Short Circuit Protection Circuit:

Power supplies used in laboratories are subject to overloads and short circuits. Short Circuit Protection Circuit by means of current limiting circuits is necessary in such equipment to prevent the destruction of components when an overload occurs. Transistor Q₇ and resistor R₁₀ in Fig. 17-13(a) constitute a current limiting circuit.

When the load current (I_L) flowing through resistor R₁₀ is below the normal maximum level, the voltage drop V_R10 is not large enough to forward bias the base-emitter junction of Q_7. In this case, Q₇ has no effect on the regulator performance.

When the load current reaches the selected maximum (I_L(max)), V_R10 biases Q₇ on. Current I_C7 then produce a voltage drop across resistor R₁ that drives the output voltage down to near zero.

The voltage/current characteristic of the regulator is shown in Fig. 17-13(b). It is seen that output voltage remains constant as the load current increases up to I_L(max). Beyond I_L(max), V_o drops to zero, and a short-circuit current (I_SC) slightly greater than I_L(max) flows at the output.

Under this circumstance, series pass transistor Q₁ is carrying all of the short-circuit current and has virtually all of the supply voltage developed across its terminals.

So, the power dissipation in Q₁ is,

Obviously, Q₁ must be selected to survive this power dissipation.

Design of the current limiting circuit in Fig. 17-13 is very simple. Assuming that Q₇ is a silicon transistor, it should begin to conduct when V_R10 ≈ 0.5 V. Therefore, R₁₀ is calculated as,