Introduction: AC Arc Welder FAIL

Picture of AC Arc Welder FAIL

I had the idea to make an arc welder from Tim Anderson's instructable "Build a Microwave Transformer Homemade Stick/Arc Welder". On his instructable he shows how to make a welder out of two microwave transformers.

I began working on it. After many hour of work and $80 spent it was finally finished. However, when I tried welding with it, it would not even arc. After half an hour of failed attempts I accidentally got my welding rod stuck to the metal I was welding. Before I even had a chance to remove it, one of the transformers began melting down.

What did I do wrong? Should I have used larger than 10 gauge wire for the new secondary coil? Should I have wound it more times? Fewer times? Do I have my transformers wired wrong?

If you can answer any of my questions please comment below. It would be greatly appreciated.


darthskids (author)2015-01-20

The most important things for the build, in no particular order:

1) Make sure you wind the wire exactly the same way in every transformer you use. If you wind the wire clockwise, starting from the bottom going up, then they better all be wound exactly that way.

2) Ground! All transformers should be mounted to the same metal base, and that metal base connected to the green (ground) wire on the electric cord.

3) Only use thin 3/32" or 1/16" rod. Standard US outlets can't supply enough current to weld with thicker rod. The amp requirements are listed on the back of the boxes that the rod comes in. Expect 30-40 amp at best.

4) Use a single electrical cord wired to both transformers in parallel.

5) Don't solder connections. At mains current, plain old lead solder will melt and create all kinds of hazards. Use physical crimps and connectors.

DIY Dave (author)darthskids2015-01-20

thanks for the help. I probably won't be finishing the project anytime soon, but I am curious to know what I did wrong. (I always want to learn, even from my mistakes) It really does make as much sense for me to buy a cheap one or borrow one for as little as I need to weld.

darthskids (author)DIY Dave2015-01-24

Well, what cooked the transformer was most likely that they were out of phase. All that means is that the voltage on one transformer wasn't changing in sync with the other transformer.

Basically, it's like two waves. If they are in phase, they merge together going in the same direction and add their power to each other. But the more out of phase they are, it's more like two waves crashing into each other. Most of the energy in a collision like that goes up as waste heat. So when the welding rod stuck, you put a heavy load on them and poof. Two big waves crash into each other and one ends up eating all the energy from the both of them.

Why that happened? There's a lot of things that can cause it. If the wire was wound differently on each transformer. Or the connections from the wall outlet aren't also matching. All it would take is reversing the hot and common (Black and white) wires going to one of the transformers and that would put it out of phase by 180 degrees. And with two electric cords, there's no guarantee that the 120 on one cord is going to be in phase with the other cord. If each one is plugged into an outlet on a different breaker then there's a good chance they will be out of phase. Just putting two transformers too close to each other can cause problems because their magnetic fields will interact too. So I guess it may be tough to say exactly what was wrong but probably swapped hot and common wires.

(Picture) The white and red wires are connected on the same terminal on each transformer. Can't see it in the picture so well, but all the transformers are wound the same way. And the whole thing is grounded via green wire to the metal chassis.

CarlM11 (author)darthskids2016-04-19

I don't think perfect phasing error with identical transformers would do it. If out of phase, the voltage would be zero. Two identical transformers connected out of phase and sorted would deliver no current to the load, and (other than losses) draw no current from the wall.
If the transformers were not identical enough, you could have some voltage, but too little voltage to strike and maintain the arc. Shorted electrodes, even with low voltage, would cause high currents to flow, which could overheat transformers and blow breakers.
When working properly, most of the output voltage from an arc weller is expressed across the arc, creating the heat to build the melt pool. A key skill of stock welding is to keep the arc length optimized for the energy dissipation needed, while not destroying anything.

DIY Dave (author)DIY Dave2015-01-20

Would you recommend using 240 instead? Or a 120 circuit with higher amperage? You'll have to post some pictures and wiring diagrams when you're done

darthskids (author)DIY Dave2015-01-25

For yours, if you wanted to take another crack at it, stick with 110, tear the burnt wiring off the transformer. Use the same 10 gauge wire and wind it again making sure it's wound the same way.

