Lincoln Electric NA-3S, Service Manual, Page: 62 / 110

THEORY OF OPERATION
E-3 E-3
NA-3 AND NA-4
CONTROL AND LOGIC
BOARDS
In general, the control board houses two
SCR-controlled 115 VDC power supplies
that power the field and armature circuits in
the wire drive motor. Also incorporated in the
control board is a regulated 24 VDC control
circuit power supply.
The logic board incorporates a 15 VDC
regulated supply, which is derived from the
24 VDC received from the control board. The
logic board interprets and processes the
signals received from the various switches
and controls.
Upon receiving commands from the user-
operated switches or potentiometers, the
logic board sends the appropriate signal to
the control board, driving the wire feed motor
at the correct speed and in the proper
direction. When the start signal is received
by the logic board, the power source contact
relay (3CR) is energized along with the
travel relay (2CR) and the flux relay (1CR).
When the weld current closes the reed
switch (4CR), the logic board directs the
control board to change the wire drive motor
from the preset inch speed to the welding
feed speed set by the wire speed control
potentiometer. Also, output control of the
power source is transferred from the strike
setting to the weld output setting. The “Inch
Down” and “Stop” switches signal the logic
board, sending the appropriate command to
the control board, which sets the wire drive
motor to the correct mode.
The “Inch Up” switch, which is connected
directly to the control board, dictates that the
wire drive motor reverse direction, backing
the electrode wire away from the work piece.
See Figure E.2.
FIGURE E.2 – CONTROL AND LOGIC BOARD.
TRAVEL
RECEPTACLE
TRAVEL
SWITCH
INCH UP
SWITCH
CONTROL
BOARD
LOGIC
BOARD
LOGIC CONTROL
SIGNALS
24 VDC
ARC VOLTAGE
SENSING SIGNALS
OPTIONAL
START
BOARD
OPTIONAL
CRATER
FILL
BOARD
1CR
2CR
3CR
WIRE FEED
SPEED CONTROL
POWER SOURCE
OUTPUT CONTROL
START
SWITCH
STOP
SWITCH
INCH
DOWN
SWITCH
115 VAC
115 VAC
R1
115 VAC
115 VAC
115 VAC
VARIABLE VOLTAGE
BOARD
ARC
VOLTAGE
METER
VOLTAGE
SENSING
LEADS
ELECTRODE
WORK
115 VAC 115 VAC
115 VAC
115 VAC
FROM
WELDING
POWER
SOURCE
WIRE
DRIVE
MOTOR
WELD CURRENT SENSING
REED SWITCH 4CR
2CR TRAVEL
RELAY CONTACTS
CIRCUIT
BREAKER