JPH0377071A - Current detecting device - Google Patents

Abstract

PURPOSE:To stably measure a current with high accuracy by converting a very small DC voltage across a detection resistance into an alternating current and amplifying the current, converting it into a direct current and then feeding an output voltage proportional current back, and obtaining a DC signal which is proportional to the current to be measured. CONSTITUTION:The current detecting device consists of the detection resistor 2 which converts the current IL to be measured into the voltage, a switching circuit 9 which inputs it, a transformer 10, an AC amplifier 11, an AC/DC converting circuit 12, a filter circuit 13, and a current-voltage converting circuit 14 which are connected in order, and an oscillation circuit 16 which supplies an ON/OFF signal to the circuit 9. Then the resistor 2 which is so small that its voltage drop causes no problem converts the current into the voltage, which is AC-converted by the circuit, applied to the amplifier 11 by the transformer 10, and amplified; and the voltage is converted by the circuit 12 into the direct current, which is fed back to the circuit 9 to obtain the DC signal which is proportional to the current IL to be measured. Consequently, the current detection which is tolerant to the common mode noise and stable to temperature variation and requires no adjustment becomes possible.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a current detection device comprising a circuit configuration for accurately and stably measuring direct current.

[Conventional technology]

FIG. 4 is a diagram showing the most common current detection method. In Fig. 4, 1 is a power source for supplying current 2 to electric load 3, 2 is a current detection resistor, and its resistance value is R8.
shall be. 4 is the instrumentation amplifier part 2
The output voltage V is proportional to the voltage between the two input terminals 41 and 42. 7 can be obtained at the output terminal 43.

In FIG. 4, since V r −I L・Rs,
If the amplification degree of the instrumentation amplifier 4 is A', then Vooy=A'·ILR-R, and the current (2) flowing through the electric load 3 can be measured.

Generally, a voltage drop across the detection resistor 2 is undesirable, so the resistance value R8 is set as small as possible. For example, when the maximum measured current is 100 A and the voltage drop across the detection resistor 2 is 1.0+V, R8 is 0.1 d. Kono Toki, 1. = 10A, then
V + =1 mV, which is a fairly small value as a voltage level handled by the instrumentation amplifier 4. Such power supply detectors are also used for measuring the output current of motor vehicle generators, in which case the ambient temperature of the detector varies from -40°C to +100'C. Furthermore, in a car, the car body is used as an electrical ground, so current detection must be measured on the power supply side (usually the positive side), so the potential of the detection resistor 2 is equal to the voltage of the onboard pantry. Become. Automotive power supplies usually fluctuate by more than 1 volt due to fluctuations in various on-vehicle electrical loads. Therefore, the instrumentation amplifier 4 must amplify a minute voltage such as 1 mV under common mode noise of 1 volt or more.

Under such environmental conditions, it may not be technically impossible to obtain an instrumentation amplifier that can stably amplify voltages below 1-V as described above, but when considering the cost involved, it is impossible. , not realistic.

FIG. 5 is a drawing showing the principle of a method used for the purpose of solving the above-mentioned difficulties, and this method was first published in Japanese Patent Laid-Open No. 6
This method is described in detail in JP-A No. 3-228071, JP-A-1-113674, and the like.

In FIG. 5, reference numeral 5 denotes a ferromagnetic ring having a slit-like notch in a part thereof, and a conducting wire 20 is disposed so as to pass through this. A Hall element 6 is disposed in the notch of the ferromagnetic ring 5 through which 0 flows, so as to measure the circumferential magnetic field of the notch of the ferromagnetic ring 5. 7 is a source 1 for supplying current (or voltage) to the Hall element 6; 8 is an amplifier for amplifying the output voltage of the Hall element 6; its output terminal 8;
The voltage obtained in 1 is V. 17, the magnitude of the magnetic field applied to the Hall element 6 is naturally proportional to the current IL flowing through the conductor vA20, so the relationship between IL and V0IIT is given by the following equation.

