CONTROL BOARD, FIRING BOARD,
SNUBBER/FAN, BOARD AND
RECTIFICATION
The “neutrals” of the welding secondary windings in
the main transformer are connected together, and the
six starts are connected to the six Silicon Controlled
Rectifier (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.
A portion of the firing board is a three-phase circuit.
Each phase provides two firing pulses, one for each of
the two SCRs controlled by that phase. When a gate
firing enable signal is received, the firing circuit sup-
plies the proper amount of energy to the gates of the
power SCRs. When this gate firing signal is applied at
the correct time, through the snubber/fan board, the
SCR will turn ON. The amount of ON time versus OFF
time determines the output of the machine. See SCR
Operation. At this time the reactor and capacitor cir-
cuit is brought into the machines output circuit. This
enhances the constant voltage (CV) characteristics of
the CV-655.
The control board receives current feedback informa-
tion from the shunt and voltage feedback information
from the welding output terminals. This feedback
information is processed on the control board. The
control board compares the commands of the output
control potentiometer (or remote control device) with
the feedback information and sends the appropriate
output control signal to the firing board.
A tapped output choke is connected to the two posi-
tive output terminals. This large inductor stores ener-
gy, which provides current filtering for the welding out-
put of the CV-655. Two positive output terminals are
provided. One is connected to the tap lead in the
choke, thus providing a lower inductance. The other
utilizes the entire choke for higher inductance arc char-
acteristics.
The snubber/fan board furnishes protection to the
SCR bridge from the transient voltages. It also moni-
tors the thermal sensor and activates the fan motor
when cooling is necessary.
THEORY OF OPERATION
E-3
E-3
CV-655
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NOTE: Unshaded areas of Block Logic Diagram are the subject of discussion.
FIGURE E.3 – CONTROL BOARD, FIRING BOARD, SNUBBER/FAN BOARD AND RECTIFICATION
CONTROL
TRANSFORMER
R
E
C
O
N
N
E
C
T
CONTACTOR
115VAC
MAIN
TRANSFORMER
CONTROL
BOARD
SHUNT
OUTPUT
CONTROL
SWITCH
POSITIVE
TERMINAL
NEGATIVE
TERMINAL
F
E
E
D
B
A
C
K
F
E
E
D
B
A
C
K
SCR BRIDGE
42VAC
115VAC
42VAC
T
E
R
M
I
N
A
L
S
T
R
I
P
TO OPTIONAL
METER BOARD
LOW
FIRING
BOARD
SNUBBER
FAN
BOARD
CONTROL
BOARD
TRANSFORMER
CR1
CHOKE
14-pin
Amphenol
115VAC
Receptacle
Fan
Motor
Fan
Thermal
Sensor
42VAC 10VAC10VAC
32VAC
32VAC
32VAC
SIGNAL
G
A
T
E
S
I
G
N
A
L
GATE SIGNAL
OUTPUT
CONTROL
CAPACITORS
OUTPUT
CONTROL
R
E
A
C
T
O
R
