In Britain the mains frequency is 50Hz, that is the flow of electricity in a wire reverses 50 times every second. (Hertz, abbreviated Hz, is the term for 'cycles per second'; thus 50Hz simply means 50 cycles per second).

Figure 1 shows the effect of placing a rectifier across the output of a transformer.


Figure 1. Operation of a full wave rectifier.

The two diagrams indicate the path of electric current at two instants 1/100th of a second apart. It will be seen that the rectifier output is always of the same polarity but due to the rise and fall in the ac input voltage as it reverses direction the dc voltage also rises and falls. (Because their average values are zero, alternating voltages and currents are denoted by their Root Mean Square (RMS) values as shown in the top graph of the second sketch. The theory of this is somewhat complex and is outside the scope of this article).

Rectifiers consist of one or more diodes, which are electronic devices which pass current in only one direction. A single diode will allow forward current to pass but will block the reverse, the process being termed half wave rectification because current flows for only half of each alternating current cycle. lbo diodes fed by a centre tapped transformer will allow current to pass during the whole cycle but the most common arrangement is four diodes forming what is called a bridge circuit, either arrangement providing what is known as full wave rectification.

The DC voltage output of a rectifier is not constant but follows the wave form of the AC input. With half wave rectification the voltage is zero for alternate cycles and for a given AC input voltage the average DC voltage is approximately half that of full wave rectification. The graphs in Figure 2 show the voltage for both cases.

Before the development of electronic controllers made the practice obsolete, some proprietary controllers incorporated a switch in one arm of the rectifier bridge to provide a choice of either full or half wave rectification. This was intended to improve the slow running of some types of motor which, because of their electromagnetic design, would not run smoothly at slow speed on a constant voltage. However, half wave rectification causes additional heating of the motor windings and brush gear combined with greater mechanical stresses. It should therefore be used with caution and in particular should not be employed to control coreless motors.

Figure 2. Comparison of full wave and half wave rectification.