Lincoln Electric IDEALARC DC-1000 Service Manual page 31

E-3
FIGURE E.3 - OUTPUT, RECTIFICATION, CONTROL AND FEEDBACK
OUTPUT RECTIFICATION, CONTROL AND
FEEDBACK
The neutrals of the Main Transformer secondary wind-
ings are connected together and the six starts are con-
nected to the six SCR assemblies to form a six phase
output. This six phase AC output from the Main
Transformer secondary is rectified and controlled
through the SCR bridge. Output current is sensed at
the shunt, and output voltage is monitored at the weld-
ing output terminals. This feedback information is
processed in the control board. The control board com-
pares the commands of the Mode switch and the
Output Control Potentiometer (or Remote Control) with
the feedback information and sends the appropriate
signal to the Firing Board.
The Firing Board is a three phase circuit. Each phase
provides two firing pulses, one for each of the two
Silicon Controlled Rectifiers (SCR) controlled by that
phase. The firing circuit supplies the proper amount of
energy to the gates of the power SCRs. When this gate
signal is applied, at the correct time, the SCR will turn
"ON". The amount of "ON" time versus "OFF" time
determines the output of the machine. See SCR
Operation.
THEORY OF OPERATION
The Pilot Relay signals the Firing Board circuit to sup-
ply gate pulses to the SCR Bridge. Closing of the Pilot
Relay (a "dry" closure of leads #2 and #4) also brings
the Latching Resistor into the machine output circuit.
The Latching Resistor provides a pre-load for the SCR
Bridge.
In later models (above code 9500) a Choke and sepa-
rate 500 amp output terminal is provided to enhance
lower current arc characteristics, especially for sub-
merged arc and GMAW procedures below 450 amps.
NOTE: Unshaded areas of Block Logic
Diagram are the subject of discussion
IDEALARC ® DC-1000
E-3