Lincoln Electric LN-25 SVM179-B Service Manual page 42

E-4
ELECTRODE
INPUT
CONTACTOR CURRENT TRANSDUCER
INPUT
21
1
14 PIN AMPHENOL
21
DIGITAL
RUN IN
DISPLAY
BOARD
FEEDBACK AND CONTROL CIR-
CUITS
The wire speed pot provides command signal to the
control board indicating the desired wire speed. The
tachometer transforms the motor RPM to a digital fre-
quency that is fed back to the control board.
When operating in constant voltage (CV) mode, the
control board monitors the feedback signal, compares
it to the command signal and delivers the appropriate
voltage to the wire drive motor.
When operating in constant current (CC) mode, a vari-
able wire speed is desirable to compensate for the
varying arc voltages associated with the constant cur-
rent process. To accomplish this, the control board
monitors the command voltage, the feedback signal
from the tach and the arc voltage.
THEORY OF OPERATION
FIGURE E.4 — FEEDBACK & CONTROL CIRCUITS
SOLENOID
POLARITY
V M
CONTROL BOARD
CC
COLD
CV
FEED
NOTE: Unshaded areas of Block Logic
Diagram are the subject of discussion
LN-25™ PRO
67
TIMER OPTION
MOTOR
MOTOR
OVERLOAD
ARMATURE
TACH FEEDBACK
GUN TRIGGER
TRIGGER GAS
INTERLOCK PURGE
These three factors are monitored and compared, then
the appropriate armature voltage is applied to the wire
drive motor.
When the Run-in Switch is on, the wire speed is
reduced until an arc is struck. When it is off the run-in
wire speed is the same as the welding wire feed speed.
Pressing the cold feed button activates the drive rolls
but neither the power source nor the gas solenoid will
be activated.
FEEDPLATE
TACH
GUN
REMOTE
WIRE
VOLTAGE
SPEED
E-4