Explain class B commutation with neat circuit diagram

Ans:

Explanation: 

• This is also a self commutation circuit in which commutation of SCR ’T’ is achieved automatically by L and C components. 

•  When the DC supply is applied to the circuit, the capacitor charges with upper plate positive and lower plate negative up to the supply voltage Edc. When the SCR is triggered, the current flows in two directions, one through Edc + –T – RL – Edc - and another commutating current through L and C components. 

•  Once the SCR is turned ON, the capacitor starts discharging through C+ – L – T – C-. When the capacitor is fully discharged, it starts charging with a reverse polarity. Hence a reverse voltage applied across the SCR which causes the commutating current to oppose load current IL. 

•  When the commutating current is higher than the load current, the SCR will automatically turn OFF and the capacitor charges with original polarity. 

•  The desired frequency of ON/OFF depends on the values of L and C. This type of commutation is mostly used in chopper circuits. 

Note: Waveform is optional.