Controlled Transistor Series Regulator With Overload and Short-Circuit Protection

Posted on Feb 5, 2014

If the load resistance RL is reduced or load terminals are shorted acciden ­tally, a very large load current will flow. It may destroy the pass transistor Q1, diode or possibly some other component. Fuse protection will not prove adequate because the transistor may get damaged in a very small fraction of a second The current limit ­ing circuit con

Controlled Transistor Series Regulator With Overload and Short-Circuit Protection
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sists of a transistor Q3 and a re ­sistor R5 (approxi ­mately 1 ohm) connected between base and emitter terminals of transistor Q3. With normal load current, transistor Q3 remains off because the voltage drop across resistor R5 is small (less than about 0. 7 V necessary for making the transistor Q3 on). Under this condition, the circuit works normally, as described above. With the excessive load current (exceeding 0. 6/1, that is, 0. 6 A or 600 mA) the voltage drop across R5 becomes large enough to turn transistor Q3 on. The collector current of transistor Q3 flows through R3 thereby decreasing the base voltage of transistor Q1. This results in reduction of the conduction level of transistor Q1. Thus further increase in load current is prevented. Figure summarizes the current limiting. When load resistance RL is infinite, the output voltage is regulated and has a value of VREQ. The load current IL is zero for this operating condition. When RL decreases, the load current IL increases upto the point where RL becomes equal to RL(min) At this minimum load resist ­ance, IL equals 600mA and VBE equals 0. 6V. Beyond this point, transistor Q3 turns on and the current limiting sets in. Further decrease in RL produces decrease input voltage, and regulation is lost. When RL is zero, the load current IL is limited to a value between 600 m A and 700 m A. The load current with shorted-load terminals is symbolized as ISL. When the load...

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