When you connect the windings together, make sure to connect them together in series... i.e. the windings have a beginning and ending, connect the ending bit of wire from one transformer to the beginning bit of wire on the other transformer. So you'd have the first bit of wire (That you started the winding) connected to your clamp, and then the last bit of wire on the 2nd transformer connected to the rod holder.

After that, only use one power cord and make sure the wires connect the same way on both the transformers. If they're facing the same direction, the white plugs should both be on one side, and the black plugs on the other.

If you get 40vac or so once it's plugged in, you should be ready to go for 1/16" or so welding rods.

darthskids (author)DIY Dave2015-01-24

In retrospect, I'd just get a cheapo harbor freight special welder rather than mess around with using these transformers.

On my work in progress, it trips the breakers constantly on a regular 15 amp breaker. Hooked it up to 30 amp. Doesn't trip breakers anymore, but it is now more of a plasma cutter than a welder. The only way it works at all on 15 amp, is if you use only two transformers, they both need to be putting out as close to 20v as possible each (so 40vac when in series), and then it will only work with 1/16" or so rod.

On mine, I picked up a big IGBT and a couple heat sinked bridge rectifiers and plan on fixing it up nicely with controllable amperage , steady dc rather than AC, etc. But by the time I get it sorted out, it wouldn't have been any cheaper or easier than buying a machine. And I came about two inches away from killing myself with the thing... sooo, I don't think the time, and risk to life and limb is worth saving some money and getting an unregulated, dangerous welder.

shintashi (author)2016-04-18

there's a DIY Jacob's ladder video on Youtube that shows how microwave transformers can overheat when abused.

it is essentially the same principle. Your rods have to also be thin enough to melt. Thicker rods in my limited experience required more power, and the rods you used might have pulled too hard on the transformers. I'm no expert. Hope this helps.

r01walker (author)2015-12-25

further their is a duty rating on welders. the box has no ventilation. was the box real hot? i had a harbor freight welder with a 9% duty cycle. a couple of 9 inch 110v fans i put on the transformer to increase the duty cycle.

r01walker (author)2015-12-25

um no facts. on small low power welders it sometimes takes a minute to get them to arc. lots of sticking. by the rod sticking you had power. a fuse would have saved the transformers. without a volt amp measurements the rest is conjecture. if you game rebuild and be somewhat conservative in your tests. if the turns are backwards, or the two transformers are wired backwards. could mean a world of difference in functionality.

RehabIsForQuitters (author)2015-04-12

I pondered building one of these myself. It, ultimately, came down to if I want to build one or not. You mention an $80 cost and probably lots of time and lots of time for something that failed and will need more work. If you want to build one for the sake of building one then that's great. These technologies are getting cheaper every day. I found a perfectly good stick welder on Amazon for a great price of $119. Not that far off from what you spent and how much your time costs. I am way happier with the manufactured one considering that it'd be way better than what I could put together in my garage. Only a 10% duty cycle but I' also not welding armored warships either so it's more than adequate for the job. Consider the following link before spending more money and time on it:

I'd thought about making one too, but in the end I picked up an old Lincoln 220 tombstone stick welder for $100. They're built like tanks and are great for general and even some heavy work.

darthskids (author)2015-01-20

Still need any help with it?

I'm in the process of building one and it has been a good learning experience.

On the other hand, this build is deceptively difficult and dangerous. If all you want is a welding machine then you are much better off saving your money for a proper one.

For example, a standard US receptacle can only supply 15 to 20 amps of current at 110v. That works out to 1755 to 2300 watts of power. Unfortunately, two typical microwave transformers will draw 2000 to 3000 watts of power and trip the circuit breaker.

There is also an electrocution hazard from the transformers because the bases are hot. For safety, it is very important to have the transformers and case grounded via the green wire on the electric cord.

