Short Circuit Protection

Short Circuit Protection- The main debit of any series controller is that the pass transistor can be destroyed by inordinate loading current if the loading is accidentally shorted.

To avoid such an eventuality, a current limiting circuit is add to a series controller as shown in Fig-1.

A current limiting circuit consists of a transistor
(Q₃) and a series resistor (R₄) that's connect between base and emitter outstations.

(i) When the loading current is normal, the voltage across R₄ (= voltage across base-emitter of Q₃) is small and Q₃ is * off.
Under this condition, the circuit works as described before.

(ii) If loading current becomes inordinate, the voltage across R₄ becomes large enough to turn on Q₃.

The collector current of Q₃ flows through R₃, thereby dwindling the base voltage of Q₂.

The drop in base voltage of Q₂ reduces the conduction of pass transistor ( i.e., Q₂), precluding any further increase in loading current.

Therefore, the loading current for the circuit is limit to about 700 mA.

Example-1. In the series feedback voltage regulator shown in Fig-1, R₁ = 2 kΩ,
R₂ = 1 kΩ, V_Z = 6 V and V_BE = 0.7 V. What is the regulated output voltage?

Solution.

∴ Regulated output voltage, V_out = A_CL (V_Z + V_BE)

= 3 (6 + 0.7) = 20.1 V

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