Lincoln Electric SVM123-A service manual Output Rectification, CONTROL, and Feedback

E-3 THEORY OF OPERATION E-3

Return to Section TOC	Return to Master TOC	FIGURE E.3- Output, Rectification, Control and Feedback
Return to Section TOC	Return to Master TOC

		OUTPUT RECTIFICATION,
TOC	TOC	CONTROL, AND FEEDBACK
		The neutrals of the Main Transformer secondary	proper amount of energy to the gates of the power
Section	Master
		windings are connected together and the six starts	SCRs. When this gate signal is applied, at the cor-
		are connected to the six SCR assemblies to form a	rect time, the SCR will turn “ON”. The amount of
to	to	six phase output. This six phase AC output from the	“ON” time versus “OFF” time determines the output
		Main Transformer secondary is rectified and con-	of the machine. See SCR Operation.
Return	Return
		trolled through the SCR bridge. Output current is	supply gate pulses to the SCR Bridge. Closing of
		sensed at the shunt, and output voltage is moni-	The Pilot Relay signals the Firing Board circuit to
		tored at the welding output terminals. This feedback	the Pilot Relay (a “dry” closure of leads #2 and #4)
		information is processed in the control board. The
			also brings the Latching Resistor into the machine
		control board compares the commands of the Mode
			output circuit. The Latching Resistor provides a
		switch and the Output Control Potentiometer (or
			pre-load for the SCR Bridge.
		Remote Control) with the feedback information and
		sends the appropriate signal to the Firing Board.	In later models (above code 9500) a Choke and
		The Firing Board is a three phase circuit. Each	separate 500 amp output terminal is provided to
			enhance lower current arc characteristics, espe-
TOC	TOC	phase provides two firing pulses, one for each of
			cially for submerged arc and GMAW procedures
		the two Silicon Controlled Rectifiers (SCR) con-
			below 450 amps.
		trolled by that phase. The firing circuit supplies the
to Section	to Master
Return	Return	NOTE: Unshaded areas of block logic diagrams are the subject of discussion.
		IDEALARC DC-1000