61 is provided with four switch circuits SW61 to SW64 for controlling on / off of current. Among these, a series connection circuit of SW61 and SW63 and a series connection circuit of SW62 and SW64 are provided. In addition, they are connected in parallel. The terminal L1 of the exciting coil L is connected to a connection node between the contact terminal of SW61 and the contact terminal of SW63, and the terminal L2 is connected to a connection node between the contact terminal of SW62 and the contact terminal of SW64. .

FIG. 7 is a signal waveform diagram showing the operation of the conventional excitation circuit.
The excitation signals SA and SB are composed of pulse signals having excitation frequencies having a complementary phase relationship. Among these, SA controls SW61 and SW64, and SB controls SW62 and SW63.
Therefore, as shown at time T60, when SA rises and SB falls, SW61 and SW64 are turned on, and SW62 and SW63 are turned off. As a result, a path of SW61 → terminal L2 → excitation coil L → terminal L1 → SW64 → Tout → constant current circuit 62 is formed as a path of input current input from VP via D60 and Tin, and the excitation current Iex The polarity is switched.

  On the other hand, as shown at time T61, when SA rises and SB rises, SW61 and SW64 are turned off and SW62 and SW63 are turned on. As a result, the SW62 → terminal L1 → excitation coil L → terminal L2 → SW63 → Tout → constant current circuit 62 path is formed as the path of the input current, and the polarity of the excitation current Iex is switched.

  Here, when the polarity of the excitation current Iex is switched, a counter electromotive voltage is generated in the inter-terminal voltage VL between both ends L1 and L2 of the excitation coil L due to the self-inductance of the excitation coil L. For example, when the excitation current Iex is controlled to be switched from the L1 → L2 direction to the L2 → L1 direction at time T60, the voltage of L2 is reduced by the back electromotive voltage generated between both ends L1 and L2 of the excitation coil L. It becomes higher than L1. At this time, since L1 is connected to the ground potential GND via the SW 64 and the constant current circuit 62, the high voltage generated at L2 is charged to the capacitive element C via DB.

  On the other hand, when the excitation current Iex is controlled to be switched from the L2 → L1 direction to the L1 → L2 direction at time T61, the back electromotive voltage generated at both ends of the excitation coil L causes the voltage of L1 to be higher than L2. . At this time, since L2 is connected to the ground potential GND via the SW 63 and the constant current circuit 62, the voltage generated at L1 is charged to the capacitive element C via DB.

  In this way, when the polarity of the excitation current Iex is switched, the back electromotive voltage generated from the excitation coil L is charged to the capacitive element C. Therefore, the charging voltage VC of the capacitive element C is supplied from the power supply potential VP via the diode D60. In a period higher than the supplied voltage, current is supplied from the capacitive element C to the switching circuit 61. Thereby, larger electric power can be supplied to Tin of the switching circuit 61, and the delay time until the exciting current Iex reaches the maximum value from the switching timing of the exciting signals SA and SB is shortened. Therefore, compared with the case where the counter electromotive voltage of the exciting coil L is not used (the broken line waveform in FIG. 7), the rising (falling) of the exciting current Iex can be accelerated even at a high exciting frequency.

JP-A-2-122221 Japanese Patent No. 40000491

In such a conventional technique, the capacitive element C is connected to the current amount force terminal Tin of the switching circuit 61 to which the input current from the power supply potential VP is input. Here, when VP is directly applied to Tin, even if the charging voltage VC of the capacitive element C becomes higher than VP due to the back electromotive voltage of the exciting coil L, it flows backward to the VP side and is absorbed.
For this reason, in the prior art, it is necessary to connect the diode D60 in series between VP and Tin. However, when an input current is supplied from VP to Tin, the input applied to Tin due to the voltage drop of D60. As a result, the excitation coil L cannot be efficiently driven by the power supply potential VP.

A voltage drop Vf of a general PN junction diode is about 0.6V, and a Schottky diode having a low voltage effect Vf is about 0.3V. Therefore, when an input current is supplied from VP to Tin, a voltage of VP−Vf is applied to Tin.
Here, there is no problem when VP is a sufficiently high voltage, but a two-wire electromagnetic flow meter that receives power supply from a host device via a pair of signal lines, or a battery type that operates with an installed battery. In the electromagnetic flow meter, VP decreases to about 2V. For this reason, even when a Schottky diode is used as D60, the voltage applied to Tin is 1.7 V, and the power supplied to the exciting coil L is reduced by 15%.

  The present invention is for solving such a problem, and even when the rise of the excitation current is made faster by utilizing the back electromotive voltage of the excitation coil, the excitation coil can be efficiently driven by the power supply potential. The purpose is to provide a total excitation circuit.

In order to achieve such an object, an excitation circuit according to the present invention supplies an excitation current from an excitation circuit to an excitation coil arranged outside the measurement tube, thereby allowing fluid flowing in the measurement tube to flow. The electromotive force perpendicular to the magnetic field from the excitation coil generated in the fluid by the excitation is detected from a pair of electrodes arranged in the measurement tube, and the flow rate value of the fluid is measured based on the electromotive force. An excitation circuit used in an electromagnetic flow meter that performs input current flowing from a current input terminal to a current output terminal to one end or the other end of the excitation coil based on an excitation signal composed of a pulse signal having an excitation frequency. A switching circuit for switching and supplying the exciting current, a capacitive element connected between the current input terminal and the ground potential, and a back electromotive voltage generated from the exciting coil are rectified. A diode bridge for charging the capacitive element; a detection circuit for detecting a detection cutoff period in which a charging voltage charged to the capacitive element is higher than the power supply potential; and outputting a detection cutoff signal; A first control circuit for controlling connection / disconnection between a potential and the current input terminal, and flowing a current from one contact terminal connected to the power supply potential side to the other contact terminal connected to the current input terminal side. A first MOSFET including a parasitic diode, and based on the detection cutoff signal from the detection circuit and a forced cutoff signal indicating a forced cutoff period having a certain time length from the switching timing of the excitation signal, In the detection cutoff period and the forced cutoff period, the first MOSFET is controlled to be in an OFF state, and in a period other than the detection cutoff period and the forced cutoff period There are and a cutoff circuit for controlling the first MOSFET to the on state.

