Lincoln Electric LN-25 Service Manual page 36

E-3
ELECTRODE
INPUT CABLE
P
O
S
L W
A I
R T
TO WORK
I
C
SENSING LEAD T
H
Y
FEEDBACK AND CONTROL
CIRCUITS
The combination of the range switch and wire
speed control provides a command voltage for
the control board.
transforms the motor RPM to a digital voltage
frequency that is Òfed backÓ to the control board.
When operating in the constant voltage (CV)
mode, the control board monitors the Òfeedback
signalÓ, compares it to the command voltage and
delivers the appropriate armature voltage to the
wire feed motor . In this manner , a constant wire
feed speed is maintained. See Figure E.2. When
the LN-25 is connected to a constant current
(CC) power source, a variable wire feed speed
NOTE: Unshaded areas of block logic diagrams are the subject of discussion.
THEORY OF OPERATION
FIGURE E.2 Ð CONTROL CIRCUITS.
COLD
OPTIONAL
INCH
CONTACTOR
SWITCH
BOARD
VOLT
METER
CONTROL
BOARD
WIRE FEED
MODE SWITCH
SOLENOID
PC BOARD
The hall ef fect module
OPTIONAL
CONTACTOR
CONDUCTOR
ARMATURE VOLTAGE
RPM FEEDBACK
RANGE
SWITCH
WIRE SPEED
CONTROL
GAS
SOLENOID
is desirable to compensate for the varying arc
voltages associated with the constant current
process. To accomplish this, the control board
monitors the command voltage, the feedback
signal from the hall ef fect module and the arc
voltage. These three factors are monitored and
compared, and then the appropriate armature
voltage is applied to the wire feed motor
TRIGGER CIRCUIT
When the gun trigger is closed, the control board
is ÒsignaledÓ to apply armature voltage to the
drive motor and to activate any auxiliary circuits
that may be incorporated within the LN-25. See
Figure E.2.
LN-25
GUN CABLE
BLOCK
GUN
M
O TRIGGER
HALL
T
CONNECTOR
O
EFFECT
R
MODULE
E-3
.