Speed control for HF MIG welders

I bought a Harbor Freight “Dual MIG 131/2” welder several years ago for a particular project which I then did without using the welder. I recently decided I really needed to learn to use the welder properly. It is actually well made in many ways, but the slowest wire feed rate was far too fast at least for me as a learner. But since the speed is electronically controlled it seemed that surely I could fix that! First I had to work out the schematic for the control system. The PC board in this particular welder is labeled CS-321 but I see in the user manuals that Harbor Freight has online that the overall schematic is very much the same for many of their MIG welders, with other PC boards labeled CS-320 or something else that sounds similar and are connected to the rest of the machine in exactly the same way. So what I found ought to apply to most of their current machines as well as this older one. This machine runs on 120V AC, but for the 220V welders the diagram in the user manual implies that either their PC board also runs on 120V from taps on the primary of the main transformer or in other cases runs on 220V but connects in the same way as this one, so the analysis below should still be very close to correct. Here is what I found on the CS-321 PC board: (There were two resistors labeled R8, I have renamed one R6 for this purpose which fits with the way the labels are arranged on the board.

C1 100uF 63V

C2 10uF 63V

C3 100uF 63V

D1 1N4001

D2 1N4001

D3 1N4001

D4 1N914

DZ1 ZD, voltage unknown

M1 Wire Feed Motor

Pot 1K

Q1 MPS A42

R1 22 ohm 1/4W

R2 3.3K 1/4W

R3 3.3K 1/4W

R4 10ohm 1/4W

R5 4.7ohm 5 W

R6 4.7K 1/4W

R7 220ohm 1/2W

R8 10ohm 1/4W

R9 see notes below

RL1 Hongfa HF2100, 24V DC coil and 30A 120V/220V contacts

SCR BT151

T1 120V/24V 22MA transformer

Pins P3, P6, P7, P8, and P10 connect to a cable and thus to the rest of the welder. There are also push-on connectors on top of the relay that are in the primary wiring of the main power transformer of the welder. Pin P1 is shown on the schematic in the user manual but in fact there is no connection there, instead it is also connected to one of the push-on connections and thus to the power transformer.

Pins P1 (really through one of the push-on connectors on top of the relay) and P3 have 120V AC applied whenever the welder is turned on, regardless of the power setting. Thus there will be 24V AC at the secondary: Diode D1 with capacitor C1 and resistor R1 thus make a DC power supply that will energize the relay coil whenever P6 is connected to P8. Pins P6 and P8 are connected together when the trigger switch in the torch is pressed: P6 goes to the welding wire (the positive side of the welding voltage when using gas, the negative side when using fluxed wire), and P8 goes to the switch in the torch which connects it to the welding wire. Hence the relay energizes when the trigger is pressed, and its contacts complete the connection of 120V AC to the primary of the main welder power transformer. The exact configuration of the primary, which portion of the windings the AC is applied to, depends on the power settings: This portion of the wiring is shown in the diagram in the user manual.

As soon as AC power is applied to the main power transformer, and rectified into pulsating DC by the large rectifiers in the welder (again shown on the user manual diagram), there will be 17-20V DC applied between pin 6 (+) and pin 10 (-). (The exact voltage will depend on the power settings, how much arc current is flowing, etc.) The rest of the circuitry takes that pulsating DC and drives the wire feed motor. The potentiometer is the front-panel wire-speed control, and the circuitry around transistor Q1 supplies pulses to turn on the SCR at some point in the pulse cycle depending on the potentiometer setting. Feedback of effective motor voltage, smoothed by C3, is applied across the potentiometer and a variable (with speed setting) portion of that is applied to the transistor, giving some speed regulation independent of the voltage that varied with the welding power setting.

In my unit resistor R9, which connects from the moving contact on the potentiometer to the high speed end of the same potentiometer, was 390W to begin with. That means the tap on the potentiometer was never more than 390W from the high speed end no matter where the control was set. I tried various values for R9: I expect it could be omitted entirely, but it may provide some protection for the base-emitter voltage of Q1. I wound up using a 4700W resistor. Now I have nice speed control. I never have needed to set it below 1, but it works even below 1 should I ever need extremely slow wire feed.

Bob Wilson