JPH03277117A - Overcurrent protective circuit - Google Patents

Abstract

PURPOSE:To cut off the current supply to the load when a device gets in an over-current condition and an over current is detected and to resume the current supply automatically after elimination of the cause of the over current by making up the device from an oscillation circuit, a circuit to detect an over current and a flip-flop circuit. CONSTITUTION:In the case that a current is less than a rated value, a transistor Q1 is in an OFF condition and a transistor Q2 in an OFF condition. When an over current flows in the load, the transistor Q1 is switched ON by a voltage drop in a resistor R1 with a 'd' point at the 'L' level and an 'e' point at the 'H' level. Then, the transistor Q2 turns OFF followed by the cutoff of the current supply to the load. When the resistor Q1 turns OFF, the 'd' point comes to the 'H' level, however, the 'e' point is held at the 'H' level until the 'L' level pulse is applied to a 'c' point. With the 'L' level pulse applied to the 'c' point from an IC 1 for a timer, the 'e' point changes to the 'L' level and the transistor Q2 turns ON and finally the current supply to the load is resumed.

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Field of Industrial Application This invention relates to an overcurrent protection circuit used in power supplies and the like.

(1)) Conventional technology in general: A DC voltage stabilizing power supply circuit used as a device has a voltage stabilizing circuit that stabilizes the output voltage in response to fluctuations in the input voltage, as well as a voltage stabilizing circuit that stabilizes the output voltage in response to fluctuations in the input voltage. An overcurrent protection circuit is provided to protect against

A typical overcurrent protection circuit connects a switching element in series in the load current path and detects the load supply current.
There are two types: one is equipped with an i detection circuit and turns off the switch element when the load supply current exceeds a certain value, and the other is a stop type that reduces the load supply voltage when an overcurrent is detected and returns to the original setting when the overcurrent condition is released. There is an automatic return drooping type that returns to voltage.
Either method is adopted depending on the load or the purpose of overcurrent protection.

(C) Problems to be Solved by the Invention Among the above-mentioned overcurrent protection circuits, the operation stop type has -
Once the overcurrent condition occurs, the power must be turned on again even if the overcurrent condition is restored. Automatic return droop type does not have such inconvenience, but the load supply voltage drops significantly below the rated voltage in an overcurrent condition, so
It cannot be used for loads that operate normally only at the rated voltage and malfunction or break down at low voltages.

An object of the present invention is to provide an overcurrent protection circuit that always supplies a rated voltage when supplying power supply voltage to a load, cuts off the load supply current by detecting an overcurrent, and then automatically restores it when the cause of the overcurrent is removed. There is a particular thing.

(Means for Solving Problem d1) The overcurrent protection circuit of the present invention is set by an oscillation circuit that generates a rectangular wave signal of a constant period, a circuit that detects an overcurrent, and a detection output of the overcurrent detection circuit. , a flip-flop that is reset by the output signal of the oscillation circuit, and a switch element that is connected in series in a load current path and is conductive in the reset state of the flip-flop.

(e) Function In the overcurrent protection circuit of the present invention, the oscillation circuit generates a rectangular wave signal with a constant period, and the flip-flop is set by the overcurrent detection of the overcurrent detection circuit and reset by the output signal of the oscillation circuit. Ru. Further, a switch element is provided in series in the load current path, and conducts in the above-mentioned free-knob flop state. Therefore, in the normal state when the flip-flop is reset, the above-mentioned switch element becomes conductive and the load current is supplied, but if the overcurrent detection circuit detects an overcurrent state, the flip-flop is set, and accordingly, the above-mentioned The switch element enters a cutoff state, and the load and power supply circuit enter an overcurrent protection state. The flip-flop is reset at regular intervals by a rectangular wave signal output from an oscillation circuit. When the flip-flop is in the reset state, the switching element becomes conductive again and the rated voltage is supplied to the load. If the cause of the overcurrent condition has not been eliminated at this point, the flip-flop is again set by the detection output from the overcurrent detection circuit, and the switch element is turned off. After the cause of the overcurrent condition is removed, the flip-flop is set by the rectangular wave signal output from the oscillation circuit, and the switch element becomes conductive. After that, the overcurrent detection circuit does not detect an overcurrent condition, so current supply to the load continues. In other words, it automatically returns.

In this way, in the overcurrent protection state, the load supply current is completely cut off, and it automatically recovers as soon as the cause of the overcurrent state is removed (more precisely, at the first point in time when a square wave signal is output from the oscillation circuit). be done. Therefore, although it is an automatic reset type, it can also be applied to loads that cannot be supplied with a power supply voltage other than the rated voltage due to the characteristics of the load.

