Printed Circuit Board Functions - Lincoln Electric SVM138-A Service Manual

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PRINTED CIRCUIT BOARD
FUNCTIONS
POWER BOARD
The input power (115VAC) is applied to the power
board, and from that the power board develops the
variable DC armature voltage for the permanent mag-
net wire drive motor. The SCR circuitry, used to con-
trol the motor armature voltage, is also contained on
the power board.
The relay used to activate motor armature voltage, the
gas solenoid, and power source output (#2 and #4) is
housed on the power board.
The shut down, start response and trigger interlock cir-
cuitry, in addition to the rectified and regulated power
supplies used for the LN-9 GMA control circuitry, are
included in the power board design.
CONTROL BOARD
The control board provides a reference voltage to the
wire speed control and voltage control potentiometers,
allowing SET (pre-set) signals to be produced.
The control board also powers and processes the feed-
back signals from the hall effect device. It compares
the actual speed information to the SET speed com-
mand signal and develops firing pulses that drive the
motor armature SCR circuitry, located on the power
board.
Acceleration and run-in rates for wire feed speed are
controlled by the control board as well as the SET speed
and actual speed signals for the digital meter circuitry.
THEORY OF OPERATION
VOLTAGE BOARD
The voltage board compares the SET voltage values to
the actual voltage values and makes any necessary
correction by sending the appropriate signal to the
Lincoln CV power source. If for any reason the arc
voltage cannot be controlled to the SET voltage, the
voltage board interrupts the LN-9 GMA trigger path,
and the unit shuts down. The response (Fast or Slow)
to changes in arc voltage is also controlled by the volt-
age board, as well as the SET and actual arc voltage
signals for the digital meter circuitry.
TRIGGER BOARD
The trigger transformer supplies 24VAC to the trigger
board where it is rectified and regulated to 20VDC.
This DC voltage is used in the gun trigger circuit to ini-
tiate the welding operation.
HALL EFFECT DEVICE
A square wave frequency signal, proportional to motor
speed, is generated by the hall effect device and sent
to the control board.
METER BOARD
The meter board contains signal scaling and power
supply circuitry for the digital meter.
LN-9 GMA Wire Feeder
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