Power supply protection: Short circuit protection circuit

Introduction

Electronic circuits and PCBs have two properties which sometimes can lead to problems. They are small and very delicate. Electronic components are fragile and can be damaged by small over current or over voltage. On the other hand due to small size of electronic circuits, the connections wires or paths are very close’ to each other. Therefore possibility of short circuit or other faults is higher. As a result it is very important to provide protection against short circuits and over current in electronic circuits. Power supply is integral part of electronic circuit. The protection of power supply is as important as any other component. Therefore this article introduces a very simple power supply protection mechanism which provides protection against short circuit. As soon as the short circuit is removed, the circuit restores to its Pre-fault position. Indications are also provided to showing normal mode and short circuit mode. This short circuit protection can be applied to any of the following power supplies
Dual Voltage power supply
Dual Polarity Power supply 

Components & Connection diagram for short circuit protection circuit

• Electronic Power supply
• Resistors (10k, 1k, 470, 270 0)
• Transistor (N-P-N BC547, P-N-P BC546AP)
• LED (Two different colors)
• Capacitors (1uF, 0.1uF)

The above components are arranged as shown in the following figure.

Red LED lights up when the short circuit occurs while Yellow LED is on during normal operation mode. To simulate short circuit, a momentary push button is added between the protected terminals (D E).

Simulation and Explanation of power supply protection

The short circuit protection for power supply is simulated in ISIS Proteus.

Normal mode.

In normal mode the transistor Q2(BC547) is run through R1, R4 and LED. Through this path, a positive voltage is applied at the base of Q2. When Q2 turns on, the collector current of Q2 turns on the transistor Q1 (BC546AP). When Q3 conducts, +5V appear on the protected supply terminal D. Now Point DE can be used to power electronic circuits. Yellow LED D2 also turns on and remains On to show the normal operating mode of power supply. It can be seen in the following figure

5V appear on Point D and C indicating that both Q2 and Q3 are conducting. Yellow LED is ON showing normal mode of short circuit protection for power supply.

Short Circuit Mode

When short circuit occurs at point D and E, 0 Volts appears (due to short circuit) on point C which is base of transistor Q2. As a result Q2 cut off. Q2 turns off Q1 isolating point A, output of power supply from protected terminals C & D. Due to isolation, short circuit on Point DE is not seen at point A and B. Hence actual power supply is saved from short circuit at D and E. To verify this, the momentary push button is pressed to simulate short circuit. The result is shown in the figure.

Now 0 volts can be seen at D indicating short circuit. However 5V can be still observed at Point A i-e output of actual power supply. It means short circuit is not being seen by the actual power supply. Red LED is glowing showing short circuit mode of power supply protection mechanism. The above circuit is made for the 5V supply and the resistor values are taken according to this voltage. In order to modify this circuit for other voltages use the following design equations

R_B = (H_FE X V_s)/(1.3 X I_L)

Where, R_B = Base resistances of transistors of BC556AP and BC547 H_FE = 200 for BC556AP and 350 for BC547. Switching Voltage Vs = 5, 1.3 = Safety factor, I_L = Collector-emitter current (10mA to 100mA for BC547)