Half wave rectifiers
When a single rectifier unit is placed in series with the load across an ac supply, it converts alternating voltage into uni-directional pulsating voltage, using one half cycles of the applied voltage, the other half cycles being suppressed because it conducts only in one direction. Unless there is an inductance or battery in the circuit, the current will be zero, therefore, for half the time. This is called half-wave rectification. As already discussed, diode is an electronic device consisting of two elements known as cathode and anode. Since in a diode electrons can flow in one direction only i.e. from cathode to anode so the diode provides the unilateral conduction necessary for rectification. This is true for diodes of all types-vacuum, gas-filled, crystal or semiconductor, metallic (copper oxide and selenium types) diodes. Semiconductor diodes, because of their inherent advantages are usually used as a rectifying device. However, for very high voltages, vacuum diodes may be employed.
The half-wave rectifier circuit using a semiconductor diode with a load resistance RL but no smoothing filter is given in figure. The diode is connected in series with the secondary of the transformer and the load resistance RL, the primary of the transformer is being connected to the ac supply mains.
Working of a Half wave rectifier:
The ac voltage across the secondary winding changes polarities after every half cycle. During the positive half-cycles of the input ac voltage i.e. when upper end of the secondary winding is positive w.r.t. its lower end, the diode is forward biased and therefore conducts current. If the forward resistance of the diode is assumed to be zero (in practice, however, a small resistance exists) the input voltage during the positive half-cycles is directly applied to the load resistance RL, making its upper end positive w.r.t. its lower end. The waveforms of the output current and output voltage are of the same shape as that of the input ac voltage.
During the negative half cycles of the input ac voltage i.e. when the lower end of the secondary winding is positive w.r.t. its upper end, the diode is reverse biased and so does not conduct. Thus during the negative half cycles of the input ac voltage the current through and voltage across the load remains zero if the reverse current, being very small in magnitude, is neglected. Thus for the negative half cycles no power is delivered to the load.
Thus the output voltage developed across load resistance RL (VL) is a series of positive half cycles of alternating voltage, with intervening very small constant negative voltage levels, It is obvious from the figure that the output is not a steady dc, but only a pulsating dc wave. Since only half-cycles of the input wave are used, it is called a half-wave rectifier.
Power Supply Specifications. The most important characteristics which are required to be specified for a power supply are given below :
1.The required output dc voltage.
2.The average and peak currents in the diode.
3.The peak inverse voltage (PIV) of each diode.
5.The ripple factor.
Advantages and Disadvantages of Half wave rectifier:
(i)Advantages: Simple circuit and low cost.
(ii)Disadvantages:1. The output current in the load contains, in addition to dc component, ac components of basic frequency equal to that of the input voltage frequency. Ripple factor is high and an elaborate filtering is, therefore, required to give steady dc output.
(iii)2.The power output and, therefore, rectification efficiency is quite low. This is due to the fact that power is delivered only half the time.
(iv)3.Transformer utilization factor is low.
(v)4.DC saturation of transformer core resulting in magnetizing current and hysteresis losses and generation of harmonics.
The type of supply available from a half-wave rectifier is not satisfactory for general power supply.This type of supply can be satisfactory for some particular purposes such as battery charging.