To cause the motor to run continuously, the transistor which is in series with it will be short-circuit from emitter to collector, or incorrectly constantly turned on by a signal on the base. You would need to trace the motor wiring to confirm this. This second component will get switched on when the motor is not under power, to short it and bring it to a rapid halt, preventing run-on of the MIG wire spool. It is possible that one transistor is in series with the motor to control the speed, and the other is across the motor terminals ( i.e.
Either of these two devices could easily run a typical wirefeed motor, at 24 or 48 volts and 2 or 3 Amps. The MJ15015 specs are: 120 volts, 15 Amps, and 180 Watts. The MJ900 is rated at 80 volts, 8 Amps, and 90 Watts. The normal digital meter test procedure may not show the typical results on a Darlington Pair. That is a pair of transistors in one case, with the amplified current of one feeding into the base of the other, so they switch easily and quickly. The MJ900 is the equivalent of the NTE244, and it is a PNP Darlington Pair. The two power transistors that you found in your welder explain why I discovered data about two alternative components - there are in fact two different ones used in the machine. Snap-on YA212 & 15TG10 Replacement MIG Gun 15' For Machine Models: FM-140, FM-140A, YA-204, YA-212, YA-212A, MM140SL and MM250SL Direct-Connect Cable 2-Trigger Plugs Snap-on YA212 & 15TG10 Style Replacement MIG Gun (15') Item 012NS029 Regular price: 389.00 Welding Direct Price 298.
