Controlled Transistor Series Regulator with Short Circuit Protection:

Controlled Transistor Series Regulator with Short Circuit Protection – If the load resistance R_L is reduced or load terminals are shorted accidentally, a very large load current will flow in the circuit shown in Fig. 43.6. 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.

To avoid this situation, a current limiting circuit is added to a series regulator, as illustrated in Fig. 43.7.

The current limiting circuit consists of a transistor Q₃ and a resistor R₅ (approximately 1 Ω) 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 earlier. With the excessive load current (exceeding 0.6/1 i.e., 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 43.8 summarizes the current limiting. When load resistance R_L is infinite, the output voltage is regulated and has a value of V_REG. The load current I_L is zero for this operating condition. When R_L decreases, the load current I_L increases up to the point where R_L becomes equal to R_{L (min)}. At this minimum load resistance, I_L equals 600 mA and V_BE equals 0.6 V. Beyond this point, transistor Q₃ turns on and the current limiting sets in. Further decrease in R_L produces decrease in output voltage, and regulation is lost. When R_L is zero, the load current I_L is limited to a value between 600 mA and 700 mA. The load current with shorted load terminals is symbolized as I_SL. When the load terminals are shorted in Fig. 43.7, the voltage across resistor R₅ is

where V_BE is typically between 0.6 a nd 0.7 V.

The minimum load resistance where regulation is lost can be estimated with the following equation

The exact value of R_{L (min)} will be slightly less or greater than this.

The simple current limiting circuit also has a drawback of large power dissipation across the series pass transistor. With a short across the load, almost all the input voltage appears across the pass transistor. So the pass transistor has to dissipate approximately

where V_BE is the base-emitter voltage of Q₃, the current-limiting transistor.