JPH034126Y2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an overcurrent limiting circuit that detects overcurrent and limits the current flowing to a load.

2. Description of the Related Art Conventionally, current limiting circuits that protect a load by detecting an overcurrent flowing through the load and limiting the current include those shown in FIGS. 1, 2, 3, and 4, for example.

The current limiting circuit shown in FIG. 1 includes a switching transistor 2 on the power supply line to the load 1, a resistor R1 and a constant current diode DC connected between the base and collector of the transistor 2, and
When the load current flowing between the emitter and collector of transistor 2 increases, the voltage across resistor R1 increases due to the increase in base current, and when it reaches the specified voltage, thyristor 3 is triggered and shoots across the emitter and base of transistor 2. Transistor 2 is cut off at this point. In addition, in Figure 2, PUT4 is used instead of thyristor 3 in Figure 1. Similarly, when the base current increases, the gate potential of PUT4 decreases, and the anode potential becomes higher than the gate potential by more than a specified value. It turns on and shoots between the base and collector of transistor 5, cutting off transistor 5.

However, in the circuits shown in Figures 1 and 2, if the load current is cut off by the trigger of thyristor 3 or PUT 4, the circuit will not be restored unless the power supply is cut off, resulting in temporary overcurrent due to noise etc. There is a problem in that even if the power supply is activated, a recovery operation must be performed to cut off the power supply each time.

FIG. 2 shows transistor 6 and resistors R1, R
It is equipped with a relay 7 operated by a current detection circuit consisting of 2 and R3, and the relay contact 7a of the relay 7 is connected in parallel with a current limiting resistor R4. The voltage drop across resistor R3 is divided by resistors R1 and R2 and supplied to the base of transistor 6, and when the collector current of transistor 6 due to an increase in load current reaches the operating current of relay 7, relay 7 operates to close the relay contact. 7a open and current limiting resistor R4
The current flowing through the load 1 is limited by intervening.

However, in the circuit shown in Figure 3, the voltage between the base and collector of transistor 6 changes significantly depending on the temperature, so the base current of transistor 6 that operates the relay fluctuates, making the operating point unstable, and the relay Since the base current of transistor 6 must be selected to be above a certain value since relay 7 is operated, the overcurrent range in which current can be limited is restricted, and furthermore, when relay 7 is operated, relay contact 7a is opened. Since there is a time delay before the current limit is reached, the maximum current flows through the load during this delay time, resulting in a problem that the responsiveness of the current limit is low.

FIG. 4 shows a conventional current limiting circuit using a switching transistor 8 and a current detection transistor 9. When the base-emitter voltage of the transistor 9 determined by the current detection resistor R5 exceeds a specified value, the transistor 9 is turned on. and transistor 8
By shooting between the base and emitter of the transistor 8, the transistor 8 is cut off and the current of the load 1 is limited.

However, in this circuit as well, as in the circuit shown in Figure 3, the base-emitter voltage of transistors 8 and 9 changes significantly depending on the temperature, and the operating point for detecting overcurrent and limiting current is not stable, and the current is limited by the mutual feedback operation of transistors 8 and 9, so the circuit impedance changes linearly until the current is limited, and the load current changes to the specified current for current limiting. In the case where the previous value was reached, there was a risk that the transistors 8 and 9 would be balanced before reaching the circuit impedance for current limiting, resulting in an unreliable current limiting operation state.

The present invention was developed in view of these conventional problems, and it is possible to easily set the operating point of current limit, is not affected by temperature, and does not significantly reduce the circuit impedance until current limit is applied. It is an object of the present invention to provide an overcurrent limiting circuit which suppresses the change and can automatically cancel current limiting if the load current returns to normal after current limiting.

To achieve this objective, the present invention provides a bridge circuit that includes the load as a bridge resistor, detects changes in bridge balance due to overcurrent, turns off a switching element, and connects a current limiting resistor connected in parallel to this switching element. The current flowing through the load to the load is limited.

