E-5
E-5
PRECISION TIG 275
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DC WELDING OUTPUT
When the polarity switch is placed in either DC position, the
AC voltage from the main transformer secondary is applied
to the SCR bridge. The SCR bridge and choke circuits are
connected in the conventional full wave bridge and filter
configuration, resulting in a controlled DC output. Since
the choke is in series with the negative leg of the bridge
and also in series with the welding load, a filtered DC is
applied to the output terminals. The bypass board pro-
tects the internal circuitry from interference.
When the machine is in the DC mode, the background cir-
cuitry provides an added boost of voltage to the output
terminals. This circuitry is controlled by the control board.
THEORY OF OPERATION
FIGURE E.5 – DC WELDING OUTPUT
POWER
SWITCH
RE
CO
NN
EC
T
P
AN
EL
OPTIONAL POWER
FACTOR
POLARITY
SWITCH
BYPASS BOARD
WORK
FRONT
ELECTRODE
REAR
ELECTRODE
78
V
AC
X1
X2
AC
AC
DC
DC
SHUNT
CH
O
KE
HIGH FREQUENCY
TRANSFORMER
HIGH VOLTAGE
TRANSFORMER
THERMOSTAT
115 VAC
115 VAC
11
5 V
AC
FAN
GA
TE
LE
AD
S
FE
ED
BA
CK
GAS
SOLENOID
11
5 V
AC
11
5 V
AC
LED's
THERMOSTAT
20 VAC
20 VAC
CONTROL BOARD
BACKGROUND
CIRCUIT
FROM
CONTROL
BOARD
63
.5
VA
C
CORRECTION
CAPACITORS
REAR GANG
SCR
BRIDGE
ADVANCED CONTROL
PANEL RECEPTACLE
MINIMUM
OUTPUT
CONTROL
MAXIMUM
OUTPUT
CONTROL
PROTECTION
BOARD
REMOTE
RECEPTACLE
METER
STICK
TIG
SWITCH
BALANCE
CONTROL
POST
FLOW
LOCAL/REMOTE
SWITCH
FIGURE E.6 DC WELDING CURRENT GENERATION
G
G
G
G
CHOKE
ELECTRODE
WORK
DC
PRIMARY
10