And using individual electrical cords for each transformer is an accident waiting to happen. As soon as one cord is plugged in, it will induce a high voltage current in the prongs of the other cord.

faacuunndoo (author)2013-02-03

you dont need two cables going to the power supply. I'm going to make a spot welder with the microwave oven transformer that I found. I'm using 10 mm wire, you are using a very thin wire. Research some more it looks great.

DIY Dave (author)faacuunndoo2013-02-03

Thanks for the comment. I'm going to do as much research as I can before I rebuild it. I'll probably use a lot heavier wire too.

dmfe (author)2013-01-27

i have a bit of a problem. i made an A.C arc welder but the when im welding the fire doesn't come out smooth... my primary wire is 2mm copper wire and my secondary wire is 4mm... pliz i really need your help.

how about i double the 2mm to make 4mm and then in the secondary i double the aluminium flat bars i have of 7mm to make 14mm?

DIY Dave (author)dmfe2013-02-03

I don't really know but here's an idea. You could add am arc stabilizer-see step 6 in this instructable

Energymadandy (author)2013-01-05

Dave, your bio says it all....

DIY Dave (author)Energymadandy2013-01-17

Thanks for the comment.

My grandpa was a jack of all trades and helped all his relatives with their DIY projects. He wasn't afraid to work on anything. That's one of his many expression he would tell everyone. "Can't" can't do anything until "try" comes along and does it

ipod135 (author)2012-12-02

did you wire it in series or in parallel? also how many turns of ten gage wire did you use?

PheonixZip (author)2012-10-03

I absolutely love the tool box idea, when mine is complete I intend on following suit.

Now I have my transformers prepped and I'm not trying to spend money here, and I've used up my 10ga wire on new lighting and outlets in my garage. So I have 3 sets of 8ga jumper cables (stranded) or 12/14ga (solid) idr but about a good 50' left. Reccomendations on what I should do and how many wraps? Thank you.

Eternal_Tristan (author)2011-09-22

I'm a little alarmed to hear that there are people out there building these things who don't have a multimeter. You MUST have a multimeter to successfully complete this project. You need to be able to check the voltage of your transformers. Make sure the meter is set to AC Volts (NOT AMPS) and check the voltage. If it is too low you need to wind more transformers and stack them up. I have found that microwave transformers come in many different sizes. Some of them are a lot smaller than the ones used in the instructable. On second thought, if you don't have a multimeter this might not be a safe project for you. You should have a good idea of how electricity work before playing with this kind of fire power.
Practice the one hand always in a pocket rule. This is probably the most dangerous configuration of scrap electrical components possible. Play it safe and get some help like T Bomber.

DIY Dave (author)Eternal_Tristan2011-09-22

Thanks for the comment. I do have a multimeter, but I didn't know if the voltage/current output on the welder would damage the multimeter (Once I ruined a multimeter by testing an electric fence charger). The transformers I used were fairly large, and came from high wattage microwaves. Also, I am experienced at wiring, I just don't know that much about transformer or electronics.

snotty (author)DIY Dave2012-04-17

The voltage is fine for pretty much any mulitimeter. The amperage is a different story though. I only ever tested the voltage of the two transformers separately and together. The amperage would have been too high and blown my meter, actually an AA battery would also blow my meter's fuse. So you don't need a fancy meter to test voltage.

Eternal_Tristan (author)DIY Dave2011-09-22

Basically almost any multimeter can handle standard wall voltage (120VAC), and you should be expecting less than 120VAC because you are wiring the transformer as a step down transformer. You should be expecting no more than 20v to 50v out of one of your transformers. My guess is that your voltage is even lower since you can't get an arc. Any time you have more windings on the input and fewer windings on the output of a transformer you have a "step down transformer." A step down transformer basically takes the electricity from the input and steps the voltage down while proportionately increasing the available current. An arc welder is essential a high current, low voltage power supply.

dph84 (author)Eternal_Tristan2011-12-14

I know, I started reading this out of passing interest and I've just become more and more alarmed by the different ways people are misinterpreting what's going on.

