January 26

Converting an inexpensive AC Welder to DC Service…

About 9 years ago I converted my standard Lincoln AC “Buzz Box” Welder into DC Service. I did this by building a massive full wave bridge rectifier. It has provided great service and welds very nice – DC welding is acutally quite a bit easier than AC welding, and in fact, this DC full wave Bridge is clean enough for Tig welding should I ever get the bug to do that.

Here is how I did it. (Excepted from my old original website hosted on Comcast – and it is still there even though I haven’t been a customer in over 9 years!)

Welding can be done with AC but for better welds and for work on thinner sheets of metal, DC is needed. Lincoln sells a inexpensive AC arc welder that can handle as much as 225 amps of current at about a dollar per amp. A DC welder of this current can cost twice as much, and a Mig welder of this current is way more. I decided to buy the cheap Buzz Box and then convert it to do what I want. First I built this DC arc conversion. Next I will build a Mig and Tig extension and then I will have a fairly complete welding system for very little money.

The basic parts – check out those massive diodes!

I acquired these four 300-amp,200-volt diodes from Ebay for $7.00 apiece. They originally sold for $90.00 each. You can get this stuff cheap and there. The old transformer will be used as the choke coil (see circuit diagram).

Close up of the diode, these suckers are Huge! The measure in at 3 inches across.

The heat sinks for the diodes and the cooling fans in the back ground.

The heat sinks are prepared.

Using copper strapping to make the connections have as low resistance as possible for the massive welding current.

The terminals and heat-sinks must be connected such that the heavy current (as much as 225 amps) can flow unrestricted, and so that cooling is possible in both air or in a oil bath, and so that the whole is structurally sound. To insure all of this I used both aluminum and copper strapping scavenged from the transformer shown earlier. Notice how the aluminum is wrapped with the copper.

Below is a diagram of the full wave bridge. Again, while the circuitry is simple, the actual physical construction is challenging due to the heat dissipation requirements and the current carrying requirements.

Schematic Layout

Completed Rectifier in the welding cart.

A view of the completed air cooled full wave bridge mounted on its nylon header. The header is made from a nylon kitchen cutting board. The box is an old main-frame computer power supply box.


The finished DC rectifier showing connection end


Rectifier showing Choke and cooling fans


This welder has given me 9 years of great service – never overheated, and I have never had any issues with the rectifiers. A great way to build an awesome system and save money. Your just not going to be able to buy something of this ruggedness without spending a ton more money.


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Posted January 26, 2014 by ccrossadmin in category Build your own, Equipment

17 thoughts on “Converting an inexpensive AC Welder to DC Service…

  1. WT Myers

    This is exactly what I would like to do.
    Is it as simple as it appears to be?
    I have an older Lincoln AC 225S.

    1. ccrossadmin Post author

      Hey WT, glad to have you!

      Yes this is fairly simple, the hardest part is securing the Diodes of that size and watt-handling capacity. These were from Russia about 10 years ago – you probably can still get these from Ebay if you look hard enough. Normally this is not how it is done in modern equipment, today they use switching power supplies and the jack the voltage and frequency up real high, then step it down with toroid transformers just like in modern computer supplies.

      The great thing is, if you can get these cheap enough, you now have a fully functional welder supply for TIG welding!

  2. Robert

    Dear David McKee

    My presence here is a result of a search for information on converting an older stick welder to dc. output.

    This site and content are impressive and inspiring.
    Thank you for finding the time and having the desire to share.

    Yours sincerely

    Melbourne, Victoria, Australia.

    1. ccrossadmin Post author

      Thanks Robert! I hope you found something interesting. I converted that welder over 10 years ago and it is still going strong. Have a great day!

    1. ccrossadmin Post author

      It was an old junk transformer I got for a couple of bucks from somebody – I barely remember as that was over 10 years ago that I built the DC converter. Basically I cut off the original windings and just rand some of my own with heavy cable as shown. It acts as a choke to keep the current spikes from overheating the main transformer in the Buzz Box.

      You could probably get away with wrapping several turns of heavy cable around some soft Iron such as old train spikes or a stack of angle iron.

      Hope that helps!

  3. Fábio Guarita

    Hi! Thanks for the useful information! I expect to do the same with my ESAB welding AC transformer.

    I just bought a fully ready 300Amps rectifying bridge from AliExpress for around 25 USD with free shipment from China. (if any of you readers want to find it look for: “MFQ300A” on AliExpress).

    That model is a controlled bridge built with thyristors, I hope to be able to control the duty cycle of the output with a microcontroller. My idea is to control the power on the welding arc the same way a dimmer control the luminosity of a lightbulb.(and maybe also stabilize the arc but that’s a future project 😀 )

    Congratulations for the awesome projects! Bye!

  4. Tom

    I have a 1970 miller 225 amp a/c machine that i built a rectifier for in the 80s using banks of 40 amp diodes. It seems to work well and never seems to overheat I’ve always intended to build a proper choke coil for it from scraps if I can. Can someone provide specs and plans or instructions for such a choke.

    1. ccrossadmin Post author

      I did not really design the choke I made. I basically had a shot transformer from an old battery charger from a mechanics setup (that big one in the picture). I just wrapped heavy cable around the core until I could not get any more in and it has done the job great. Not sure how critical stuff is for this kind of setup really. If you were building a precise tig welder, that would be another matter!

    1. ccrossadmin Post author

      Anything that can handle 200 amps or more of current. In that choke I actually used 2 150 amp stiff cable lines in parallel. For the rest of the box I used 400 amp jumper cable. Of course when they rate that stuff it usually can take twice the rating for at least some period of time.

  5. Wayne Marion

    Thank you for posting this conversion. I was wondering if there is any dangerous stored energy in the choke after the welder is turned off?
    What size wire did you use to wrap around the iron core, and is bigger conductor size better?
    For my conversion I’m considering cutting 2″ off the end of a 4″ diameter cast iron pipe to use as a core. Does this sound feasible to you?
    My welder is a 110 volt, 120 amp flux core wire feed with transformer.

    1. ccrossadmin Post author

      I used 2 strands of very large gauge wire wrapped about 20 times. I don’t remember the gauge but I got it at Lowes. You should always discharge and choke or capacitor before working on anything, however if you just touch the welding tip to ground you should be ok.


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