Page 29 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

Rectifiers

Objectives: At the end of this lesson you shall be able to
• state the purpose of rectifier in power supply circuit
• explain the working of half-wave, full-wave and bridge rectifier circuit
• state the need of filter circuit to rectifier circuits
• state the different types filter circuit for rectifiers.

Most of the electronic equipment, both entertainment and professional, need DC voltage for operation. The
power supply converts AC supply voltage into DC. Diodes are used as rectifier in a power supply circuit.
Half wave rectifier: This simplest form of AC to Dc converter is by using one diode such an Ac to Dc converter is
known as half-wave rectifier as in Fig 1.

Fig 1

A diode D1 and a load resistance RL in series are connected across the secondary of a step down transformer
(Fig 1(a). The transformer steps up or steps down the supply voltage as needed. Further the transformer isolates
the power line and reduces the risk of electrical shock. During the positive half-cycle of the input line frequency,
(Fig 1b) the diode anode is made positive with respect to the cathode. The diode D1 conducts because it is
forward-biased. Current flows from the positive end of the supply through diode D1 and RL to the negative
terminal of the input. During this period of time, a voltage is developed across RL. The polarity of the voltage is
as indicated in Fig 1C.
During the negative half cycle of AC input line frequency, the diode is reverse-biased. Practically no current flows
through the diode and the load RL and there is no voltage output.
DC output: The voltage drop across the forward biased diode is low, because the resistance of the forward-biased
diode is very low. Ge diode drops 0.3V and Si diode drops 0.7V. Ignoring the small voltage drop across the diode.
We can find the relationship between AC input and DC output voltage.
The AC input wave-form is shown in Fig 1b.
V = 0.707 V
rms p

In Fig 1C, the DC output is shown. The diode produces only half cycle of the Ac input. The average value of this
half wave is the DC output voltage.

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CITS : Power - Electrician & Wireman - Lesson 60-69