VO1+T= K IL+ VCl −
・"(1) In equation (1), K is a constant and is determined by the dimensions of the magnetic ring 5, the sensitivity of the Hall element 6, and the amplification degree of the amplifier 8.■. is the offset voltage, It is determined by the residual magnetism of the ring 5, the unbalanced voltage of the Hall element 6, and the offset voltage of the amplifier 8.The method shown in FIG. 5 is different from the method shown in FIG. This is extremely useful because there is no need to worry about voltage drop due to the detection resistor 2, but there are many ways to keep K and V in equation (1) at stable constant values. Among the elements that determine the constant K, the Hall element 6
The sensitivity varies greatly from component to component and is highly temperature dependent. Furthermore, the unbalanced voltage of the Hall element 6, which is a factor that determines the offset voltage (2o), also varies widely from component to component and has temperature dependence. For this reason, in the current detector according to the method shown in Fig. 5, the Hall element 6
Some adjustment is required in the manufacturing process to compensate for variations in sensitivity and unbalanced voltage, and temperature compensation also needs to be performed. The temperature dependence (temperature characteristics) of the sensitivity and unbalanced voltage of the Hall element 6 will show the same tendency if the type (model name) of the Hall element 6 is the same, but it is guaranteed that they have exactly the same temperature coefficient. Therefore, even if temperature compensation is performed, temperature dependence cannot be completely eliminated.

[Problem to be solved by the invention]

As mentioned above, in the method of converting the current to be measured into voltage using a detection resistor, it is necessary to amplify the minute DC voltage in the presence of large common mode noise, and it is necessary to amplify the small DC voltage in the presence of large common mode noise. The method of measuring with a Hall element requires adjustment to reduce the effects of variations in the sensitivity of the Hall element and unbalanced voltage, and also requires compensation for the temperature characteristics of the above two parameters, and complete compensation is not possible. There were some issues such as difficulty.

The present invention has been made to solve the above-mentioned problems, and an object thereof is to obtain a current detection device that is resistant to common mode noise, does not require adjustment, and is stable against temperature changes.

[Means to solve the problem]

The current detection device according to the present invention converts current into voltage using a detection resistor whose resistance value is sufficiently small so that voltage drop does not become a problem, and further converts this voltage into AC using a switching circuit, and then leads it to an AC amplifier using a transformer. By amplifying AC, converting the AC amplified signal back to DC, and feeding it back to the switching circuit in the form of current, a DC signal proportional to the magnitude of the current to be measured is obtained. .

[For production]

The current detection device of the present invention converts a DC voltage proportional to the current to be measured into an AC voltage having an amplitude proportional to the current, and converts the DC voltage into an AC voltage having an amplitude proportional to the current to be measured. After amplification, it is converted to DC using an AC/DC conversion circuit, and this is then fed back as a current to the input.

〔Example〕

An embodiment of the present invention will be described below with reference to the drawings. 1st
In the figure, 1.2.3 is a power supply, a detection resistor, and an electric load connected in series to form one closed loop,
A current iL flowing to the electrical load 3 via the detection resistor 2 connected to the non-grounded side of the power source 1 is the current to be measured. Reference numeral 9 denotes a switching circuit, and the control input is a DC voltage V applied between an input terminal 91 connected to one end on the high potential side of the detection resistor 2 and a common terminal 92 connected to the other end on the low potential side. It switches synchronously with the pulse signal applied to the terminal 93, and generates an alternating current signal (pulse train) e between the output terminal 94 and the common terminal 92 with an amplitude proportional to v3. Between the input terminal 91 and the output terminal 94, an input resistor 95 and a load resistor 96 having a resistance value R, whose connection point is a terminal 99, are connected in series. A switching element 97 such as a PNP transistor whose base is connected to the control input terminal 93 has its base connected to the terminal 99 and its collector connected to the common terminal 92. Bypass resistor 98 is connected between base common terminals 92 of switching element 97 . In addition, the switching circuit 9 is designated by reference numerals 91 to 9.
It is composed of 9 elements.

10 is a transformer for removing common mode noise, with an output terminal 94 and a common terminal 92 as input terminals on the primary side, one end of the secondary side being grounded, and the other end connected to the amplifier body 110 for removing DC components. It is connected to the input end of an AC amplifier 11 which is connected in series with a coupling capacitor 111. 1
2 is an AC/DC converter circuit, in which the collector of a switching element 122 whose emitter is grounded, such as an NPN (NPN) transistor, is connected to the output terminal 112 of the AC amplifier 11 via a resistor 121, and the base thereof is connected via a resistor 123. It is connected to the output terminal of an oscillator 15, which will be described later. 13 is a filter circuit, which includes an input resistor 131, a capacitor 132 .
It consists of an operational amplifier 133, known as an integrator in the shell.