To everyone--> because I'm forcing myself to stop reading more comments on this intructable, out of the need for less worry in my life-- PLEASE understand that this instructable is intrinsically very SIMPLE, and yet there are a few key aspects of it that are dangerous to anyone who would be working on the circuit, be they professional or amateur. Get someone's help who at least kinda sorta knows what they are doing. Discharge that large capacitor in the microwave and test it with a multimeter to ensure that you got rid of all the voltage. Sometimes there is quite a bit remaining after the first "pop!" Be CAREFUL and fully understand the phase (think of it kinda like polarity) of the transformers, and if you don't have a clue what that means then just skip this build. You might spend a lot less time finding someone who will gladly show you how to weld, than you would on this project BTW.

rjnerd (author)2011-09-08

Are you sure you have the transformers connected with the correct phasing? If you got the phase wrong, they will fight each other. What is the open circuit voltage for each transformer, and for the combined?

Second, do you have any experience with a stick welder? Difficulty getting a rod started, and rods sticking are the norm when first starting out. Look around for a class or another weldor, to get some time on known working welders. New welder and weldor (welder is the machine, weldor the person) together mean you can't tell where the problem is.

What kind of rod are you using? How has it been stored? Did you try cleaning off the end of the rod, to make sure it would make contact? (you might try a "bounce" start" to see if you can get an arc that way. hold the stinger a little loosely, and tap straight up and down, the idea is to get it to "recoil" so it won't stick - its one way to deal with crud on the tip of the rod, or when a rod burns "into" the flux coating - look at the end, and you will see what amounts to a tube of flux, that will need to get broken off before the rod can make contact. You need to learn regular "match" starting, as its hard to control the bounce start, it often bounces out of arcing distance, but it is a good way to "warm up" a stubborn starting rod)

jakdedert (author)rjnerd2012-03-03

"Are you sure you have the transformers connected with the correct phasing? If you got the phase wrong, they will fight each other. What is the open circuit voltage for each transformer, and for the combined?"

BINGO!!!! Give the man a prize!

This was my first thought on reading the OP. Not nearly enough space has been devoted to this aspect. It's a shame, because the topic is dear to my heart. I'd build the welder in a heartbeat. I learned how to weld when I was ten years old; but my neighbors would absolutely freak out if I started arc welding on my back porch (the only suitable spot for doing it).

That said, if one isn't sufficiently knowledgeable to understand the basic electricity involved, one shouldn't be fooling with high-current AC. Sorry, kids. DO NOT try this at home without reading a book--even an article--on AC fundamentals. It looks like fun, but you can kill yourself, or burn your house down.

DIY Dave (author)rjnerd2011-09-08

No, I'm not sure if they are in the correct phasing. I don't remember right off hand what the voltage is, but I think I have it written down somewhere, I'll see if I can find it.

No, I don't have experience using a welder, but I had my dad try it (he's an experienced weldor) and he couldn't get it to work either.

I'm not sure what kind of rod it is. It has been stored for a while in a sealed container made for welding rods.

snotty (author)2011-12-11

Hmm. My first try melted down too.

I used stranded wire that was too thin and got overloaded.

My second failure involved wrapping too tightly and cutting the insulation on a sharp corner of the transformer. This caused a short circuit. I fixed it by filing down the sharp edges around the transformer's hole and re-winding it.

I've made two of these welders. They both work but can only use thin rods (around 1/16") and they still trip the circuit breaker after a few seconds. Also they get pretty warm.

I used three smaller transformers in my first welder and two larger ones in my second welder.

The primaries (connected to the wall plug) are in parallel and the secondaries (the ones we rewind with thick wire) are in series. You should have 30 volts minimum.

DIY Dave (author)snotty2011-12-11

Thanks for the help. Do you remember what size wire you used for the new secondary winding on the transformers? Also you said on the first failure you used standed wire, so did you end up using solid conductor wire to make the working welders?

snotty (author)DIY Dave2011-12-15

For my working welders I used the wire recommended in the original instructable. I think it was 10ga solid wire.

tansit234 (author)2011-11-22

One tip, have the transformer cores 90 degrees to each other so the mag field doesn't bleed over between the two.
I waited and watched craigslist (for a long time) until a Miller Dialarc 250 AC/DC came up for $ the $350 co-pay when my Dad threw his back out helping me. Don't lean over when you're man handling 180 pounds (1/2).