  According to another configuration example of the excitation circuit, the cutoff circuit includes a drain terminal connected to the power supply potential, a source terminal connected to the current input terminal, and a current flowing from the drain terminal to the source terminal. Based on the first MOSFET comprising a P-channel MOSFET including a parasitic diode, the detection cutoff signal and the forced cutoff signal, the first MOSFET is turned off during the detection cutoff period and the forced cutoff period. And a second MOSFET made of an N-channel MOSFET that blocks reverse flow of the charging voltage with respect to the power supply potential.

  Further, another configuration example according to the excitation circuit further includes a voltage regulator that generates a constant voltage potential from the power supply potential and supplies the constant voltage potential to the current input terminal instead of the power supply potential, The detection circuit detects a period in which the charging voltage is higher than the constant voltage potential as the detection cutoff period and outputs the detection cutoff signal. The cutoff circuit has a source terminal connected to the constant voltage potential, and a drain terminal. Is connected to the current input terminal, and includes the first MOSFET including an N-channel MOSFET including a parasitic diode that flows current from the source terminal to the drain terminal. In the detection cutoff period and the forced cutoff period, By turning off the first MOSFET, the reverse flow of the charging voltage with respect to the constant voltage potential is cut off. That.

  According to another configuration example of the excitation circuit, the switching circuit includes a first switch circuit in which one contact terminal is connected to the current input terminal and is turned on / off according to the excitation signal. , One contact terminal connected to the current input terminal, a second switch circuit that is turned on / off in the opposite phase to the first switch circuit, and one contact terminal at the one end of the excitation coil A third switch circuit connected to the current output terminal and operating on / off in a phase opposite to that of the first switch circuit, and one contact terminal of the excitation coil A fourth switch circuit connected to the other end and having the other contact terminal connected to the current output terminal and performing on / off operation in the same phase as the first switch circuit, and the excitation circuit includes: Determined from the power supply potential A voltage regulator for generating a voltage potential and supplying the constant voltage potential to the current input terminal instead of the power supply potential; and supplying the discharge current from the capacitive element to the one end of the exciting coil. A first high-breakdown-voltage switch circuit that performs on / off control in the same phase as the switch circuit; and supply of the discharge current from the capacitive element to the other end of the exciting coil in an opposite phase to the first switch circuit. A second high-breakdown-voltage switch circuit that performs on / off control, and the detection circuit detects a period during which the charging voltage is higher than the constant voltage potential as the detection cutoff period, and outputs the detected cutoff period as the detection cutoff signal. The interrupting circuit has a source terminal connected to the other contact terminal of the first switch circuit, and a drain terminal connected to the one contact terminal of the third switch circuit. The first MOSFET including an N-channel MOSFET including a parasitic diode that flows current from the source terminal to the drain terminal, the source terminal is connected to the other contact terminal of the second switch circuit, and the drain terminal is A second MOSFET comprising an N-channel MOSFET connected to the one contact terminal of the fourth switch circuit and including a parasitic diode that allows a current to flow from the source terminal to the drain terminal; In the detection cutoff period and the forced cutoff period, by turning off the first MOSFET and the second MOSFET, a reverse flow of the voltage at the one end or the other end of the exciting coil with respect to the constant voltage potential is achieved. It is intended to block.

According to the present invention, in the forced cutoff period and the detection cutoff period, the first MOSFET is turned off and the power supply potential and the current input terminal are shut off. Therefore, even if the charging voltage becomes higher than the power supply potential, the power supply Backflow to the potential side and absorption are prevented. On the other hand, during a period other than the above, the first MOSFET is turned on to connect between the power supply potential and the current input terminal, so that an input current from the power supply potential is input to the current input terminal.
Therefore, compared with the conventional case where a diode for preventing backflow is provided between the power supply potential and the current input terminal, the voltage drop generated for preventing backflow can be greatly reduced. As a result, a power supply potential of sufficient voltage is used, such as a two-wire electromagnetic flow meter that receives power supply from a host device via a pair of signal lines, or a battery-type electromagnetic flow meter that operates with an installed battery. Even if this is not possible, the exciting coil can be efficiently driven by the power supply potential.

It is a circuit diagram which shows the structure of the excitation circuit concerning 1st Embodiment. It is a signal waveform diagram which shows operation | movement of the excitation circuit concerning 1st Embodiment. It is a circuit diagram which shows the excitation circuit concerning 2nd Embodiment. It is a circuit diagram which shows the excitation circuit concerning 3rd Embodiment. It is a block diagram which shows the structure of a general electromagnetic magnetometer. It is a circuit which shows the conventional excitation circuit. It is a signal waveform diagram which shows operation | movement of the conventional excitation circuit.

Next, embodiments of the present invention will be described with reference to the drawings.
[First Embodiment]
First, an excitation circuit 10 for an electromagnetic flowmeter according to a first embodiment of the present invention will be described with reference to FIG. FIG. 1 is a circuit diagram showing a configuration of an excitation circuit according to the first embodiment.

The excitation circuit 10 is a circuit for supplying an excitation current to the excitation coil in the electromagnetic flow meter.
In general, in an electromagnetic flowmeter that measures the flow rate of a fluid having electrical conductivity, excitation is performed in which polarity is alternately switched to an excitation coil that is arranged so that the direction of magnetic field generation is perpendicular to the flow direction of the fluid flowing in the measurement tube. Supply current, detect the electromotive force generated between a pair of electrodes arranged in the measuring tube perpendicular to the magnetic field generated from the excitation coil, amplify the electromotive force generated between the electrodes, and then sample and signal processing By doing so, the flow rate of the fluid flowing in the measuring tube is measured. The overall configuration of the electromagnetic flow meter is the same as that in FIG. 5 described above, and a detailed description thereof is omitted here.

In FIG. 1, the excitation circuit 10 includes a switching circuit 11, a constant current circuit 12, a detection circuit 13, a cutoff circuit 14, a diode bridge DB, and a capacitive element C.
As shown in FIG. 6 described above, the constant current circuit 12 includes, for example, an emitter follower circuit including a transistor Q, an operational amplifier OP, and a resistance element R. The constant current circuit 12 includes a current output terminal Tout of the switching circuit 11 and a ground potential GND. This is a general constant current circuit that is connected in between and supplies an input current with a constant current from the power supply potential VP to the switching circuit 11.

