JPH02280617A - Overcurrent prevention circuit - Google Patents

Abstract

PURPOSE:To prevent overcurrent from keeping on and to protect a power source and peripheral circuits by reducing the output of power supply in the occurrence of overcurrent. CONSTITUTION:When output current increase and exceeds the tolerance, a photocoupler P1 becomes conducting and the gate voltage of a thyristor SCR is raised to conduct the current because the current will flow to a resistance R5 through a resistance R7. A photocoupler P2 will thereby be conducted, which drops reference voltage VP as a result and the output voltage at an output terminal 3 will be OV. This state will continue until a resetting switch 4 is turned ON and the thyristor SCR is turned OFF.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an overcurrent prevention circuit, and particularly to an overcurrent prevention circuit used in a power supply circuit.

[Traditional technique #i]

Conventionally, as shown in FIG. 4, in this type of overcurrent prevention circuit, when the output current I from the output terminal 3 flows abnormally, the current detection resistor R4 and the current supply transistor TrL
The transistor TrS connected between the transistor TrS becomes conductive and limits the base current of the transistor Trl, so that the current does not exceed a certain level.

[Problem to be solved by the invention]

In the conventional overcurrent prevention circuit described above, a current exceeding a certain level does not flow, but on the contrary, a certain current is allowed to flow.
When used in an environment where people are not constantly observing, for example,
When S product circuit products are used in BT tests, etc., the current is left flowing for a long time, causing the product to heat up and, in the worst case, pose a risk of fire or multiple When conducting a BT test on an individual product, there is a drawback that the current supply capacity of the original power supply becomes insufficient, and the product cannot be tested and the test must be repeated.

[Means to solve the problem]

The overcurrent prevention circuit of the present invention includes a current detection resistor inserted in series with a current supply line, a current detection circuit connected in parallel to the resistor, and an output of the current detection circuit when an overcurrent flows. The thyristor is activated by the thyristor, and a current stop circuit that lowers the reference voltage and prevents output current from flowing out when the thyristor becomes conductive.

〔Example〕

Next, the present invention will be explained with reference to the drawings.

FIG. 1 is a circuit diagram of a first embodiment of the present invention.

As shown in Fig. 1, resistors R and R2 are inserted in series between the power supply terminal 1 and the ground terminal of the power supply VCC, and the operation is performed to connect the non-inverting input terminal to the connection node between the resistors R and R2. An amplifier 2, a resistor R5 whose one end is connected to the output terminal of the operational amplifier 2, and a current supply transistor T whose collector is connected to the power supply terminal 1 and whose base is connected to the other end of the resistor R3.
r l, and a power supply V divided by resistors R1 and R2.
For a normal power supply circuit that uses CC as the reference voltage VP, a current detection resistor R4 is provided between the emitter of the current supply transistor T r 1 and the output terminal 3, and a resistor R4 is also provided.
A photocoupler P1 as a current detection circuit is connected in parallel to the photocoupler P1, and a thyristor SCR is made conductive by the output of the photocoupler P1.
a resistor R5 for maintaining conduction of the thyristor SCR;
A photocoupler P2 is added as a current stop circuit that lowers the voltage value of the reference voltage by conducting the thyristor SCR and lowers the output current (2) from the output terminal 3 to prevent overcurrent from flowing. Further, the reset switch 4 is for cutting off the conduction of the thyristor SCR and restoring the circuit to normal condition, and the reset switch 4 is used to turn off the conduction of the thyristor SCR and restore the circuit to normal.
is used to indicate that an overcurrent has flowed and the output current has been cut off.

FIG. 2 is a waveform diagram for explaining the operation of the first embodiment shown in FIG. The operation of the first embodiment will be explained below with reference to FIG.

During normal operation at time T1, the output current is within the allowable range, so the voltage across the resistor R4 is also low. For this reason, the photocoupler P1 is in a non-conducting state, and therefore the thyristor SCR
And the photocoupler P2 is also in the cut-off state and the output terminal 3
The output voltage V0 of the glass has a normal value.

Next, when the current value exceeds the permissible range during the time period T2 during which the output current I increases, the voltage across the resistor R4 increases and exceeds the threshold voltage V7H (Habo IV) of the photocoupler P1. becomes conductive, and the current flows through the resistor R7.
The voltage flows through the resistor R5, and the gate voltage of the thyristor SCR increases, causing the thyristor SCR to become conductive. When the thyristor SCR becomes conductive, current also flows to the photocoupler P2, and the phototransistor on the secondary side of the photocoupler P2 becomes conductive, lowering the reference voltage VP, and the output voltage of the output terminal 3 becomes oV, and the output current ■ stops.

During the period of time T3 after the photocoupler P2 becomes conductive, no output current flows, and even if the photocoupler P becomes non-conductive, the thyristor SCR remains in contact until the reset switch 4 is connected.
Since is conductive, photocoupler P2 also maintains conduction, and as a result, the output voltage V. will be maintained at Ov. Further, the photodiode PD remains lit to indicate that the power has been turned off.

FIG. 3 is a circuit diagram of a second embodiment of the present invention.

As shown in FIG. 3, the difference between the second embodiment and the first embodiment shown in FIG. 1 described above is that the photocoupler P1 as a current detection circuit is replaced with a PNP type transistor Tr2. When an overcurrent flows, the voltage across the resistor R4 at which the protection circuit operates is approximately IV in the photocoupler Pl of the first embodiment, while it is approximately 0 in the case of the second embodiment.
．． 7■ I can do it.

〔Effect of the invention〕

As explained above, the present invention reduces the output of the power supply when an overcurrent flows and further maintains that state.
Since it is possible to prevent the output current from continuing to flow, it has the effect of protecting the original power supply and surrounding circuits.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram of a first embodiment of the present invention, and FIG. 2 is a circuit diagram of a first embodiment of the present invention.
A waveform diagram for explaining the operation of the first embodiment shown in FIG.
The figure is a circuit diagram of a second embodiment of the present invention, and FIG. 4 is a circuit diagram of an example of a conventional overcurrent prevention circuit. 1... Power supply terminal, 2... Operational amplifier, 3... Output terminal, 4... Reset switch, ■... Output current,
P. P2...Photocoupler, PD...Photodiode, R
1-R7...Resistor, SCR...Thyristor, Trl
~Tr 3...transistor, VCC...power supply,
VP...Reference voltage, ■. ...Output voltage. Agent Patent Attorney Uchihara

Claims (1)

[Claims] A current detection resistor inserted in series with a current supply line, a current detection circuit connected in parallel to the resistor, a thyristor activated by the output of the current detection circuit when an overcurrent flows, and the thyristor. a current stop circuit that lowers a reference voltage when the current becomes conductive to prevent output current from flowing out.