Hereinafter, embodiments of the present invention will be described based on the drawings.

FIG. 5 is a circuit diagram showing an embodiment of the present invention.

First, to explain the configuration, the current limiting circuit of the present invention has four terminals, including a primary terminal with terminals 10 and 11 to which current is input, and a secondary terminal with terminals 12 and 13 to which the load 1 is connected. It is configured as a circuit network, and one end of a current limiting resistor R10 is connected to one terminal 10 of the primary terminals, and this current limiting resistor R10
The emitter and collector (electrode of the circuit) of a transistor 14 which operates as a switching element are connected in parallel with the transistor 14. Of course, the transistor 14 has a base terminal as a control terminal, and a bias resistor R12 is connected between the collector and the base.

The other end of the current limiting resistor R10 is the transistor 14.
It includes the load 1 as a bridge resistor together with the emitter side of the resistor 1, and is connected to the connection point of the second resistor R20 and the third resistor R30 in a bridge circuit including the second resistor R20, the third resistor R30, and the fourth resistor R40. As for the terminals 12 and 13 serving as the secondary terminals, one terminal 12 is first connected to the third resistor R30 of the bridge circuit. Further, the terminal 13 is connected to a series circuit of a second resistor R20 and a fourth resistor R40 forming one side of the bridge circuit, and is further connected to the terminal 11 on the primary terminal side.

In this way, in order to detect the bridge balance of the bridge circuit formed by the resistors R20, R30, R40 and the load 1, the connection point between the third resistor R30, which is one side of the bridge, and the load 1, that is, the terminal 12
and a second resistor R2, which is the other side of the bridge circuit.
The connection point between 0 and the fourth resistor R40 is the photocoupler 15.
The light emitting diode PD is connected to the light emitting diode PD, in which the potential V1 at the connection point between the second resistor R20 and the fourth resistor R40 is the potential at the connection point between one of the secondary terminals 12, that is, the third resistor R30 and the load 1. Light is emitted by the current that flows when the voltage exceeds V2. still,
A diode D is provided on the signal line connected from the connection point of the bridge circuit to the light emitting diode PD of the photocoupler 15 to prevent backflow to the light emitting diode PD.

Phototransistor provided in photocoupler 15
The collector of PT is connected to the base of the transistor 14 which operates as a switching element, and the phototransistor PT is activated by light emission from the light emitting diode PD.
When the transistor 14 is turned on, the transistor 14 is turned off.

Next, the operation of the embodiment shown in FIG. 5 will be explained.

First, in a steady state in which the current flowing to the load 1 via the transistor 14 and the resistor R30 is below the specified current value set for applying current limitation, one terminal 12 of the secondary side terminals, that is, the bridge circuit The potential V2 at the connection point between the third resistor R30 and the load 1 is the same as the potential V2 at the connection point between the second resistor R20 and the fourth resistor R40.
The potential at the connection point of the transistor PT is approximately equal to or higher than the potential V1, and as a result, no current flows to the light emitting diode PD of the photocoupler 15, so the phototransistor PT is turned off, and the transistor 14
A load current is supplied to the load 1 by conduction.

Next, if the load current exceeds the specified value,
The potential V2 at the terminal 12 is lower than the potential V1 at the connection point between the second resistor R20 and the fourth resistor R40, so that a driving current flows through the light emitting diode PD of the photocoupler 15 to emit light, turning on the phototransistor PT. Turning on phototransistor PT pulls the base of transistor 14 to zero volts, cutting transistor 14 off.
When the transistor 14 is turned off, the current to the load 1 flows through the current limiting resistor R10 connected in parallel to the transistor 14, and the current limiting resistor R10
The current is limited to a small amount determined by the resistance value of , and the load 1 is protected from overcurrent.

Of course, if the cause of the overcurrent in load 1 is resolved and the normal state returns after current limitation, the potential of terminal 12 will change.
V2 rises and the light emitting diode PD stops emitting light, and the phototransistor PT is turned off, turning on the transistor 14 again and returning to the state of supplying a normal load current.

