Need help replacing potentiometer with Arduino control

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Joined Jul 9, 2015
I am working on modifying a 90A Harbor Freight welder to use shielding gas and have fully adjustable DC output. I decided to adjust the AC voltage on the input side of the transformer as the current would be much lower.

I decided not to go with a variac and instead use something solid state as the size is much smaller and I was hoping to keep all of my components inside of the cabinet if possible.

I purchased a 75A AC voltage regulator from eBay for about $20 to use as the transformer input voltage adjustment. Upon arrival I realized that under no load that it put out 120v all of the time so I have a 50W 300ohm resistor coming that will put a small load in it at all times.

With that problem out of the way I need to figure out how to replace the case mounted potentiometer with something I can control via a Sainsmart brand knockoff Arduino Mega R3.

At first I wondered if there was something that existed like a digital potentiometer. Sure enough there is. But the problem is that the potentiometer appears to have AC voltage and digital pots only work with DC. Someone suggested a MOSFET but I'm not clear if they meant as the potentiometer or to replace the entire regulator.

I don't know if this will work on my application or not and if it will, then I don't know how I should construct the circuit. I'm looking for any and all ideas anyone has.

If I continue using the regulator I already have the modification can't take up much more space. I'm open to buying something else instead of using the regulator I already have but I would like to keep it as cheap as possible and definitely under $50.

It also has to be smaller than one of those plastic pencil boxes and the smaller the better. I want it to go as low as 25 or 30 VAC and up to full voltage. Bonus points if it can go a little above supply voltage like some variacs do.

The only other idea I had was to take the pot that is installed in the voltage regulator, couple it to a servo motor, and control it with the Arduino.

I didn't have any small test wires handy to check the voltages with the pot plugged in but I measured it with the plug off and got AC voltage. The stock potentiometer is 250k ohms

I greatly appreciate any advice you can provide. Thanks

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Joined Oct 11, 2012
You need to post a link to the regulator.

I assume/guess the regulator is actually an SCR in perhaps a dimmer type circuit which means it's chopping the mains in a PWM type fashion so it's really controlling the average power transfer not the voltage.

75A is a really big current.