  The switching circuit 11 has a complementary phase relationship output from a signal processing unit (not shown) of the converter, and is based on an excitation signal SA and an excitation signal SB that are pulse signals having an excitation frequency. The input current supplied from the current input terminal Tin to the current output terminal Tout supplied by the current circuit 12 is switched and supplied to the terminal L2 (one end) or the terminal L1 (other end) of the exciting coil L as an alternating exciting current Iex. It has a function to do.

  The diode bridge DB has a function of rectifying the counter electromotive voltage generated between the terminals L1 and L2 (both ends) of the exciting coil L and charging the capacitive element C. The AC terminals of DB are connected to L1 and L2, respectively, the plus terminal is connected to one end of the capacitive element C, and the minus terminal is connected to the ground potential GND. If this DB is constituted by a Schottky diode, the voltage drop in the forward direction in each diode constituting the DB can be reduced.

The capacitive element C is connected between the current input terminal Tin and the ground potential GND, and has a function of charging the counter electromotive voltage rectified by DB.
The detection circuit 13 detects a period in which the charging voltage VC charged in the capacitive element C is higher than VC as a detection cutoff period TY that should block between the power supply potential VP and the current input terminal Tin, and detects the detection result. It has a function of outputting as a cutoff signal SY.

  The interruption circuit 14 controls connection / interruption of the power supply potential VP and the current input terminal Tin by using two contact terminals, and from one contact terminal connected to the VP side to the other contact terminal connected to the Tin side. It has a MOSFET (first MOSFET) Q1 including a parasitic diode (first parasitic diode) through which a current flows, and based on a detection cutoff signal SY from the detection circuit 13 and a forced cutoff signal SX from the outside, Q1 Has a function of ON / ON control.

  The forced cut-off signal SX is output from a signal processing unit (not shown) of the converter, and has a certain time length from the switching timing of the excitation signals SA and SB between the power supply potential VP and the current input terminal Tin. Is a control signal instructing as a forced cutoff period TX to cut off.

  The parasitic diode is also called a built-in diode, and is a diode that is parasitic inside a general MOSFET. When a current flows between the drain and the source, the MOSFET can flow a current not only in the forward direction but also in the reverse direction. At this time, there are two current paths in the reverse direction, one is a path composed of a drain-source channel, and the other is a path composed of a parasitic diode. For this reason, when the MOSFET is in the OFF state, the parasitic diode operates as a normal diode in the reverse direction.

  In the present embodiment, the switching circuit 11 includes a switch circuit SW1 (first switch circuit) in which one contact terminal is connected to the current input terminal Tin and is turned on / off in response to the excitation signal SA. A contact terminal is connected to Tin, and a switch circuit SW2 (second switch circuit) that is turned on / off in a phase opposite to that of the switch circuit SW1 according to the excitation signal SB, and one contact terminal is the other contact of SW1 A switch circuit SW3 (third switch circuit) which is connected to the terminal and L2 and whose other contact terminal is connected to the current output terminal Tout and which is turned on / off in a phase opposite to that of SW1 according to SB; Is connected to the other contact terminal of SW2 and L1, and the other contact terminal is connected to Tout, and is turned on / off in the same phase as SW1 according to SA. And a road-SW4 (fourth switch circuit).

  The cutoff circuit 14 includes a P-channel MOSFET (first MOSFET) including a parasitic diode whose drain terminal is connected to the power supply potential VP, source terminal is connected to Tin, and current flows from the drain terminal to the source terminal. Q1, a constant voltage diode ZD1 having a cathode terminal connected to the gate terminal of Q1 and an anode terminal connected to the source terminal of Q1, and a resistance element (a first element connected between the gate terminal and the source terminal of Q1) N-channel MOSFET having a drain terminal connected to the gate terminal of Q1 through a resistor element (second resistor element) R2 and a source terminal connected to the ground potential GND (second element). MOSFET) Q2 and a diode D having an anode terminal connected to the gate terminal of Q2 and a cathode terminal connected to the detection cutoff signal SY A diode D2 having an anode terminal connected to the gate terminal of Q2, a cathode terminal connected to the forced cutoff signal SX, one end connected to VP, and the other end connected to the gate terminal of Q1, the anode terminal of D1, and D2 And a resistance element (third resistance element) R3 connected to the connection node of the anode terminal. If these D1 and D2 are composed of Schottky diodes, the voltage drop in the forward direction in these diodes can be reduced.

The detection circuit 13 includes a resistance element R4 having one end connected to one end of the capacitive element C, a resistance element R5 having one end connected to the other end of R4 and the other end connected to the ground potential GND, and a cathode terminal. Is connected to one end of R5, a constant voltage diode ZD2 whose anode terminal is connected to GND, a resistance element R6 whose one end is connected to the power supply potential VP, one end is connected to the other end of R6, and the other end is GND And the inverting input terminal is connected to the connection node of the other end of R4, one end of R5, and the anode terminal of ZD2, and the non-inverting input terminal , And an operational amplifier OP1 that is connected to a connection node between the other end of R6 and one end of R7 and outputs a detection cutoff signal SY from an output terminal.
In this case, by setting the resistance ratio of R4 and R5 and the resistance ratio of R6 and R7 to be equal to the ratio of the peak value of VC and VP, OP1 using VP and GND as the operation power supply, VP And VC can be compared.

  As described above, in this embodiment, the cutoff circuit 14 for controlling connection / cutoff between the power supply potential VP and the current input terminal Tin is provided, and the charging voltage VC of the capacitive element C indicated by the detection cutoff signal SY is VP. In both of the higher detection cutoff period TY and the forced cutoff period TX having a certain time length from the switching timing of SA and SB indicated by the forced cutoff signal SX, the power supply potential VP and the current input terminal Tin are cut off. In other periods, the power supply potential VP and the current input terminal Tin are connected.

  As a result, in the detection cutoff period TY and the forced cutoff period TX, Q1 is turned off and the gap between VP and Tin is cut off, so that even if VC becomes higher than VP, it flows back to the VP side and is absorbed. It is prevented. On the other hand, during a period other than TY and TX, Q1 is turned on and VP and Tin are connected, so that an input current from VP is input to Tin.

