Introduction
This topic covers rectification, the conversion of an alternating current (AC), which reverses direction periodically, into a current that flows in one direction only. It is done with semiconductor diodes. The one-way (half-wave) rectifier uses one half of each cycle, and the two-way (full-wave) rectifier uses both halves. Rectified current is the basis of the direct-current supplies of medical equipment.
The diode as a rectifying element
A semiconductor diode is a p-n junction. It conducts well in the forward direction, when the anode is positive relative to the cathode and the voltage exceeds the threshold of about 0.6 to 0.7 V for silicon. In the reverse direction it blocks, and only a very small leakage current flows until the breakdown voltage is reached. An ideal diode is therefore a switch that closes for one polarity and opens for the other, which is what makes rectification possible.
One-way (half-wave) rectification
A single diode is placed in series with the load and the AC source. During the positive half-cycle the diode conducts and the load receives a current pulse; during the negative half-cycle the diode blocks and the load receives no current. The output is a train of pulses with one pulse per cycle, at the mains frequency of 50 Hz in Europe.
- Average output voltage: U_{avg} = U_m / \pi, where U_m is the peak voltage of the source.
- The output is strongly pulsating, and the ripple is large. The output is zero for half of the time, so the power delivered is low.