Short Circuit Protection


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.

 

Short Circuit Protection

 





Fig-1




(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, VZ = 6 V and VBE = 0.7 V. What is the regulated output voltage?

Solution.

∴ Regulated output voltage, Vout = ACL (VZ + VBE)

= 3 (6 + 0.7) = 20.1 V

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