  Here, the on-resistance of a general MOSFET is 1 Ω or less, and there are some having a resistance of 100 mΩ or less. Therefore, even if the on-resistance of Q1 is 1Ω and a current of 50 mA is supplied, the voltage drop at Q1 is 0.05 V or less. For this reason, compared with the case where the diode for backflow prevention is used between VP and Tin like the past, the voltage drop in Q1 can be reduced significantly. Thereby, the exciting coil L can be efficiently driven by the power supply potential VP.

[Operation of First Embodiment]
Next, the operation of the excitation circuit 10 according to the present embodiment will be described with reference to FIG. 1 and FIG. FIG. 2 is a signal waveform diagram showing the operation of the excitation circuit according to the first embodiment.

  As shown at time T0, when SA rises and SB falls, SW1 and SW4 are turned on and SW2 and SW3 are turned off. Thereby, a path of SW1 → terminal L2 → excitation coil L → terminal L1 → SW4 → Tout → constant current circuit 12 is formed as a path through which the input current from Tin flows, and the polarity of the excitation current Iex is switched.

At this time, at time T0, VC is discharged to the exciting coil L and slightly lower than VP before that, so that the detection cutoff signal SY from the detection circuit 13 is at a high level (VP or high impedance state). The detection cutoff period TY is not shown.
On the other hand, at time T0, the forced cutoff signal SX is at a low level (GND potential), indicating the forced cutoff period TX. As a result, the gate potential of Q2 is lowered and turned off via the diode D2, and the gate potential of Q1 becomes equal to the source potential via R1, so that Q1 is also turned off. When VC is lower than VP, VP is supplied to Tin by the parasitic diode of Q1.

Further, when the polarity of the excitation current Iex is switched and controlled at time T0, a counter electromotive voltage is generated between both ends L1 and L2 of the excitation coil L due to the self-inductance of the excitation coil L.
For example, at time T0, the excitation current Iex is switched from the previous L1 → L2 direction to the L2 → L1 direction, so that the voltage of L2 is lower than L1 due to the back electromotive voltage generated between the terminals L1 and L2. Get higher. As a result, a negative voltage is generated as the inter-terminal voltage VL between the terminals L1 and L2, and this back electromotive voltage is charged to the capacitive element C via DB.

Thereby, when VC exceeds the voltage obtained by subtracting the voltage drop Vf of the parasitic diode of Q1 from VP, the input current from VP to Tin stops, and the discharge current from VC to Tin begins to flow.
Thereafter, at time T1 when VC further rises and reaches VP, SY from the detection circuit 13 changes to low level (GND potential) to indicate the detection cutoff period TY. As a result, the gate potential of Q2 is lowered and turned off, and Q1 is turned off until time T3 when the generation of the counter electromotive voltage ends and VC becomes lower than VP.

  The time length of TX has a time length from the switching timing of SA and SB, here time T0 to time T1 at least when VC reaches VP and SY is output. Therefore, in the period from time T0 to time T1, Q1 is maintained in the off state even if VC does not reach VP and SY is not output. For this reason, VC does not flow backward to VP, and the back electromotive voltage can be sequentially charged to C. Note that TX changes to a high level (VP or high impedance state) at time T2 after time T1 and before time T3.

  In this manner, Q1 is turned off during the period from time T1 to time T3, and the reverse flow of VC with respect to VP is prevented, so that the discharge current from VC is supplied from Tin to L2 via SW1. As a result, power larger than the input current from VP can be supplied to L2, and the delay until the excitation current Iex reaches the maximum value from the switching timing of the excitation signals SA and SB is shortened. Therefore, compared with the case where the counter electromotive voltage of the excitation coil L is not used (the broken line waveform in FIG. 2), the fall (rise) of the excitation current Iex can be accelerated.

  On the other hand, as shown at time T4, when SA falls and SB rises, SW1 and SW4 are turned off and SW2 and SW3 are turned on. Thereby, as a path through which the input current from Tin flows, a path of SW2 → terminal L1 → excitation coil L → terminal L2 → SW3 → Tout → constant current circuit 12 is formed, and the polarity of the excitation current Iex is switched.

At this time, at time T4, VC is discharged to the exciting coil L and slightly lower than VP before that, so the detection cutoff signal SY from the detection circuit 13 is at a high level (VP or high impedance state). The detection cutoff period TY is not shown.
On the other hand, at time T4, the forced cutoff signal SX is at the low level (GND potential), indicating the forced cutoff period TX. As a result, the gate potential of Q2 is lowered and turned off via the diode D2, and the gate potential of Q1 becomes equal to the source potential via R1, so that Q1 is also turned off. When VC is lower than VP, VP is supplied to Tin by the parasitic diode of Q1.

Further, when the polarity of the excitation current Iex is switched at time T4, a counter electromotive voltage is generated between both ends L1 and L2 of the excitation coil L due to the self-inductance of the excitation coil L.
For example, at time T4, the excitation current Iex is controlled to be switched from the L2 → L1 direction to the L1 → L2 direction, so that the voltage of L1 is lower than L2 due to the back electromotive voltage generated between the terminals L1 and L2. Get higher. As a result, a positive voltage is generated as the inter-terminal voltage VL between the terminals L1 and L2, and this back electromotive voltage is charged in the capacitive element C via DB.

Thereby, when VC exceeds the voltage obtained by subtracting the voltage drop Vf of the parasitic diode of Q1 from VP, the input current from VP to Tin stops, and the discharge current from VC to Tin begins to flow.
Thereafter, at time T5 when VC further rises and reaches VP, SY from the detection circuit 13 changes to low level (GND potential) to indicate the detection cutoff period TY. As a result, the gate potential of Q2 is lowered and turned off, and Q1 is turned off until time T7 when generation of the counter electromotive voltage is finished and VC becomes lower than VP.

  The time length of TX has a time length from the switching timing of SA and SB, here, time T4 to time T5 when at least VC reaches VP and SY is output. Therefore, in the period from time T4 to time T5, even if VC does not reach VP and SY is not output, Q1 is maintained in the off state. For this reason, VC does not flow backward to VP, and the back electromotive voltage can be sequentially charged to C. Note that TX changes to a high level (VP or high impedance state) at time T6 after time T5 and before time T7.

