JPH044813B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a short circuit protection circuit in a signal transmission circuit using a photocoupler.

Prior Art and Its Problems As shown in Figure 1, the photo coupler signal transmission circuit connects the input end of the photo coupler PC between input terminals 1 and 2 via a protective resistor R1, and outputs an output current to the output side of the photo coupler PC. A transistor TR1 which amplifies and performs a switching action is connected. Resistor R2 is the base-emitter resistance of transistor TR1, and is a photocoupler PC.
It absorbs the dark current of the transistor PCTR and prevents the transistor TR1 from malfunctioning. Also, the Zener diode ZD is a Zener diode for protecting the element. A power source 5 and a load 6 are connected in series between the output terminals 3 and 4 as shown.

In the conventional example shown in Figure 1, when an input signal is applied between input terminals 1 and 2, the photo coupler PC
The light emitting diode of is driven, the photocoupler transistor PCTR is turned on, and the transistor
TR1 is also turned on and load 6 is driven by power supply 5. However, if the load 6 is not connected and the output terminals 3 and 4 are directly connected to the power supply 5, and the transistor of the photocoupler PC becomes conductive, a short circuit current flows between the output terminals 3 and 4, and the transistor TR1 is destroyed. I end up. Also load 6
Even if the transistor TR1 is connected, if the load 6 becomes short-circuited due to deterioration or the like, the output transistor TR1 will be destroyed. In order to prevent such destruction of the output transistor, a circuit has been proposed that detects short-circuit current and stops driving the light emitting diode of the photo coupler PC via another photo coupler. However, such a circuit has a disadvantage in that it has a complicated configuration and requires a plurality of photo couplers, which increases the price.

Purpose of the Invention The present invention has been made in view of the problems of the conventional signal transmission circuit, and uses the base terminal of the transistor on the light receiving side of the photocoupler to prevent the photoisolation output from short-circuiting. The purpose of the present invention is to provide a short circuit protection circuit that protects an output element.

Structure and Effects of the Invention The present invention provides a photocoupler whose light-receiving side circuit is composed of a photocoupler transistor whose base is an external terminal, and which has an output control terminal that switches on and off the load current based on the operation of the photocoupler transistor. A transmission circuit,
an output current detection resistor connected in series to the output control element; and a three-terminal switching element with a self-holding function connected in parallel to the output control element and triggered when the voltage of the current detection resistor reaches a predetermined value. , wherein the base of the photocoupler transistor is connected to a three-terminal switching element,
The present invention is characterized in that the photocoupler transistor is turned off when the three-terminal switching element is conductive.

According to the present invention having such features, when a large current flows through the output control element due to a load short circuit or the like, the three-terminal switching element is triggered and the photocoupler transistor is turned off. As long as current is supplied from the power supply, the three-terminal switching element continues to operate even after the load short-circuit condition ends, so no signal is transmitted from the input side to the output side of the photo coupler, protecting the output control element. It becomes possible. The present invention does not use a photo coupler for purposes other than signal transmission to protect against such short circuits, so the configuration is simple and it is possible to ensure short circuit protection in the output signal transmission circuit.

DESCRIPTION OF THE EMBODIMENT FIG. 2 is a circuit diagram showing an embodiment of the present invention. In this figure, the same parts as in the conventional example are shown using the same reference numerals. In the case of this embodiment as well, the input end of the photo coupler PC is connected between input terminals 1 and 2 via the protective resistor R1 as in the conventional example.
The transistor PCTR on the output side is output terminal 3,
4 are connected in series together with a resistor R2 that absorbs dark current. This photocoupler transistor
The point that a switching transistor TR1 is provided at the emitter of PCTR is also similar to the conventional example described above. Now, in the present invention, a resistor R3 is connected to the emitter of the transistor TR1 to detect its output current.
will be provided. The connection end with the emitter is a normal P-gate thyristor via resistor R4.
Connected to the gate of SCR1. thyristor
The SCR1 is connected in parallel with the switching transistor TR1 between the output terminals 3 and 4, together with a resistor R5 that provides a holding current to the thyristor SCR1. And the photo cutlet transistor PCTR
The base of is connected to the thyristor via diode D1.
Connected to the anode of SCR1. Zener diode ZD is connected between output terminals 3 and 4 as in the conventional example to protect each element against surge voltage, and capacitor C is connected to thyristor SCR1.
This is provided to improve the dv/dt withstand voltage.

