Regulated Power Supply with Short Circuit Protection

The simple way of providing over current protection is to use another transistor that controls the series transistor. The circuit is shown in Figure

Regulated Power Supply with Short Circuit Protection

In this circuit a current sensing resistor is connected in series with the series transistor. The voltage drop across the current sensing resistor, R₄ will be the base emitter voltage of the transistor T₃.

The value of this resistance R₄ is chosen to be small. For example we connect a 1 W resistor and if the load current is less than 500 mA; the voltage across the resistor will be less than 0.6 V. Therefore T₃ will not function. The circuit works normally. If the load current increase beyond 600 mA the voltage drop across the resistance R₄ , will be sufficient to turn on the transistor T₃.

The collector current of T₃ flows through R₃. It decreases the base voltage to the series transistors T₂. This in turn decreases the output voltage preventing further increase in load current. If we represent circuited load current to be I_SC, the voltage drop across R₄ is I_SC R₄. This is equal to the V_BE of the transistor T₃. Therefore;

V_BE = I_SCR₄ ----------------- (1)

I_SC = V_BE/R₄ ----------------- (2)

By properly choosing the value of resistance R4, the level of current limiting can be adjusted.
The disadvantage of this circuit is that large power dissipation takes place at the series transistor, when there is a short circuit at the output terminals.