  In this way, Q1 is turned off during the period from time T5 to time T7, and the reverse flow of VC with respect to VP is prevented, so that the discharge current from VC is supplied from Tin to L1 via SW2. As a result, power larger than the input current from VP can be supplied to L1, and the delay until the excitation current Iex reaches the maximum value from the switching timing of the excitation signals SA and SB is shortened. Therefore, compared with the case where the counter electromotive voltage of the exciting coil L is not used (the broken line waveform in FIG. 2), the rising (falling) of the exciting current Iex can be accelerated.

[Effect of the first embodiment]
As described above, in this embodiment, the cutoff circuit 14 for controlling connection / cutoff between the power supply potential VP and the current input terminal Tin is provided, and the charging voltage VC of the capacitive element C indicated by the detection cutoff signal SY is the power supply. In both the detection cutoff period TY higher than the voltage VP and the forced cutoff period TX having a fixed time length from the SA and SB switching timing indicated by the forced cutoff signal SX, the power supply potential VP and the current input terminal Tin are cut off. In other periods, the power supply potential VP and the current input terminal Tin are connected.

  Specifically, the switching circuit 11 switches and supplies the input current flowing from Tin to Tout as Iex to L1 or L2 of L based on SA and SB, and DB generates a counter electromotive voltage generated from L. Rectifying and charging to C connected between Tin and GND, the detection circuit 13 compares VC and VP, detects TY where VC is higher than VP, outputs it as SD, and shuts off circuit 14 Includes a parasitic diode that controls connection / disconnection of VP and Tin by two contact terminals and flows current from one contact terminal connected to the VP side to the other contact terminal connected to the Tin side. Based on SY and SX indicating TX having a certain length of time from the switching timing of the excitation signal, Q1 is controlled to be off in SY and SX, and Q1 is turned on in other periods It is obtained so as to control the.

As a result, Q1 is turned off in TY and TX, and the gap between VP and Tin is blocked, so that even if VC is higher than VP, it is prevented from flowing back and being absorbed into the VP side. On the other hand, during a period other than TY and TX, Q1 is turned on and VP and Tin are connected, so that an input current from VP is input to Tin.
Therefore, as compared with the conventional case where a diode for preventing a backflow is provided between VP and Tin, a voltage drop generated for preventing a backflow can be greatly reduced. As a result, a VP having a sufficient voltage cannot be used, such as a two-wire electromagnetic flow meter that receives power supply from a host device via a pair of signal lines or a battery-type electromagnetic flow meter that operates with an installed battery. Even in this case, L can be driven efficiently by VP.

  In the present embodiment, SW1 and SW4 are controlled to be turned on / off based on SA, and SW2 and SW3 are controlled to be turned on / off based on SB. However, the present invention is not limited to this. If the ON / OFF operation relationship between these switch circuits is the same as described above, the combination of SA and SB and these MOSFETs may be determined in accordance with the control logic of the MOSFETs constituting each switch circuit.

  In the present embodiment, the case where SX is supplied from the signal processing unit of the converter has been described as an example, but the present invention is not limited to this. As described above, SX becomes low level (GND potential) at the timing of switching between SA and SB, and high level (VP or high impedance state) within a certain period until SY returns to high level (VP or high impedance state). ). Therefore, for example, a time constant circuit or a sequential circuit may be provided in the detection circuit 13 to generate SX as described above from SA and SB. At this time, by taking the logical sum of SX and SY, a control signal indicating the operation state of Q1 in FIG. 2 is generated and output from the detection circuit 13 to the gate terminal of Q2 instead of SX and SY. It may be.

[Second Embodiment]
Next, an excitation circuit 10 according to a second embodiment of the present invention will be described with reference to FIG. FIG. 3 is a circuit diagram showing an excitation circuit according to the second embodiment, and the same or equivalent parts as those in FIG. 1 are given the same reference numerals.
In the first embodiment, the case where connection / cutoff between the power supply potential VP and the current input terminal Tin is controlled using a P-channel MOSFET has been described as an example. In the present embodiment, a case where connection / cutoff between VP and Tin is controlled using an N-channel MOSFET will be described.

In FIG. 3, the excitation circuit 10 further includes a voltage regulator REG that generates a constant voltage potential VR from the power supply potential VP and supplies VR to the current input terminal Tin of the switching circuit 11 instead of VP.
The REG may be a general linear regulator such as a three-terminal regulator that generates a constant voltage potential by applying a reference voltage to the control terminal of the transistor, or a switching regulator such as a DC-DC converter.

The cutoff circuit 14 has a Q11 (first MOSFET) made of an N-channel MOSFET including a parasitic diode whose source terminal is connected to VR, drain terminal is connected to Tin, and current flows from the source terminal to the drain terminal. In the detection cutoff period TY and the forced cutoff period TX, Q11 is turned off to have a function of blocking the reverse flow of VC with respect to VR.
More specifically, the source terminal of Q11 is connected to VR, the drain terminal of Q11 is connected to Tin, the gate terminal of Q11 is connected to the other end of the resistor element R3, the anode terminal of D1, and the anode terminal of D2. Connected to the node.

The detection circuit 13 has a function of detecting a period in which VC is higher than VR as the detection cutoff period TY and outputting the detection cutoff signal SY. In the present embodiment, VR is supplied to Tin instead of VP. Therefore, the detection circuit 13 needs to compare VC and VR. Moreover, what is necessary is just to adjust TX as needed.
In addition, about the other structure in the excitation circuit 10 concerning this Embodiment, it is the same as that of FIG. 1, and detailed description here is abbreviate | omitted.

[Operation of Second Embodiment]
Next, the operation of the excitation circuit 10 according to the present embodiment will be described with reference to FIG. For signal waveforms, refer to FIG.
When the forced cutoff signal SX becomes low level (GND potential) at time T0, the gate potential of Q11 is lowered via the diode D2, so that Q11 is turned off. Therefore, reverse flow of the charging voltage VC with respect to VR is prevented by Q11.
Further, when the detection cutoff signal SY becomes a low level (GND potential) at time T1, the gate potential of Q11 decreases via the diode D1, so that Q11 is turned off. Therefore, the backflow prevention of the charging voltage VC with respect to VR is continued by Q11.

