Rectifiers

Semiconductor diodes are commonly used to convert alternating current (a.c.) to direct current (d.c),in which case they are referred to as rectifiers. The simplest form of rectifier circuit makes use of a single diode and, since it operates on only either positive or negative half-cycles of the supply, it is known as a half-wave rectifier.


Figure .1. shows a simple half-wave rectifier circuit. Mains voltage (220 to 240 V) is applied to the primary of a step-down transformer(T1). The secondary of T1 steps down the 240V r.m.s. to 12 V r.m.s. (the turns ratio of T1 will thus be 240/12 or 20:1). Diode D1 will only allow the current to flow in the direction shown (i.e.from cathode to anode). D1 will be forward biased during each positive half-cycle (relative to common) and will effectively behave like a closed switch. When the circuit current tries to flow in the opposite direction, the voltage bias across the diode will be reversed, causing the diode to act like an open switch (see Figs.2.(a) and 2.(b), respectively).

The switching action of D1 results in a pulsating output voltage which is developed across the load resistor (RL). Since the mains supply is at 50 Hz, the pulses of voltage developed across RL will also be at 50 Hz even if only half the a.c. cycle is present. During the positive half-cycle,the diode will drop the 0.6 V to 0.7 V forward threshold voltage normally associated with silicon diodes. However, during the negative half-cycle the peak a.c.voltage will be dropped across D1 when it is reverse biased.

This is an important consideration when selectiong a diode for a particular application. Assuming that the secondary of T1 provides 12 V r.m.s., the peak voltage output from the transformer’s secondary winding will be given by:

V_pk = 1.414 x V_r.m.s = 1.414 x 12 V = 16.79 V

The peak voltage appilied to D1 will thus be approximately 17 V. The negative half-cycles are blocked by D1 and thus only the positive half-cycles appear aross RL. Note, however, that the actual peak voltage across RL will be the 17 V positive peak being suppliied from the secondary on T1, minus the 0.7 V forward threshold voltage dropped by D1. In other words, positive hafl-cycle pulses having a peak ampilitude of 16.3 V will appear across RL.