DIY Dave (author)tansit2342011-11-23

Thanks for the tips. I'm always usually careful not to use my back when I'm lifting heavy loads

lenny25 (author)2011-09-11

I've been working on the same project and off for a long time. I also tried with two transformers and melted my primary coils together. No 1. You need the heaviest gauge wire you can get your paws on. No 2. Get an ameter that can measure amperage up to 200amps, that way you can fiddle with different configurations and see what's happing to the volts and amps in the secondary circuit. You need a minimum about 30volts if you want a arc, and a fair amount of amps if you want to melt anything. No. 3 Look at other DIY welders on the web, there are stax of them. Most of them use between 4 to 12 transformers. I actually don't know how Tim Anderson's welder works without overheating. My welder has six transformers which means the load is much more distributed, and therefore it does not heat up/melt down. The problem I have is that my welder, welds at like 150amps. Which is great for heavy duty welding but it just burns holes in anything thinner than 3mm. I have tried all sorts of different configurations of the transformers' primary and secondary circuits, parallel and series and vs versa. And have not really managed to be able to bring the current down and still maintain a reasonable amount of volts for it to arc properly.

Our electricity in South Africa is 220v, and we also have a different frequency as well, which throws a big spanner in the works for me trying to follow the american guys' calculations. At the current moment I have lost interest in the project, when i am more motivated again I will try to "cool it down". For now I have made quite a few useful things with it, like a motorbike stand, and various tools.

Try thicker gauge wire, and then try another MOT or two and see what happens. Don't spend money on stuff!...That defeats the whole point of DIY... I put adverts up on gumtree asking people for their broken microwaves, and in a week, I had six microwaves! The copper i chopped out of the secondary windings, earned me enough money at the scrap yard, that I could buy all the wire I needed for my new secondaries, plus stuff like rod holders and clamps etc. I also bought my wire from a scrapyard, which meant I paid 1/10 of what it would cost me new.

switch62 (author)lenny252011-10-03

How have you got your 6 transformers wired?  Multiple MOT designs are usually a combination of series and parallel connections of secondaries. 

Also did you leave the shunts in the MOTs.  The shunts are to limit the power through the MOT, without them the MOT can draw too much power and overheat.

To reduce the welding current you actually may need to have more turns on your secondary, that means slightly thinner wire.  This will give you higher voltage with less amps.  You probably won't need as many transformers.

Tim had 20 windings to get 20V from each transformer.  With two 1000W MOTs he got 40V at 50A for welding.  That's a fairly low powered welder.

Your voltage and frequency should not cause any trouble with calculations.

Basically with a transformer Power in = Power out  and Power = V x A
So if your secondary winding gives you 20V and you have a 1000W MOT then you get P / V = 1000 / 20 = 50 Amps.
As you can see if your secondary voltage goes up you current goes down and vice versa because the maximum power stays constant.

To calculate the maximum current on the primary 1000 / 220 = 4.5 Amps

To work out your turns ratio on the MOT just put 10 turns on the secondary using any thin insulated wire, we aren't going to draw current from it.  Connect the mains to the primary and measure the voltage on the secondary, divide the voltage by 10 and you get your turns ratio.
EG secondary measures 12V that gives you 1.2V per turn. 
Remove the wire afterwards.


DIY Dave (author)lenny252011-09-13

Thanks for all your help. As you have built your welder, how many transformers did you find to work the best. Also, do you think I should run the transformers in parallel or in series?

jking14 (author)2011-09-14

When you took the wires out of the transformers, did you yank out the steel shunts also? You needed to keep those in (mostly to limit the current so you don't burn them up on a dead short).

Also you need to find the output voltage when tied together in series vs individually. They should add together (say 12V from one and 14V from the other gives 26V, which is low, but ok to weld with). If they seem to subtract (say you get 2V) you need to swap the wires on one of the MOT's. This is also very crucial to check if you plug in the two plugs into different circuits, as the may be on the same phase or opposite phase.

