Lincoln Electric SVM169-A service manual Output Rectification Feedback Control

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E-3		THEORY OF OPERATION					E-3
	FIGURE E.3 Control Board and Output Feedback Control.
						BY-PASS
						BOARD
			POLARITY				WORK
			SWITCH				ELECTRODE
		MAIN				HI-FREQUENCY
		TRANSFORMER				TRANSFORMER
	INPUT
	POWER
	SWITCH BYPASS					C
	ASBLY
						H
		H1	AC	DC+		O
		X1
						K
			SCR
						E
			BRIDGE
		H2			SHUNT	HIGH VOLTAGE
		X2
			AC	DC-		TRANSFORMER
		H3
					F	CIRCUIT
					E
					E
					D
					B
					A
					C
					K
		18VAC				115VAC
		18VAC		CONTROL		POWER
	FAN	115VAC		BOARD		"ON"
						DIGITAL
						DISPLAY
						MODE
						SWITCH
		REMOTE			THERMO-
		RECEPTACLE	THERMAL	GAS		OUTPUT
					STATS
			LIGHT	VALVE		CONTROL

AUTO	PULSE TIG	STICK	AC	PULSE POSTFLOW
BALANCE FREQ. LED		LED	BALANCE	CONTROL CONTROL
LED	LED		CONTROL

OUTPUT RECTIFICATION AND

FEEDBACK CONTROL

The AC output from the main transformer secondary is rectified and controlled through the SCR bridge. Output current is sensed at the shunt as a low voltage signal and fed back to the control board. The control board compares the commands of the mode switch, output control, AC balance control, pulse control, post- flow control, or remote control with the feedback infor- mation. The appropriate SCR gate firing signals are created by the control board and sent to the SCR bridge. The control board controls the firing of the SCRs, which control the output of the machine. See SCR Operation. The control board monitors the ther- mostats, and also controls the gas solenoid valve, ther- mal light, auto balance LED, pulse frequency LED, TIG LED, stick LED, and the digital display.

NOTE: Unshaded areas of block logic diagrams are the subject of discussion.

PRECISION TIG 185