Circuit Operation
The circuit of a single-phase fully-controlled bridge rectifier circuit
is shown in the figure above. The circuit has four SCRs. It is preferable
to state that the circuit has two pairs of SCRs, with S1 and S4
forming one pair and, S2 and S3 the other pair. For
this circuit, vs is a sinusoidal voltage source. When it is positive,
SCRs S1 and S4 can be triggered and then current flows
from vs through SCR S1, load resistor R, SCR S4
and back into the source. In the next half-cycle, the other pair of SCRs conducts.
Even though the direction of current through the source alternates from one
half-cycle to the other half-cycle, the current through the load remains unidirectional.
The main purpose of this circuit is to provide a variable dc output voltage,
which is brought about by varying the firing angle. Let vs = E
sin wt, with 0 < wt < 360o. If wt = 30o when S1
and S4 are triggered, then the firing angle is said to be 30o.
In this instance, the other pair is triggered when wt = 210o.
When vs changes from a positive to a negative value, the current
through the load becomes zero at the instant wt = p
radians, since the load is purely resistive and the SCRs cease to conduct.
After that there is no current flow till the other pair is triggered. The
conduction through the load is discontinuous.
The operation of the circuit is illustrated by animating the functioning
of this circuit. Key in the firing angle in degrees and click the button.
The source voltage and the output voltage waveforms are also displayed.
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