This filter circuit 13 has an input terminal connected to the AC/DC converter 1.
It is connected to the collector of the second switching element 122.

14 is a voltage/current conversion circuit, the emitter connected to the output terminal 143 is grounded via a resistor 142, and the collector is connected to the terminal 9 of the switching circuit 9 via a terminal 144.
9, and its base is connected to the output terminal of the filter circuit 13. Reference numeral 16 denotes a drive circuit, in which the base of a switching element 161 whose emitter is grounded is connected to the oscillator 15 via a resistor 162, and the collector is connected via a resistor 163 from an output terminal 164 to a control input terminal 93 of the switching circuit 9. ing.

2 and 3 are diagrams for explaining the operation of the switching circuit 9. FIG.

Next, the operation of this embodiment will be explained with reference to FIGS. 1 to 3. A current ■ flows from the power supply 1 to the detection resistor 2 and the electric load 3. This current IL is passed through the detection resistor 2 to the electrode V,
and the input terminal 91 of the switching circuit 9. It is applied between the common terminals 92.

In FIG. 2, a resistor 960 whose value is RE is an equivalent impedance when looking right from the terminal 99 in FIG. becomes 0 oscillator 1
The switching element 161 of the drive circuit 16 is turned on and off by a signal with a repetition frequency f of 50% and a duty ratio of 50%.
In response to this on/off, the switching element 97 of the switching circuit 9 repeats on/off at a repetition frequency f and a duty ratio of 50%. When the switching element 97 is turned on, the switching element 97
The emitter-collection voltage e, is zero. Also,
When the current IF flows from the input terminal 99 to the voltage/current conversion circuit 14 (not shown in FIG. 2) and the switching element 97 is off, the value of the output voltage e is E.

is given by the following equation.

Therefore, the voltage eII across the resistor 960 becomes a pulse train with an amplitude of E (P-P value) as shown in FIG.

This signal e containing a DC component is supplied to an AC amplifier 11 via a transformer 10.

The transformer IO removes common mode noise caused by fluctuations in the voltage of the power supply l, e.

is transmitted to the AC amplifier 11. The signal e0 obtained at the output terminal 112 of the AC amplifier 11 is connected to the coupling capacitor 111.
This is an AC signal from which DC components such as offset voltage generated in the amplifier main body 110 are removed. The output e0 of the AC amplifier 11 is given by the following equation.

In equation (2), G is a constant determined from the impedance of the transformer 10, the coupling ratio, the input impedance of the AC amplifier 11, and the amplification factor. The AC/DC converter circuit 12 which receives the output signal e0 of the AC amplifier 11 repeatedly turns on and off the switching element 122 at a repetition frequency f and a duty ratio of 50% in accordance with the output of the oscillator 159.
The AC/DC conversion circuit 12, which performs phase detection or synchronous detection, operates synchronously with the control input from the oscillator 15, thereby converting the input AC signal e0 from the AC amplifier 1 into a DC signal, reflecting its phase as well. . The output voltage e of the AC/DC conversion circuit 12 is zero if the switching element 122 is on, and L-G4t.
The value becomes (Vs 11R+)/(Rt+R+).

Here, K is the resistance 121. It is a constant determined by the value of the resistor 131, and its absolute value is smaller than 1.

The filter circuit 13 acts as an integrator, and attenuates the AC component of the input from the AC/DC converter circuit 12 so that most of it is lost, leaving only the DC component. When the DC amplification degree of the filter circuit 13 is A and the output voltage (DC) is E, Es is given by the following equation.

L −4+ ・G4z・(Vs −1y・R+)/ (
Rx +R+)... (3) Guided to the conversion circuit 14 and connected to the terminal 99 of the switching circuit 9
Feedback current to flow into 1. is generated, and a voltage output Eout is generated at the output terminal 143.
In the voltage/current conversion circuit 14, the feedback current IF and the output voltage Eout are given by the following equation.

Eout= EI VIE --(
4) Iy=(Em-Vmz)/Ro=Eooy/R
o -- (5) Here, VIE is the base-emitter voltage of the transistor serving as the switching element 141.