(fl Embodiment) An example of an overcurrent protection circuit according to an embodiment of the present invention and the waveforms of each part thereof are shown in FIGS. 1 and 2.
N is a power input terminal, OUT is a power output terminal, and S is a terminal that outputs a determination result as to whether the load supply current is within a normal range. Resistor R1 is used for detecting load current,
Its value is selected so that when the current supplied from 0LIT via transistor Q2 reaches the maximum allowable value, the voltage across the resistor becomes 0.6V. The transistor Q1 is in an off state when the voltage across the resistor R1 is 0.6V or less, and in that case, the point b becomes oV. In addition, Ql, R1, R
2 and R3 constitute a filtering large current detection circuit according to the present invention. ICI is a timer IC corresponding to the oscillation circuit according to the present invention, and the line C in FIG.
Outputs a rectangular wave signal as shown in . Voltage b is supplied to pins 1 and 2 of IC2, and its output pin 3 is d.
At the point, an inverted signal of point b appears. This IC2 constitutes a flip-flop (R-S flip-flop) according to the present invention by connecting pins 4, 5, and 6 and pins 1) and 12, and 13, and point d is once at "L" level. Then, even if point d subsequently returns to the "H" level, point e remains at the "H" level until an "L" level pulse is applied to point C.

The transistor Q2 corresponds to the switch element according to the present invention, and is connected to the point e through the resistor R4, so when the point e reaches the "L" level (0■), the base current is supplied through the resistor R4. Transistor Q2 becomes conductive and supplies current to output terminal OUT. Conversely, when the point e is at the "H" level (5■), the transistor Q2 is cut off.

A voltage obtained by dividing the voltage at the output terminal OUT by resistors R8 and R9 is supplied to the base of the transistor Q3. Q
Since the emitter of 3 is connected to the power supply input terminal IN (5V), if the voltage at point g becomes 4.4V or less (the voltage between the base and emitter of Q3 is 0.6V or more), Q3 turns on. Let point h be the "H" level. In addition, resistance R8
The diode D1 connected to the resistor R1, R8, R9 and the transistor Q2 acts as a temperature compensation circuit for the load supply current detection circuit, and the transistor Q3 turns off when the load supply current is below the rated range regardless of the ambient temperature. .

Now, if the current supplied from the output terminal OUT to the load is less than the rated range, the transistor Q1 is turned off and the point b becomes "
"L" level, point d becomes "H" level, point e becomes "L" level, and Q2 turns on. Also, since the load supply current is small, the voltage drop due to resistor R1 is small, and point g becomes 4,
It becomes 4V or more. Therefore, transistor Q3 is in the off state,
Point h becomes "L" level and point i becomes "H" level, which is output from terminal S (see below-rated area in FIG. 2).

When the load supply current is normal, the voltage at point g decreases to 4.4V or less due to the voltage drop caused by the resistor R1. As a result, transistor Q3 is turned on and point h becomes "H".
level. At this time, the voltage drop across resistor R1 is still 0.
Since the voltage does not reach 6V, transistor Q1 remains off. Therefore, the i point becomes the "L" level, which is output from the terminal S (see the rated load area in FIG. 2).

When an excessive current flows through the load, the voltage drop across resistor R1 exceeds 0.6V, and as a result, transistor Q1 turns on, and point d becomes "L" and point e becomes "H", turning transistor Q2 off. , this cuts off the load supply current.

Due to this interruption, the voltage drop across resistor R1 becomes O, and Q
l is turned off and point d becomes "H" level, but the level of point e remains "H" until a "L" level pulse is applied to point 0.
Maintain the level. During the period when the level of the 0 point is the "H" level, the i point becomes the "H" level, which is output from the terminal S. When a "L" level pulse is applied to point 0, point e changes to "L" level, transistor Q2 is turned on, and current supply to the load is restarted. If the load remains in the overload state at this time, the operation of turning on the transistor Q1 and cutting off the transistor Q2 is repeated (see the overload region in FIG. 2).

In this way, if the load supply current is within the rated range, the current is supplied, and if there is an overload condition such as a short circuit, the current supply is interrupted until the cause is removed. When the overload condition is resolved, the system automatically returns to normal operation.

The circuit shown in Figure 1 has a judgment result output function that indicates whether or not the load supply current is within the rated range (lower limit ≦ load current ≦ upper limit). It is also possible to display the load status and load status.

(g1 Effect of the invention According to this invention, the load supply current is completely cut off when an overcurrent condition occurs, so that a low voltage lower than the rated voltage is not applied to the load, resulting in malfunction or failure of the load.) Moreover, once the cause of the overcurrent condition is removed, the load current is automatically supplied after that, so manual operations such as turning off and turning on the power switch are no longer necessary.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of an overcurrent protection circuit according to an embodiment of the invention. FIG. 2 is a signal waveform diagram of each part of the circuit. ICl-IC0 for timer

Claims (1)

[Claims] (1) an oscillation circuit that generates a rectangular wave signal with a constant period, a circuit that detects excessive current, and a flip-flop that is set by the detection output of the overcurrent detection circuit and reset by the output signal of the oscillation circuit; An overcurrent protection circuit comprising a switch element connected in series in a load current path and conductive when the flip-flop is in a reset state.