  Thereafter, when SX is at a high level (VP or high impedance state) and the detection cutoff signal SY is also at a high level (VP or high impedance state) at time T2, the gate potential of Q11 is set to the source via R2. Since VP is higher than VR which is the potential, Q11 is turned on. Therefore, VR is supplied to Tin via Q11. Note that REG generates VR in advance in which the voltage drop from VP to VR has a voltage difference equal to or greater than the gate-source voltage at which Q11 is turned on.

On the other hand, when the forced cut-off signal SX becomes low level (GND potential) at time T3, the gate potential of Q11 decreases via the diode D2, and thus Q11 is turned off. Therefore, reverse flow of the charging voltage VC with respect to VR is prevented by Q11.
Further, when the detection cutoff signal SY becomes a low level (GND potential) at time T4, the gate potential of Q11 is lowered via the diode D1, so that Q11 is turned off. Therefore, the backflow prevention of the charging voltage VC with respect to VR is continued by Q11.

  Thereafter, when SX is at a high level (VP or high impedance state) and the detection cutoff signal SY is also at a high level (VP or high impedance state) at time T5, the gate potential of Q11 is set to the source via R2. Since VP is higher than VR which is the potential, Q11 is turned on. Therefore, VR is supplied to Tin via Q11.

[Effect of the second embodiment]
As described above, the present embodiment further includes the voltage regulator REG that generates the constant voltage potential VR from the power supply potential VP and supplies VR to the current input terminal Tin instead of VP. It has Q11 which consists of N channel MOSFET including a parasitic diode which is connected to VR, drain terminal is connected to Tin, and current flows from the source terminal to the drain terminal, and in the detection cutoff period TY and the forced cutoff period TX, Q11 Is turned off to block the reverse flow of the charging voltage VC with respect to VR.
Therefore, the same operational effects as those of the first embodiment can be obtained, and the circuit configuration of the cutoff circuit 14 can be simplified as compared with the first embodiment.

[Third Embodiment]
Next, an excitation circuit 10 according to a third embodiment of the present invention will be described with reference to FIG. FIG. 4 is a circuit diagram showing an excitation circuit according to the third embodiment, and the same or equivalent parts as those in FIG. 1 are given the same reference numerals.
In the first and second embodiments, the case where the cutoff circuit 14 is provided between the power supply potential VP and the current input terminal Tin has been described as an example. In the present embodiment, a case where the cutoff circuit 14 is provided inside the switching circuit 11 will be described.

  In FIG. 4, a switching circuit 11 has one contact terminal connected to Tin, a switch circuit SW1 (first switch circuit) that operates on / off according to SA, and one contact terminal connected to Tin. In accordance with SB, the switch circuit SW2 (second switch circuit) that is turned on / off in the opposite phase to SW1, one contact terminal is connected to L2, the other contact terminal is connected to Tout, A switch circuit SW3 (third switch circuit) that performs on / off operation in the opposite phase to SW1 according to SB, one contact terminal is connected to L1, the other contact terminal is connected to Tout, and SA Accordingly, a switch circuit SW4 (fourth switch circuit) that performs on / off operation in the same phase as SW1 is provided.

  The interruption circuit 14 includes an N-channel MOSFET including a parasitic diode that has a source terminal connected to the other contact terminal of SW1, a drain terminal connected to one contact terminal of SW3, and a current flowing from the source terminal to the drain terminal. (First MOSFET) Q21 and a parasitic terminal including a source terminal connected to the other contact terminal of SW2, a drain terminal connected to one contact terminal of SW4, and a current flowing from the source terminal to the drain terminal, An N-channel MOSFET (second MOSFET) Q22, an anode terminal connected to the gate terminal of Q21, a cathode terminal connected to the forced cutoff signal SX, and an anode terminal connected to the gate terminal of Q21; A diode D22 whose cathode terminal is connected to the detection cutoff signal SY and one end of Q21 A resistor element R21 connected to the gate terminal, the other end connected to VP, an anode terminal connected to the gate terminal of Q22, a cathode terminal connected to the forced cutoff signal SX, and an anode terminal Q22 And a diode D24 whose cathode terminal is connected to the detection cutoff signal SY.

The excitation circuit 10 further includes a voltage regulator REG that generates a constant voltage potential VR from the power supply potential VP and supplies VR to the current input terminal Tin of the switching circuit 11 instead of VP.
The REG may be a general linear regulator such as a three-terminal regulator that generates a constant voltage potential by applying a reference voltage to the control terminal of the transistor, or a switching regulator such as a DC-DC converter.

  In addition, the excitation circuit 10 is a high voltage switch that controls on / off of the supply of the discharge current from the capacitive element C to the terminal L2 (one end) of the excitation coil L in the same phase as the switch circuit SW1 based on the excitation signal SA. Circuit (first high withstand voltage switch circuit) SW31 and a high withstand voltage switch circuit that controls on / off of supply of the discharge current to the terminal L1 (the other end) from C to L in the opposite phase to SW1 based on the excitation signal SB. (Second high voltage switch circuit) SW32.

The detection circuit 13 has a function of detecting a period when the charging voltage VC of the capacitive element C is higher than VR as the detection cutoff period TY and outputting it as a detection cutoff signal SY. In the present embodiment, VR is supplied to Tin instead of VP. Therefore, the detection circuit 13 needs to compare VC and VR. Moreover, what is necessary is just to adjust TX as needed.
In addition, about the other structure in the excitation circuit 10 concerning this Embodiment, it is the same as that of FIG. 1, and detailed description here is abbreviate | omitted.

  Thus, in the present embodiment, the capacitive element C is disconnected from the current input terminal Tin of the switching circuit 11, and Q21 and Q22 are provided between the terminals L1 and L2 of the exciting coil L and SW1 and SW2 of the switching circuit 11. In the detection cut-off period TY and the forced cut-off period TX, the reverse flow of the voltages L1 and L2 with respect to VR is cut off by turning off Q21 and Q22.

