Lincoln Electric IDEALARC DC-600 Service Manual page 41

E-3
FIGURE E.3 Output, Rectification, Control and Feedback
CONTROL
TRANSFORMER PROTECTION
DEVICES &
CIRCUTS
115 VAC
(
CONTACTOR
HOLD-IN
MAIN
TRANSFORMER
R
E
C
O
N
N
E
C
T
CONTACTOR
TERMINAL
STRIP
REMOTE
TO
CONTROL
CONTROL
BOARD
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 and voltage is
sensed at the shunt and output terminals. This feed-
back information is processed in the control board.
The control board compares 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
THEORY OF OPERATION
115 VAC
FAULT PROTECTION
)
FIRING BOARD
1
1
5
V
A
C
FAN
IDEALARC
OUTPUT
CONTROL
CONTROL
BOARD
F
E
E
PILOT
D
RELAY
B
A
C
K
SHUNT
SCR BRIDGE
CHOKE
energy to the gates of the power SCRs. When this
energy 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.
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.
A Choke is connected between the neutral connection
of the Main Transformer secondaries and the negative
output terminal. This large inductor stores energy and
provides filtering for the output of the DC-600.
® DC-600
E-3
MODE
SWITCH
POSITIVE
TERMINAL
F
E
E
PILOT
D
RELAY
B
A
C
K
LATCHING
RESISTOR
NEGATIVE
TERMINAL