April 19th 2015

Rectifiers:

Electronics circuits need DC voltage for biasing. The commercial power supply for consumer is in the form of AC. Hence there is a need to convert AC voltage to DC voltage.

A circuit which converts AC voltage onto DC voltage is called rectifier. The unidirectional conducting property of a diode is used in rectification.

Diode Rectifiers:

For reason associated with economics of generation and transmission, the electric power available is usually an AC supply.

The supply voltage varies sinusoidally and has a frequency of 50 Hz. It is used for lightning, heating and electric motors.

But there are any applications where DC supply is needed. When such a DC supply is required, the mains AC supply is rectified by using diodes.

The following three rectifier circuits can be used:

Half Wave rectifier
Full wave Rectifier
Bridge Rectifier

VI Characteristic of PN Junction Diode

Half Wave Rectifier:

The fig below shows that the circuit diagram of half wave rectifier.

It consists of a transformer, a diode and a load resistor. Depending on the output of DC voltage required, a transformer of proper turn’s ratio N₁:N₂ is selected.
The primary coil of a transformer is connected to the AC mains and the secondary coil to a load resistance RL through the diode D. The voltage of the secondary coil of the transformer is the input for the rectifier.
Let V=V_msinωt be the instantaneous sinusoidal voltage of frequency (=50Hz) appearing at the secondary coil of the transformer.
During the positive half-cycle of the input voltage, the diode D is forward biased and it conducts the current through R_L. Since the diode offers negligible resistance in the forward bias, the entire secondary voltage of transformer appears across R_L as output voltage.
During the negative cycle of the sine wave, the diode D is reverse biased. Hence no current flows in the circuit and no voltage develops across R_L.

Only the positive half cycle of the input appears across the load. The input AC voltage I converted into a pulsating DC voltage as shown in waveform in the figure. This process is called half wave rectification.

The output voltage is unidirectional, pulsating and intermittent. The output voltage contains a DC component and an AC component known as ripple.

Energy Bands in Solids

Peak Inverse Voltage:

The maximum negative voltage appearing across the diode when it is not conducting is called peak inverse voltage denoted by PIV.

PIV = V_m

Ripple Factor:

Ripple factor is defined as the ratio of the RMS value of the AC component of load current to the average value of load current. It is a measure of imperfection in the DC output.

Ripple Factor γ = RMS value of AC component / Average value of current

= I_ac / I_dc = 1.21

Efficiency Rectification:

The efficiency of rectification is defined as the ration of the DC output power to the input AC power.

Rectified Efficiency = O/p DC power / Input AC power

= P_dc / P_ac~ 40.6%

Voltage Regulation:

The regulation is a measure of the variation of the DC output voltage as a function of the DC current and is defined as

%Regulation = {(V_{no load} – V_full_Load) / V_{full load}} x 100

For an ideal power supply the % regulation will be zero.

Disadvantage of Half Wave Rectifier:

Since the ripple factor = 1.21 is greater than C value, the half wave rectifier is a poor device for the conversion of AC voltage into DC voltage.
Efficiency of rectification is very low (= 0.406)
It has a low transformer utilization factor.

This post first appeared on My Tech Info, please read the originial post: here