  As a result, in the detection cutoff period TY and the forced cutoff period TX, Q21 and Q22 are turned off, and the gap between SW1, SW2 and L1, L2 is cut off, so that the voltage of L1, L2 becomes higher than VR. Is also prevented from flowing back into the VR side and being absorbed. On the other hand, during a period other than TY and TX, Q21 and Q22 are turned on and SW1 and SW2 are connected to L1 and L2, so that an input current from Tin is supplied to L1 and L2.

[Operation of Third Embodiment]
Next, the operation of the excitation circuit 10 according to the present embodiment will be described with reference to FIG. For signal waveforms, refer to FIG.

  When the forced cutoff signal SX becomes low level (GND potential) at time T0, the gate potential of Q21 decreases via the diode D21 and the gate potential of Q22 decreases via the diode D22, so that Q21, Q22 Is turned off. Therefore, reverse flow of charging voltage VC with respect to VR is prevented by Q21 and Q22.

Further, when the detection cutoff signal SY becomes low level (GND potential) at time T1, the gate potential of Q21 decreases via the diode D22, and the gate potential of Q22 decreases via the diode D23. , Q22 is turned off. Therefore, the backflow prevention of the charging voltage VC with respect to VR is continued by Q21 and Q22.
At time T0, SW31 is turned on by SA and SW32 is turned off by SB, so that a discharge current is supplied from C to SW2 via SW31.

Thereafter, when SX is at a high level (VP or high impedance state) and the detection cutoff signal SY is also at a high level (VP or high impedance state) at time T2, the gate potential of Q21 is set to the source via R21. Since VP is higher than VR which is the potential, Q21 is turned on. Similarly, since the gate potential of Q22 becomes VP higher than VR which is the source potential via R22, Q22 is turned on.
Therefore, VR supplied to Tin is supplied to L2 from SW1 through Q21. Note that REG generates VR in advance in which the voltage drop from VP to VR has a voltage difference equal to or greater than the gate-source voltage at which Q11 is turned on.

  On the other hand, when the forced cutoff signal SX becomes low level (GND potential) at time T3, the gate potential of Q21 decreases via the diode D21 and the gate potential of Q22 decreases via the diode D22. , Q22 is turned off. Therefore, reverse flow of charging voltage VC with respect to VR is prevented by Q21 and Q22.

Further, when the detection cutoff signal SY becomes a low level (GND potential) at time T4, the gate potential of Q21 decreases via the diode D22 and the gate potential of Q22 decreases via the diode D23. , Q22 is turned off. Therefore, the backflow prevention of the charging voltage VC with respect to VR is continued by Q21 and Q22.
At time T3, SW31 is turned off by SA and SW32 is turned on by SB, so that a discharge current is supplied from C to SW1 through SW32.

Thereafter, when SX becomes high level (VP or high impedance state) and detection cutoff signal SY also becomes high level (VP or high impedance state) at time T5, the gate potential of Q21 is set to the source via R21. Since VP is higher than VR which is the potential, Q21 is turned on. Similarly, since the gate potential of Q22 becomes VP higher than VR which is the source potential via R22, Q22 is turned on.
Therefore, VR supplied to Tin is supplied to L1 from SW2 via Q22.

[Effect of the third embodiment]
As described above, the present embodiment further includes the voltage regulator REG that generates the constant voltage potential VR from the power supply potential VP and supplies VR to the current input terminal Tin instead of VP. Q21 made of an N-channel MOSFET including a parasitic diode that is connected to the other end of SW1, the drain terminal is connected to L2, and a current flows from the source terminal to the drain terminal, and the source terminal is connected to the other end of SW2. Q22 made of an N-channel MOSFET including a parasitic diode that has a drain terminal connected to L1 and flows a current from the source terminal to the drain terminal, and Q21 and Q22 are turned off in the detection cutoff period TY and the forced cutoff period TX By setting the state, the reverse flow of the charging voltage VC with respect to the VR is cut off.

  Therefore, the same operational effects as those of the first embodiment can be obtained, and even when a high counter electromotive voltage is generated from the exciting coil L as compared with the first embodiment, the on-resistance of the high breakdown voltage MOSFET is used. Since the reduction of the excitation current can be suppressed, the excitation coil L can be driven efficiently.

That is, according to the present embodiment, since the capacitive element C is disconnected from the current input terminal Tin of the switching circuit 11, a high counter electromotive voltage generated in the exciting coil L is transferred from C 1 through SW 1 to SW 1. Q21 and Q22 can prevent the application of the high back electromotive force generated in the exciting coil L to the contact terminals of SW1 and SW2 while avoiding the application to the contact terminal of SW2.
For this reason, since it is not necessary to use high voltage MOSFETs as SW1 and SW2, MOSFETs with low on-resistance can be used. Thereby, reduction of the excitation current Iex in SW1 and SW2 can be suppressed, and as a result, L can be efficiently driven by the constant voltage potential VR.

  The high counter electromotive voltage generated in the exciting coil L is also applied to the contact terminals of SW3 and SW4. However, these contact terminals generally serve as the drain terminals of MOSFETs (N-channels). Even if it does not have a breakdown voltage specification, the MOSFET is not damaged.

  Further, the high counter electromotive voltage generated in the exciting coil L is also applied to the contact terminals of SW11 and SW12. However, since SW11 and SW12 have a high breakdown voltage specification, there is no fear of damage. At this time, the on-resistances of SW11 and SW12 have a relatively large value as compared with the switch circuit of the normal specification. With the on-resistances of SW11 and SW12, discharge is performed from the capacitive element C to the exciting coil L in the exciting current Iex. Only the discharge current that has a margin in power is reduced. Therefore, it is possible to efficiently supply the input current component supplied from VR to L with little power margin without being reduced.

  Further, in the present embodiment, since the capacitive element C is disconnected from the current input terminal Tin of the switching circuit 11, the path that flows backward from C to VP is only the path via SW1 and SW2. Therefore, in place of the forced cutoff period TX specified by the forced cutoff signal SX, an excitation stop period for controlling both SA and SB to a low level (GND potential) is provided, and SW1 and SW2 are simultaneously turned off. Good. Thereby, in the period until VC rises to VP, the backflow from C to VP can be prevented and the forced cutoff signal SX can be omitted even if Q21 and Q22 remain on.

