Miller Electric S-21E, Technical Manual

Page: 14 / 48

WARNING: ELECTRIC SHOCK can kill;
TANGLED WELDING WIRE can touch case
causing welding power source open-circuit
voltage to be present on case if gun trigger is
pressed.
• Do not touch wire feeder case ifgun trigger is
pressed and wire does not feed.
• If wire stops feeding, turn off welding power
source, and determine the cause.
• Correctanyhub tension,jammed wire, orgun
liner damage problems before trying to
continue welding.
6-2. SHUTTING DOWN
1. Stop welding.
2. Place POWER switch in the OFF position.
3. Shut down welding power source or generator.
4. Turn off shielding gas at source, if applicable.
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WARNING: HIGH CONCENTRATION OF
SHIELDING GAS can harm health or kill.
• Shut off gas supply when not in use.
SECTION 7- THEORY OF OPERATION
7-1. THEORY OF OPERATION
IMPORTANT:
The following Theory Of Operation is
written in steps which match the circled numbers on Dia-
grams 7-1 and 7-2.
Wire Feeder Without Options (Diagram 7-1)
The 14-pin plug PLG100 on the end of the inter-
connecting cord provides 24 volts ac input power
from the welding power source and provides con-
tactor control to the welding power source.
2. POWER switch 51 provides on/oft control of 24
volts ac input power to the wire feeder.
3. Circuit breaker CB1 provides overload protection
for the wire feeder.
4. Motor Board PCi regulates the speed of wire
drive motor Ml as set by WIRE SPEED control
R32. PCi also controls gas valve 051 and weld-
ing power source contactor via control relay
CR1.
5. Gas valve 051 provides shielding gas during the
weld cycle.
6. Integrated rectifier SRi converts the 24 volts ac
input power to a rectified 35 volts dc.
Capacitor Cl provides filtering for the 35 volts dc.
Voltage regulator VR1 reduces the 35 volts dc in-
put voltage to 24 volts dc for control circuitry on
PCi and trigger receptacle RC1 01.
9. Trigger receptacle RC1 01 provides the trigger in-
put signal to the motor stop/start circuitry.
A.
1.
7.
8.
10. Motor stop/start circuitry provides input to the
brake turn on ramp, phase modulation, and con-
trol relay circuitries.
11. Control relay CR1 controls gas valve 051 and the
welding power source contactor through 14-pin
plug PLG100.
12. Brake circuitry transistor Q7 provides an electri-
cal path for motor current during braking.
13. Turn on ramp provides power to WIRE SPEED
control R32.
14. WIRE SPEED control R32 provides a signal to
the comparator circuitry to control the wire speed.
15. Feedback circuitry provides comparator circuitry
with a feedback signal of actual motor voltage.
16. Comparator circuitry compares feedback signal
to R32 control signal and adjusts phase modula-
tion circuitry accordingly.
17. Phase reset transistors Q5 and Q6 reset the tim-
ing of phase modulation.
18. Unijuction transistor Qi creates the phase modu-
lation from the charging of capacitor ClO.
19. Pulse transformer Ti transfers the phase modu-
lated signal to the gates of SCRi and SCR2.
20. Drive SCR’s SCRi and SCR2 turn on from the
phase modulation signal and supply power to
wire drive motor Mi.
21. Wire drive motor Ml feeds the welding wire.
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TM-1571 Page 10
S-21E