Power Supplies

Stabilised power supplies

The simplest voltage regulator or stabiliser uses a zener diode as described in Chapter 3 (Fig 3.17). Reasonably constant supplies of up to l50V can be obtained in this way, and this circuit is often used for the supply of a VFO. A more effective method is to use a transistor as a regulator (the series or pass transistor) in series with the supply to the load as shown in Fig 6.9.

Fig 6.9. Simple voltage regulator

The base of the transistor is kept at a fairly constant forward voltage by the zener diode D. If the load increases, the voltage at the emitter will tend to fall. This in turn allows the transistor to conduct more easily and thus maintain the output voltage. In order to maintain better control a balancing (bridge) circuit is used, with an extra transistor working as a DC amplifier. Let the voltage in Fig 6.10. divide equally between R2 and R3. The voltage between point B and chassis will be 6V. If the diode D is rated at 6V then there will be no difference of potential between points A and B. If the line voltage falls then the voltage across R3 will fall, but the voltage across D will remain at 6V. The voltage at A will now be higher than the voltage at B. This change of voltage can be applied to the base of a transistor and so provide a degree of control. Fig 6.11 shows a circuit using such a device

Fig 6.10 Voltage regulator - basic balancing circuit

Fig 6.11. Voltage regulator using balancing circuit

If the voltage at C increases the voltage at point B will increase. The emitter voltage of TR2 stays constant due to the action of D. Due to the increased base-emitter voltage in TR2, the collector current (and hence the voltage drop in R) will increase. The base-emitter voltage of TR1 is reduced, thus increasing the effective series resistance of TR1. This brings down the emitter voltage of TR1 and so point C returns to its normal voltage.

The small capacitors C1 and C2, about 0.1pF, prevent instability (self-oscillation) of the output voltage if the load is constantly varying. There are many refinements in regulated supplies and manufacturers' literature gives much additional information.