This in turn will slightly affect V BE and the output voltage.ģ.ěecause of reasons 1 and 2 above, any change in load will result in less than perfect regulation, therefore any change at the output will slightly change the loading on the input circuit. 2.2.2, a large change in zener current (ΔI) will cause a very small change in zener voltage (δV). Because the diode characteristic has a slope over its operating region as shown in Fig. Since base current increases with load, the current through the zener diode D Z will decrease as more current is taken by the base of Tr1. The amount of this fall is about 0.25V for a change in output current from 10mA to 1A.Ģ. This in turn causes an increase in V BE and because the output voltage V OUT = V Z - V BE any increase in V BE tends to reduce the output voltage. With this simple circuit however, regulation is not perfect, and variations in output do occur for the following reasons.ġ.Ěny increase in load current (I L) causes a small increase in base current by the ratio I L/hfe. This action counteracts the variation in load current. This regulating effect is due to the base potential of Tr1 being held steady by D Z so that any variation in emitter voltage caused by varying current flow causes a change in V BE, varying the conduction of the transistor Tr1, which will usually be a power transistor. This reduction in V BE will tend to turn the transistor off, reducing current flow, and again regulating the output voltage V OUT. If V OUT tends to rise due to reduced current demand by the load, then this will reduce V BE as the emitter voltage rises and the base voltage remains stable due to D Z.

This will provide the extra current required by the load and thus regulate the output voltage V OUT. If the output voltage V OUT falls due to increased current demand by the load, this will cause V BE to increase and as a result, current through the transistor (from collector to emitter) will increase. The emitter voltage of Tr1 will be typically about 0.7V less than the base voltage and V OUT will therefore be at a lower voltage than the base. In this circuit however, they are used to provide a stable voltage reference V Z at the base of Tr1. 2.2.1 R S and D Z form a simple SHUNT regulator as described in power supplies module 2.1.

Power transistor voltage regulator