If you wired the output of the MOT's in Parallel, that requires a little more attention as to how you wire your MOT's

DIY Dave (author)jking142011-09-14

Thanks for your help. I'm not sure what the steel shunts are, so I must have ripped them out. Would it be possible to use something in place of the steel shunts?

Also, you said I should measure the voltage output, but is it alright to use a multimeter?

jking14 (author)DIY Dave2011-09-14

You could replace the shunts, but it would have to exactly the same size shape and structure as the original. Way too difficult in my opinion. If you cannot find the old shunts, it would be easier to wire up new MOT's, or build a saturable reactor.

As for measuring the voltages, by all means use a multimeter (just have it set for AC voltage..)

T Bomber (author)2011-09-14

I just started putting together another one of these,this time with solid strand wire.However,the two transformers I have are smaller than the ones I used before(physically smaller).The voltage is the same.After one set of windings it is already painfully obvious to me that I'll never get anywhere near 20 windings on either one.If I got another transformer,how many windings of what size wire would give me the same amperage?I realize that this question has probably already been answered somewhere amongst all the comments on these welders,indirectly if not directly.

T Bomber (author)2011-09-13

I just put one together,meaning I read the instructable,aquired the microwaves, prepped the transformers and got everything lined up for assembly and an electrician friend of mine wired it up according to what I told him the instructable called for.After one minor reconfiguration,much to his suprise-it worked!Neither one of us had any trouble striking an arc and it welded fairly well on a relatively thin piece of angle iron,with good penetration.I used 1/16" rod.However,it did pop a crcuit breaker after 2 minutes(20 amps).I know nothing about electricity,thats why I had someone help me.I don't know why yours didn't work,but it seems there are several variables even with something this simple.Maybe you damaged the primary coil when you removed the old secondary?I had to junk one of my transformers when I nicked the primary coil by accident.And like I said-my friends a professional electrician and his first attempt got no results.I used #10 multistrand wire and I actually got all 20 windings on each.My next one I'm going with #10 solid.I'm also going to wind the secondary in the same direction as the primary to see what difference it makes.Also try to get thin rod specific for ac welding.Good luck-keep trying.

DIY Dave (author)T Bomber2011-09-14


Phil B (author)2011-09-08

I cannot speak to why your homebrew welder did not perform. But, at $80 laid out, you almost could have found a used welder for just a few dollars more. I have a Miller Thunderbolt 225 I bought a dozen years ago on eBay for $75 plus $50 shipping.

DIY Dave (author)Phil B2011-09-08

I know, I probably should have looked for a used one. I thought I could build one cheaper, but I kind of went overboard on the spending.

Phil B (author)DIY Dave2011-09-09

I am one who should have saved to buy one welder, but has bought a cheap welder, replaced it with a better welder, and then bought still another better welder. If I had saved and spent what I should have spent, I would have spent less in all. But, you may still be able to salvage your project. I have seen plans for homebrew welders that used parallel strands of smaller wires, like #10, to achieve the current carrying capacity of #6 cable. The advantage is that you are able to use more of the space in the metal core opening than you could with one heavier cable. If the primary windings on your microwave transformers are not burned up, you may be able to make your welder work. There is also another version of that welder which uses a rheostat on the primary to give some control of the output on the secondary windings. It is the one referenced by Tim Anderson as from Dan Hartman. You probably saw it.

rimar2000 (author)2011-09-08

Maybe you are using thick electrodes. Use the thinnest you can find.

DIY Dave (author)rimar20002011-09-08

Thanks for the suggestion

crankyjew (author)2011-09-08

i used 10 gauge wire in my microwave welder, and i just barely could get it to arc, even with 1/16" electrodes. even then it was only every 10 attempts or so. i would definitely recommend trying again with larger gauge wire. 6 or so i think.

DIY Dave (author)crankyjew2011-09-08

Thanks for the recommendation

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Bio: "CAN'T can't do anything until TRY comes along and does it" -Grandpa
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