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E-2
THEORY OF OPERATION
NOTE: Unshaded areas of block logic diagram are the subject of discussion.
OUTPUT CONTROL, RECTIFICA-
TION AND VOLTAGE FEEDBACK
The AC voltage that is applied to the main
transformer primary is controlled at the con-
trol board by two SCR’s (Silicon Controlled
Rectifiers). The SCR’s are controlled by a
pulse signal developed on the control board.
The control board compares the commands
of the arc voltage control with the voltage
feedback signal. The board circuitry then
sends a pulse to turn on the SCR’s. In this
manner, the voltage applied to the primary
of the transformer is varied and controlled.
This variable and controlled voltage is
FIGURE E.2 – Output Circuits
reflected at the transformer secondary wind-
ing and is applied to the rectifier diode
bridge. This rectified DC voltage is filtered
by the output capacitor and choke circuit
and is applied to the machine’s output ter-
minals.