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E-4 THEORY OF OPERATION
CONTROL RELAY CIRCUIT
The first contact closes to allow 115 VAC to
energize the gas solenoid. This allows
shielding gas to flow to the gun. The second
contact of the control relay closes the circuit
in the welding power source contactor circuit.
This signals the welding power source to
provide power to the gun. The last set of
contacts on the control relay disables the
motor dynamic braking circuit and closes the
motor power supply circuit. See Figure E.3.
When the control relay closes the motor
contacts, the SCR power circuit supplies a
DC voltage to the wire drive motor. This
voltage is dependent on the setting of the
wire speed control. The faster the speed
setting, the higher the voltage that is applied
to the wire drive motor. The gearbox
transfers the power of the rotating motor to
the drive roll. On a four-roll wire feeder, the
gearbox also splits the drive power to two
separate drive rolls.
When the trigger is released, the control
relay is de-energized. This opens the
contacts which de-energize the gas
solenoid, open the welding power source
contactor circuit, and stop the wire drive
motor. The dynamic braking circuit engages
to rapidly stop the wire drive motor and
prevent wire overrun.
LN-7 GMA WIRE FEEDER
FIGURE E.3 – CONTROL CIRCUITS.
TRIGGER
TRANS-
FORMER
24 VAC
8.8 VAC
115 VAC
CIRCUIT
BREAKER
OPTIONAL
METER KIT
OPTIONAL
TIMER OR
BURNBACK KIT
WIRE
SPEED
CONTROL
CONTROL
BOARD
WIRE DRIVE
MOTOR
5-95 VDC
GUN
TRIGGER
SCR
POWER
CIRCUIT
FROM
WELDING
POWER
SUPPLY
DIODE
RECTIFIER
NETWORK
115 VAC
GAS
SOLENOID
TO WELDING
CONTACTOR
CURRENT
LIMITING
CIRCUIT
115 VAC
GROUND
LEAD
PROTECTOR
CR1
CONTROL
RELAY
24 VDC
NOTE: Unshaded areas of block logic diagrams are the subject of discussion.