Power electronics digital notes b. Tech III year

SCR
s 1 through 4 are gated on. The 
SCR
s are turned on in pairs by 
simultaneously applying signals to the gate terminals of 
SCR
s 1 and 4 or 
SCR
s 2 and 3. If 
SCR
s 
1 and 4 happen to be the first two switched on a current will flow from the positive terminal of 
the source through negative terminal of the source. This will establish a left to right, plus to 
minus voltage relationship on the load. 
Simultaneously, the left terminal of capacitor C1 will be charged positively with respect to the 
right negative terminal. The steady-state load current through the various components is 
determined nearly completely by the impedance of the load. Chokes 1 and 2 and
SCR
s 1 and 4 
present very low steady-state drops and therefore nearly all the source voltage appears across the 
load. Conduction of 
SCR
s 1 and 4 will continue to the end of the half cycle, at which p oint the 
gates are removed from 
SCR
s 1 and 4 remain in conduction along with 
SCR
s 2 and 3 that have 
now been turned on. If it were not for chokes 1 and 2, the action of turning on the second set 
of 
SCR
s would place very low impedance and therefore momentar ily prevent the source from 
being short-circuited. 

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P a g e
Capacitor C1 now discharges with a current which flows into the cathode of
SCR
1 
through 
SCR
2 in a forward direction back to the negative terminal of the capacitor. This 
direction of current flow causes 
SCR
1 to become non-conductive provided that the reverse 
current through the 
SCR
is of sufficient duration for the 
SCR
to again become blocking. C1 
simultaneously discharges through