FIG. 6 is a circuit diagram showing another embodiment of the present invention, and this embodiment is characterized by using an operational amplifier 16 instead of the photocoupler 15 in FIG. The connection point between the second resistor R20 and the fourth resistor R40 in the bridge circuit is connected as an input, and one terminal 12 on the secondary terminal side, that is, the third
An oscillation prevention capacitor is used to connect the connection point between the resistor R30 and the load 1 as an input, connect the output of the operational amplifier 16 to the base of the transistor 14, and prevent oscillation due to feedback connection from the input side to the output side of the operational amplifier 16. C1 is connected.

According to the embodiment shown in FIG. 6, in the photocoupler 15 shown in FIG. 5, in order to drive the light emitting diode PD to emit light, the potential V2 at the bridge connection point exceeds a predetermined value with respect to the potential V1 at the bridge connection point. Sometimes the current is limited, but by using the operational amplifier 16, when the potential V1 of the bridge connection point to the negative input terminal is slightly exceeded by the potential V2 of the other bridge connection point, the output of the operational amplifier 16 becomes high. level, the current can be limited by cutting off the transistor 14, and by using the operational amplifier 16, the detection accuracy for changes in bridge balance can be increased, further improving the responsiveness of the cutoff operation to overcurrent. I can do it.

Next, to explain the effects of the present invention, changes in bridge balance due to overcurrent in a bridge circuit that includes a load as a bridge resistor are detected and a switching element connected in parallel with a current limiting resistor is turned off. Since it is only necessary to provide circuit means for detecting the bridge balance and the bridge balance, the circuit configuration is not too complicated, and the breaking current value can be easily set by appropriately determining the value of the bridge resistance in the bridge circuit. Since the circuit detects overcurrent, the cutoff current value does not change due to temperature changes, and the current limiter detects changes in bridge balance and cuts off the transistor, so the circuit impedance remains unchanged until the current limiter is applied. If there is almost no change and the cause of the overcurrent on the load side is resolved after current limiting, the bridge balance will return to its original state and the current limiting state can be automatically released and restored. The current value for this can also be accurately set based on the bridge resistance that provides the breaking current value.

[Brief explanation of the drawing]

Figures 1, 2, 3, and 4 are circuit diagrams showing conventional overcurrent limiting circuits, Figure 5 is a circuit diagram showing one embodiment of the present invention, and Figure 6 is a circuit diagram showing another embodiment of the present invention. FIG. 1: Load, 10, 11: Terminal (primary terminal), 1
2, 13: terminal (secondary terminal), 14: transistor (switching element), 15: photocoupler,
16: Operational amplifier, R10: Current limiting resistor, R2
0: second resistor, R30: third resistor, R4: fourth resistor, PD: light emitting diode, PT: phototransistor, R12: bias resistor.

Claims (1)

[Scope of utility model registration request] A four-terminal circuit network is formed that includes a primary terminal to which a power source is connected as input and a secondary terminal to which a load is connected, one end of a current limiting resistor is connected to one of the primary terminals, and the circuit is connected in parallel with the current limiting resistor. A circuit electrode of a switching element having a control terminal is connected, and the other end of the current limiting resistor is connected to one of the secondary terminals via a third resistor on one side of the bridge circuit, and the other end of the current limiting resistor is connected to one of the secondary terminals on the other side of the bridge circuit. connected to the other of the secondary terminals through a series circuit of second and fourth resistors;
Further, the other of the primary terminal and the secondary terminal is connected, and the potential at the connection point between the second resistor and the fourth resistor of the bridge circuit is the connection point between the third resistor and the load. An overcurrent limiting circuit comprising a circuit that applies a non-conducting bias to the control terminal of the switching element when the potential exceeds the potential, and applies a conductive bias when the potential falls below the switching element.