Lincoln Electric LF-72/74 service manual Theory of Operation, Return to Section

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E-2 THEORY OF OPERATION E-2

FIGURE E.2

GENERAL DESCRIPTION & INPUT RECEPTACLE, CONTROL BOARD AND OPERATOR CONTROLS

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OPTIONAL BURNBACK TIMER

OPTIONAL REMOTE VOLTAGE CONTROL

	GAS
3.75 AMP	SOLENOID
OVERLOAD	WIRE
PROTECTION
	DRIVE
WIRE DRIVE	MOTOR
	TACK
RECEPTACLE	To Gun Trigger

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14PIN

INPUT CABLE RECEPTACLE

24/42 VAC

CONTROL BOARD

WIRE FEED	TRIGGER
	INTERLOCK
SPEED POT.	2-STEP

COLD FEED GAS PURGE

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GENERAL DESCRIPTION

The LF72 and LF74 are single head, 24vac or 42vac input, 2 roll (LF72) or 4 roll (LF74) wire feeders. They have a single calibrated control for wire feed speed WFS and an optional K2339-1 remote voltage control kit to be used in conjunction with a Lincoln constant voltage (CV) direct current DC power source.

The LF72 and LF74 offer the user 3 individual features Cold feed, trigger interlock 2 step trigger control and gas purge. Presettable burnback and postflow timers are also available with an optional K2330-1 timer kit.

The motor is equipped with a 3.75amp current over- draw protection circuit, which is indicated by a THER- MAL LED located on the case front.

INPUT RECEPTACLE, CONTROL BOARD AND OPERATOR CON- TROLS

The 24 or 42-volt input is applied to the LF72 or LF74 through the 14 pin amphenol type connector. The remote trigger leads are accessed through 14 pin receptacle. The input voltage is rectified by a full wave bridge and is sent to a switching power supply in the control board where it is filtered. This filtered DC volt- age is applied to a switching power supply. The switching power supply manufactures +15vdc and +5vdc regulated supplies for the control board circuit- ry.

The control board receives commands from the user operated controls for the wire feed speed WFS pot, the optional timer kit and the optional voltage remote control kit. It compares this feedback information with the user command signals and generates appropriate signals to control wire feed speed, arc voltage and gas flow.

The control board is also responsible for the operation of the trigger interlock, 2-step, cold feed and the gas purge features of the LF72 and the LF74.

LF-72/74 SINGLE/DUAL