Now, in this embodiment, when an input signal is applied between input terminals 1 and 2, the signal is transmitted to the output side of the photocoupler PC and makes the photocoupler transistor PCTR conductive. Therefore the output transistor
TR1 turns on, output terminals 3 and 4 become conductive, and current is supplied from power supply 5 to load 6. Now, if the load 6 is short-circuited, the short-circuit current flows to the output transistor TR1 via the output terminals 3 and 4, so the voltage of the resistor R3 increases and the thyristor SCR
1 is triggered. When the thyristor SCR1 is turned on, the base current of the photocoupler transistor PCTR given by the lighting of the light emitting diode of the photocoupler PC flows from the base terminal to the anode of the thyristor SCR1 via the diode D, and the photocoupler transistor PCTR The base current of is drawn into thyristor SCR1. Therefore, the photocoupler transistor PCTR is turned off, and the output transistor TR1 is also turned off. Therefore, the trigger signal for thyristor SCR1 disappears, but the resistor R5 continues to be applied to thyristor SCR1.
Since the protection current is supplied through the thyristor
SCR1 continues to be conductive. Therefore, photocoupler transistor PCTR and output transistor
TR1 is turned off, making it possible to protect these elements when the load 6 is short-circuited. Note that resistance R5
Since only the protection current needs to be supplied to the resistor SCR1, a relatively large resistance value may be selected. By doing this, after that input terminals 1 and 2
Even if a signal is applied between them and the Photocoupler PC is triggered, the output signal will be output from output terminals 3 and 4.
It is possible to prevent a large current from flowing due to a short circuit without being transmitted between the two. This thyristor
When the SCR 1 is turned off, it can be easily restored to its original state by turning off the power supply or inverting the input signal.

FIG. 3 is a circuit diagram showing a second embodiment according to the present invention. In this example, as an output transistor
This shows the case where a PNP transistor is used. In this figure, photocoupler transistor
The photo cutlet PCTR on the collector side of PCTR
A resistor R6 is connected to absorb the dark current of , and the photocoupler transistor conducts.
The base of PNP transistor TR2 is connected.
And output transistor TR2 is output terminal 2, 3
A photocoupler transistor PCTR is connected in parallel between the two, and a current detection resistor R7 is connected to its emitter.
is connected. A common connection end between the current detection resistor R7 and the emitter of the transistor TR2 is connected to the N gate of PUT2, which is an N gate thyristor, via a resistor R8. The capacitor C is provided to improve the dv/dt withstand voltage, as in the case of a thyristor. and N gate thyristor PUT
Resistors R9 and R10 are connected in series to the cathode side of 2. The resistor R9 is a resistor for supplying a holding current to the N-gate thyristor PUT2, and the resistor R10 is a base-emitter resistor of the transistor TR3. Transistor TR3 is an N-gate thyristor
This transistor is operated by conduction of PUT2, and its collector is connected to the base of the photocoupler transistor PCTR, and by drawing the base current, the photocoupler transistor PCTR is turned off. A Zener diode ZD is connected between output terminals 3 and 4.
Further, as shown in the figure, a power source 5 and a load 6 are connected in the same way as in the previous embodiment.

Now, in this embodiment, when a voltage is applied between input terminals 1 and 2, the photo coupler PC operates,
The photocoupler transistor PCTR and the PNP type transistor TR2 are turned on, and current is supplied from the power supply 5 to the load 6. If the load 6 becomes short-circuited, the voltage across the resistor R7 increases,
A trigger signal is applied to the N gate of N-gate thyristor PUT2, and N-gate thyristor PUT2 is turned on. Then, the transistor TR3 is turned on by the resistor R10, and draws the base current of the photocoupler transistor PCTR. In this way, photocoupler transistor PCTR
is turned off, and similarly turns off transistor TR2. In this way, it is possible to prevent the output transistor from being destroyed due to a short circuit in the load 6. Note that the N-gate thyristor PUT2 has a resistor R.
Since the holding current is supplied through 9 and R10,
Continuing the on state is the same as in the previous embodiment.

FIG. 4 shows the structure of a three-terminal switching element, which can be used as a thyristor or an N-gate thyristor by combining a PNP transistor and an NPN transistor. For example, as shown in the figure, by using the anode, cathode, and P gate terminal GP, it operates as a thyristor SCR, and by using the anode A, cathode K, and N gate terminal GN, it operates as an N gate thyristor.
Works as PUT. In this way, in addition to the thyristors SCR and PUT as the three-terminal switching element, it is also possible to use a four-terminal switching element formed by a combination of transistors.

In each of the above embodiments, in the event of a short circuit, a three-terminal switching element with a self-holding function operates, and a protection current is supplied to keep the photocoupler transistor in an off state, thereby making it possible to protect the output element.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing a signal transmission circuit using a photo coupler, FIGS. 2 and 3 are circuit diagrams of a signal transmission circuit having a short-circuit protection circuit according to the present invention, and FIG.
The figure shows the configuration of a three-terminal switching element. 1, 2...Input terminal, 3, 4...Output terminal, 5
...Power supply, 6...Load, R1 to R10...Resistance,
PC...photo coupler, PCTR...photo coupler transistor, TR1, TR2, TR3...transistor, SCR1...thyristor, PUT2...
...N-gate thyristor, D...diode, ZD
...Zena diode.

Claims (1)

[Scope of Claims] 1. Signal transmission by the photocoupler, in which the light-receiving side circuit of the photocoupler consists of a photocoupler transistor whose base is an external terminal, and has an output control element that switches on and off the load current based on the operation of the photocoupler transistor. The circuit includes: an output current detection resistor connected in series to the output control element; and a self-holding function connected in parallel to the output control element and triggered when the voltage of the current detection resistor reaches a predetermined value. a three-terminal switching element with a switch, the base of the photocoupler transistor being connected to the three-terminal switching element;
A short-circuit protection circuit for a signal transmission circuit, characterized in that the photocoupler transistor is turned off when the three-terminal switching element is conductive. 2. The short circuit protection circuit according to claim 1, wherein the three-terminal switching element is a thyristor.