[Extended embodiment]
The present invention has been described above with reference to the embodiments, but the present invention is not limited to the above embodiments. Various changes that can be understood by those skilled in the art can be made to the configuration and details of the present invention within the scope of the present invention. In addition, each embodiment can be implemented in any combination within a consistent range.

  DESCRIPTION OF SYMBOLS 10 ... Excitation circuit, 11 ... Switching circuit, 12 ... Constant current circuit, 13 ... Detection circuit, 14 ... Cut-off circuit, REG ... Voltage regulator, SW1, SW2, SW3, SW4 ... Switch circuit, SW31, SW32 ... High breakdown voltage switch circuit Q1, MOSFET (P channel), Q2, MOSFET (N channel), Q11, Q21, Q22, MOSFET (N channel), ZD1, ZD2, Constant voltage diode, D1, D2, D21, D22, D23, D24, Diode , DB ... diode bridge, C ... capacitive element, L ... exciting coil, R1, R2, R3, R4, R5, R6, R7, R11, R12 ... resistance element, Tin ... current input terminal, Tout ... current output terminal, L1 , L2 ... terminal, VP ... power supply potential, VR ... constant voltage potential, VC ... charging voltage, GND ... ground potential, SA, B ... excitation signal, SX ... forced cutoff signal, SY ... detection blocking signal, TX ... forced cut-off period, TY ... detection cut-off period, Iex ... excitation current, VL ... inter-terminal voltage.

Claims (4)

By supplying an excitation current from an excitation circuit to an excitation coil arranged outside the measurement tube, the fluid flowing in the measurement tube is excited, and from a pair of electrodes arranged in the measurement tube, the excitation An excitation circuit that is used in an electromagnetic flowmeter that detects an electromotive force orthogonal to a magnetic field from the excitation coil generated in the fluid and measures a flow value of the fluid based on the electromotive force,
A switching circuit that supplies an input current that flows from a current input terminal to a current output terminal based on an excitation signal composed of a pulse signal having an excitation frequency and that is switched and supplied as one of the excitation coils to the other end of the excitation coil;
A capacitive element connected between the current input terminal and a ground potential;
A diode bridge that rectifies a counter electromotive voltage generated from the exciting coil and charges the capacitive element;
A detection circuit that detects a detection cutoff period in which a charging voltage charged in the capacitive element is higher than the power supply potential, and outputs a detection cutoff signal;
The connection / cutoff between the power supply potential and the current input terminal is controlled by two contact terminals, and the other contact connected from the one contact terminal connected to the power supply potential side to the current input terminal side A first MOSFET including a first parasitic diode that allows a current to flow to a terminal, and a forced cutoff that indicates a forced cutoff period having a predetermined length from the switching timing of the detection cutoff signal and the excitation signal from the detection circuit; Based on the signal, the first MOSFET is controlled to be in an off state during the detection cutoff period and the forced cutoff period, and the first MOSFET is turned on in a period other than the detection cutoff period and the forced cutoff period. An excitation circuit comprising: a cutoff circuit to be controlled. The excitation circuit according to claim 1,
The interruption circuit is
The first MOSFET consisting of a P-channel MOSFET, including a parasitic diode having a drain terminal connected to the power supply potential, a source terminal connected to the current input terminal, and a current flowing from the drain terminal to the source terminal;
Based on the detection cutoff signal and the forced cutoff signal, the reverse flow of the charging voltage with respect to the power supply potential is cut off by controlling the first MOSFET to be in an off state during the detection cutoff period and the forced cutoff period. And a second MOSFET made of an N-channel MOSFET. The excitation circuit according to claim 1,
A voltage regulator that generates a constant voltage potential from the power supply potential and supplies the constant voltage potential to the current input terminal instead of the power supply potential;
The detection circuit detects a period in which the charging voltage is higher than the constant voltage potential as the detection cutoff period, and outputs the detection cutoff signal.
The cutoff circuit includes the first N channel MOSFET including a parasitic diode that has a source terminal connected to the constant voltage potential, a drain terminal connected to the current input terminal, and a current flowing from the source terminal to the drain terminal. An excitation circuit that shuts off a reverse flow of the charging voltage with respect to the constant voltage potential by turning off the first MOSFET in the detection cutoff period and the forced cutoff period. . The excitation circuit according to claim 1,
The switching circuit is
A first switch circuit having one contact terminal connected to the current input terminal and performing an on / off operation in response to the excitation signal;
A second switch circuit having one contact terminal connected to the current input terminal and performing an on / off operation in an opposite phase to the first switch circuit;
A third switch circuit that has one contact terminal connected to the one end of the exciting coil and the other contact terminal connected to the current output terminal, and is turned on / off in a phase opposite to that of the first switch circuit. When,
A fourth switch circuit that has one contact terminal connected to the other end of the exciting coil and the other contact terminal connected to the current output terminal, and is turned on / off in the same phase as the first switch circuit. And
The excitation circuit is
A voltage regulator that generates a constant voltage potential from the power supply potential and supplies the constant voltage potential to the current input terminal instead of the power supply potential;
A first high-breakdown-voltage switch circuit that performs on / off control of the supply of the discharge current from the capacitive element to the one end of the exciting coil in the same phase as the first switch circuit;
A second high-breakdown-voltage switch circuit that controls on / off of supply of the discharge current from the capacitive element to the other end of the exciting coil in a phase opposite to that of the first switch circuit;
The detection circuit detects a period in which the charging voltage is higher than the constant voltage potential as the detection cutoff period, and outputs the detection cutoff signal.
The interruption circuit is
A parasitic diode having a source terminal connected to the other contact terminal of the first switch circuit, a drain terminal connected to the one contact terminal of the third switch circuit, and a current flowing from the source terminal to the drain terminal; Including the first MOSFET comprising an N-channel MOSFET;
A parasitic diode that has a source terminal connected to the other contact terminal of the second switch circuit, a drain terminal connected to the one contact terminal of the fourth switch circuit, and allows a current to flow from the source terminal to the drain terminal; Including a second MOSFET made of an N-channel MOSFET,
The cut-off circuit turns off the first MOSFET and the second MOSFET in the detection cut-off period and the forced cut-off period, so that the one end or the other end of the exciting coil with respect to the constant voltage potential An excitation circuit characterized by blocking the reverse flow of the voltage.

Images