Lincoln Electric DC-655 service manual Down Circuits

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E-3 E-3

THEORY OF OPERATION

FIGURE E.3 – CONTROL BOARD, FIRING BOARD, SNUBBER/FAN BOARD AND RECTIFICATION

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CONTROL

TRANSFORMER

115VAC

CR2

R

E

C

O

N

N

E

C

T

CONTACTOR

CR1

42VAC

CONTROL

BOARD

TRANSFORMER

FAULT PROTECTION

							OUTPUT		ARC FORCE
							CONTROL
32VAC						OUTPUT			CONTROL
				FIRING		CONTROL SIGNAL
32VAC
32VAC				BOARD
				G	S
14-pin		115VAC	Fan	A	I
				T	G
Amphenol		Receptacle	Motor		N	Fan
				E	A	Thermal
					L	Sensor
T				SNUBBER			CONTROL
	S						BOARD		MODE
R				FAN
	T
E
M	R	115VAC		BOARD					SWITCH
I	I
N	P
A
L				GATE SIGNAL
							F
42VAC
							E
							E
MAIN							D
							B
TRANSFORMER							A
							C		POSITIVE
							K
									TERMINAL
							SHUNT	F
								E
								E
42VAC 10VAC10VAC							LATCHING	D
							RESISTOR	B
				SCR BRIDGE				A	HIGH
								C	NEGATIVE
						CHOKE		K	TERMINAL
TO OPTIONAL
METER BOARD

LOW

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CONTROL BOARD, FIRING BOARD, SNUBBER/FAN, BOARD AND RECTIFICATION

The “neutrals” of the welding secondary windings in the main transformer are connected together, and the six starts are connected to the six Silicon Controlled Rectifier (SCR) assemblies to form a six-phase output. This six-phase AC output from the main transformer secondary is rectified and controlled through the SCR bridge.

A portion of the firing board is a three-phase circuit. Each phase provides two firing pulses, one for each of the two SCRs controlled by that phase. When a gate firing enable signal is received, the firing circuit sup- plies the proper amount of energy to the gates of the power SCRs. When this gate firing signal is applied at the correct time, through the snubber/fan board, the SCR will turn ON. The amount of ON time versus OFF time determines the output of the machine. See SCR Operation. At this time the latching resistor is brought into the machine’s output circuit. The latching resistor provides a pre-load for the SCR bridge.

The control board receives current feedback informa-

tion from the shunt and voltage feedback information from the choke and welding output terminals. This feedback information is processed on the control board. The control compares the commands of the mode switch, the output control potentiometer (or remote control device) and the arc force control with the feedback information and sends the appropriate output control signal to the firing board. In the event of a “fault condition,” the control board activates the fault relay (CR2). See Protective Devices and Shut

Down Circuits.

A tapped output choke is connected between the neu- tral connection of the main transformer secondaries and the two negative output terminals. This large inductor stores energy, which provides current filtering for the welding output of the DC-655. Two negative output terminals are provided. One is connected to the tap lead in the choke, thus providing a lower induc- tance. The other utilizes the entire choke for higher inductance arc characteristics.

The snubber/fan board furnishes protection to the SCR bridge from the transient voltages. It also moni- tors the thermal sensor and activates the fan motor when cooling is necessary.

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NOTE: Unshaded areas of Block Logic Diagram are the subject of discussion.

IDEALARC DC-655