Theory Of Operation; General Description; Input Power Circuit - Lincoln Electric PRECISION TIG 375 Service Manual

E-2
FIGURE E.2 – GENERAL DESCRIPTION AND INPUT POWER CIRCUIT
FROM
CONTROL
BOARD
OPTIONAL POWER
FACTOR
CORRECTION
CAPACITORS
POWER
SWITCH

GENERAL DESCRIPTION

The Precision TIG 375 is part of a new family of indus-
trial arc welding power sources able to provide con-
stant current and single range square wave AC/DC Tig
(GTAW) with new Micro-Start
rates independent presettable minimum and maximum
output control with built-in high frequency stabilization
for continuous AC Tig welding and reliable DC Tig start-
ing. The Precision TIG 375 also has AC/DC stick
(SMAW) capabilities. This new design includes
advanced features such as a digital meter, presettable
controls, auto balance TM , fan as needed and timers for
fixed preflow and variable post flow of shielding gas. It
features a stick output terminal (front) and a universal
Tig torch connection box (rear) for simultaneous, but
separate, electrode outputs.

INPUT POWER CIRCUIT

The desired single-phase input power is connected to
the Precision TIG 375 through the power switch to the
reconnect panel located in the rear of the machine.
The machine can be configured for any one of three
input voltages (208 VAC, 230 VAC or 460 VAC) by con-
necting the jumper strap to the appropriate terminal on
NOTE: Unshaded areas of Block Logic Diagram are the subject of discussion.

THEORY OF OPERATION

Precision Tig 375
BACKGROUND
CIRCUIT
STICK OCV BOOST
CIRCUIT
FROM CONTROL BOARD
X1
X3
X2
115 VAC
THERMOSTAT
115 VAC
20 VAC
20 VAC
ADVANCED CONTROL
PANEL RECEPTACLE
MINIMUM
OUTPUT
CONTROL
MAXIMUM
OUTPUT
CONTROL
TM Technology. It incorpo-
PRECISION TIG 375
REAR GANG
POLARITY
SWITCH
BYPASS
AC DC
SCR
BRIDGE
AC DC
SHUNT
GAS
FANS
SOLENOID
CONTROL BOARD
PROTECTION STICK
METER
BOARD TIG
SWITCH
LOCAL/REMOTE
BALANCE
POST
CONTROL
FLOW
REMOTE
RECEPTACLE
the reconnect panel. When the input power switch is
turned "on," the input voltage is applied directly to the
primary winding of the main transformer.
The main transformer changes the high voltage, low
current input power to a low voltage, high current out-
put available at the main secondary winding (X1 and
X2). This 78 VAC winding supplies power to the weld-
ing arc. This 78 VAC is tapped at X3 to provide power
to the stick OCV boost circuit. In addition, four auxiliary
windings are incorporated in the main transformer. The
115 VAC winding supplies power to the 115 VAC recep-
tacle. Through the control board, it also powers the gas
solenoid, the high voltage transformer, and the cooling
fan. The cooling fan is activated only when welding cur-
rent is sensed. The 63.5 VAC winding provides power
for the DC background current. This circuit is active in
the DC TIG welding mode. The 20 VAC windings are
included in the main transformer assembly. The 20 VAC
winding is rectified on the control board and is used in
the trigger circuitry. The other 20 VAC winding is used
by the control board for phase detection. This AC volt-
age is also rectified to several DC voltages and regu-
lated to +15 VDC and +5 VDC power supplies that
operate the circuitry on the control board.
WORK
BOARD
FRONT
ELECTRODE
HIGH FREQUENCY
TRANSFORMER
REAR
ELECTRODE
HIGH FREQUENCY
INTENSITY ADJUSTMENT
HIGH VOLTAGE
TRANSFORMER
THERMOSTATS
LED's
SWITCH
E-2