Session 7

This is the second and last session we will be using to investigate the function and operation of a power supply. Any functional electronics power supply will need some form of voltage regulation. Today we will look at the process of turning a smoothed output from a bridge rectifier into a regulated voltage. It would be helpful to define the term voltage regulation before we proceed. I will do so with the aid of a diagram:


In this diagram the electrical load (resistor) and transistor, form a voltage divider. If the load resistance falls then the internal resistance of the transistor must also fall in order to maintain the output voltage at its required value. Transistors have this capability.

The following diagram shows the basic connections a transistor has and the conditions needed so that it will work in a circuit.


With the connections shown the key thing to note is that for an npn transistor to switch on the base must have a potential difference of 0.6 volts positive with respect to the base. The transistor will not switch fully on until the base voltage rises to about 0.7 volts (depends on the transistor) when this happens we say the transistor is in saturation (fully on).


The diagram above illustrates this and although it may (almost certainly will be unfamiliar) it is worth taking some time to study. The saturation point is where each curve of the graph flatlines.

For our circuit the transistor is connected in what is know as emitter follower configuration. This means that the voltage at the emitter will track/follow the voltage at the base. The only difference is that the voltage at the emitter will always be 0.6 volts lower than the voltage at the base (switch on voltage) enter the zener diode.

From earlier study (p3) we already know that a zener is manufactured to break down and conduct at low dc voltages. In our circuit example the zener is a 5v6 (5.6 volts). That means that if the voltage across the zener rises above 5.6 volts the zener will switch on (conduct) and prevent any further voltage rise across it. This means that the zener voltage will remain steady at 5.6 volts.

We us a resistor in series with the zener to maintain a suitable operating current for the zener (typically 10 mA). The absolute key thing to remember here is that the emitter voltage follows the base voltage and the base voltage is controlled by the zener.

Voila we have series pass voltage regulation, the zener is relatively immune to conditions at the emitter (within reason) and the emitter voltage follows the zener voltage despite changes in the electrical load.

This video explains further if you need.