From equations (3), (4), and (5), Eout is given by equation (6) as a function of V.

(6) Here, since G and A are sufficiently larger than 1, 1G-Rc4+ / Ro (Ri + R+) > 1 holds true, and the output voltage E of the filter circuit 13, is voltage/current (7). Typical values are R+""100Ω, R1"
I KO, K+=up, R,=10K11. C=1
Substituting 000°A = 100,000, (7)
The denominator of the second term in the equation is approximately 230, and even if it is ■□-0.7V, the value of the second term is 3■V and can usually be ignored.

Therefore, equation (7) is further simplified to obtain equation (8).

On the other hand, ■ is the measured current IL and the value R1 of the detection resistor 2.
Since it is given by the product of Eour −It XRs XRo/R+ −
...'(9). Therefore, in the constant example above, R
,=0.1ml, EouT ``'0.011 L''
'''・Oω, and when 1L-100A, Eo.

, is 1 volt. Incidentally, the voltage drop (3) across the detection resistor 2 at this time is 10+V, which is at a level that poses no problem in practice.

The biggest features of this invention are the measured current IL and the output voltage E. The UT relationship is expressed by a very simple equation (9), and the sensitivity is determined only by the resistance ratio. Note that the function of the oscillator 15 is to generate a signal for synchronously turning on and off the two switching elements 97 and 122, and it is not necessary to have performance that is very strict regarding frequency stability. The drive circuit 16 includes a switching element 97 that is at a potential on the non-grounded side (positive side) of the power supply l.
This is to turn on and off the power accurately.

In the above explanation, PNP is used as the switching element 97.
) transistor, NPN as switching element 122
) Although an example using transistors has been described, as long as these switching elements have similar functions, the same effect as the above embodiment can be obtained even if a field effect transistor or the like is used.

In addition, the on/off ratio of the switching elements 97 and 122 is set to 5.
Although the explanation has been made assuming a value of 0%, there is no change in the essence of the operation even if a value other than 50% is taken, and there is no change in the conversion formula given by equation (9).

〔Effect of the invention〕

As described above, according to the present invention, by sufficiently amplifying the minute DC voltage at the detection resistor after converting it into AC, and then feeding back the current proportional to the output voltage after converting it to DC, the three resistors are Since the configuration is such that the output characteristics are determined only by the ratio, it is extremely resistant to fluctuations in the constants of many component parts, and combined with the common mode noise removal effect of the transformer, it is suitable for harsh environmental conditions such as cars. It has the advantage of being able to provide a device for stably and highly accurate current measurement at low cost.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing the configuration of a current detection device according to an embodiment of the present invention, FIGS. 2 and 3 are explanatory diagrams for explaining the operation of the circuit in FIG. 1, and FIGS. FIG. 5 is a diagram showing the configuration of a conventional current detection device. In the figure, 1...Power source, 2...Detection resistor, 3...Electric load, 9...Switching circuit, 10...Transformer, 11...AC amplifier, 12...AC/DC Conversion circuit, 13... Filter circuit, 14... Voltage/current conversion circuit, 15... Oscillator, 16... Drive circuit, 95
... (first) resistance, 96 ... (second) resistance, 9
7...Switching element. Note that the same reference numerals in the figures indicate the same or equivalent parts.

Claims (1)

[Claims] A detection resistor that is connected to the non-grounded side of the power supply and converts the current to be measured into a voltage; a switching circuit that uses one end of the detection resistor as an input terminal and the other end as a common terminal; and a switching circuit that is connected to the output of the switching circuit. an AC amplifier whose input is the output of the transformer, an AC/DC conversion circuit whose input is the output of the AC amplifier, a filter circuit whose input is the output of the AC/DC conversion circuit, and the filter. A voltage/current conversion circuit that receives the output of the circuit as an input, and an oscillation circuit that supplies on/off signals to the switching circuit,
The switching circuit includes a first resistor whose one end is connected to the input terminal, the other end of the first resistor and the common terminal, and which is turned on and off in response to a signal from the oscillation circuit.
A switching element is provided to turn off, the output side of the voltage/current conversion circuit is connected to the connection point between the first resistor and the switching element, and one end thereof is connected to the connection point between the switching element and the first resistor. 1. A current detection device characterized in that a second resistor is connected so that the other end serves as an output terminal.