AC-225 Welder Amperage Control with SCRs

In an attempt to make my AC-225 decent at TIG welding, I added an amperage control circuit.  This allows me to plug in my homemade foot pedal and fine tune the arc as I’m welding.  Like most of my AC-225 modifications, most of the credit goes to Gregory Hildstrom for all the work he put into making this circuit and the other modifications he wrote about.  The other person who deserves much credit is Mike W who posted the original diagram.

AC-225 now with foot control!


I tried to understand how the control circuit I used works so the first few links go into how SCRs can control current and some basic control circuits.  The rest are similar to or the same circuit that I used.

SCRs Theory and Circuits:

Silicon Control Rectifier SCR Basic AC Circuit:


Mike W’s Photobucket album with his original diagram:

In case his Photobucket goes down, I saved the relevant photos to a gallery here for posterity’s sake:

Gregory Hildstrom’s article on making the circuit:


Discussion of the original Mike W design and modification to insulate the foot pedal from the 240V with a transformer (I did not do this, but my foot pedal is made of wood and if something shorts, I probably won’t die.  YMMV):

I didn’t see anyone saying they tried this design so I went with the original instead

Arc Welder output Current Control and DC Rectifier Upgrade:


A pretty cool similar setup mounting everything on the rear vents that uses a slightly simpler circuit:

Building the Circuit

The circuit diagram is essentially the same as Gregory Hildstrom’s.  I don’t want to blatantly rip off his diagrams, so I suggest checking his site out to see it and I’ll sketch up my own.  I’ve made a few changes and maybe my photos will help if someone else is making this.

The circuit is built on a 3″ x 3.5″ PCB with strips.  I ordered all the parts from Digikey for a total of $55.  After putting it together, I searched around and could have saved about $30 by getting the 470K potentiometer and the capacitors from eBay.  My soldering skills aren’t very good but after testing everything, I didn’t find any shorts and it all seemed to work.  I mounted the PCB and SCR on a piece of heavy aluminum angle I had leftover to act as a heatsink.

After making some more circuits, I suggest using PCB screw terminals instead of soldering the leads directly to the board.  I would also design a PCB and get it made for a few dollars with Fritzing and a company like

I’ve linked to the cart if you want to see all the parts from Digikey:

Or if you want a spreadsheet of the parts: AC225 Amperage Control DigiKey Spreadsheet

Connecting the leads to the project board
Using small leads to test the SCR
Testing the 470K ohm potentiometer that controls the welder current
The SCR and control board are both mounted to a heavy piece of aluminium angle

Installing the Circuit

The aluminum angle was screwed into the side of the case near the power cable and one of the power cables was disconnected from the switch and into the SCR on connector 1.  On the SCR the 2 and 3 get bridged and then gets plugged into the switch to complete the circuit.  For testing, I soldered the potentiometer into the board and gave it a whirl.  Shockingly it worked!

One A/C lead removed from the switch to route the current through the new circuit
Mounting the SCR and control board
Wiring the line to the amperage control and then back to the main power switch
Mounting the SCR and control board
The rectifier bank getting its input from the AC lugs and outputting the rectified current to the DC lugs
The welder with the amperage control and rectifiers installed
Complete view of the AC-225 welder with rectifiers, choke, and amperage control installed
Closeup of the choke connections
Closeup of the positive lead coming from the choke

Installing the Amphenol Connector

I decided to break from Gregory Hildstrom’s design by not having a remote switch on the front panel.  This is a little cheaper since the switch by itself is around $100 and the only downside is always needing to have a 470K ohm potentiometer of some kind plugged it to make the welder work.  I made a foot pedal and will probably also make a hand control for when fine control isn’t needed.

Front panel to mount the remote connector between the switch and A/C plugs
Rear of the front panel to mount the remote connector (behind the cluster of white and black wires to the left of the switch)
New Amphenol MS3106A 14S-1P (SR) and MS3102A 14S-1S connectors for $20
I finally got a stepped drill bit for holes in the panel. Much better than a forstner bit!  After this I marked the four holes and drilled them out to mount the connector with
New panel with the Amphenol connector installed.  I soldered the leads to the connector and then pushed it in from the back and used some machine screws and bolts to mount it
Everything installed from the back

With everything back together, the welder works fine and the amperage control works down as low as 15 amps.  My foot pedal is a little finicky but the control is good.

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