Picture of How to Make a Spot Welder - for Cheap!!

Step 1: You Might Be Surprised How Cheap It Can Be

Picture of You Might Be Surprised How Cheap It Can Be
Have you seen the video above?  If not, take a look now because it will help you as you go along in this instructable. 

A typical resistance Spot-Welder can range in price from about $200-$800, but with a little resourcefulness, and a bit of free time, you can make one like this for about $10 or less.

Spot welders are used to fuse thin sheets of metal together.  They are most likely used in the auto industry, as well as HVAC for welding metal ducting.  

There are a couple of videos you should see before starting on this project, because you may want some background on how the device works.

Here is how to: Make The Metal Melter

Here is what it can do:  The Metal Melter
Pondguy made it!7 days ago
This was a fun project. I was surprised at how well it worked. it has come in handy on a few projects.
JeffG23 months ago

Thanks for this project. I managed to bodge one together and to my surprise it actually works - a bit TOO well actually. So, I have a couple of questions:

1) I am using 4 AWG THHN wire and it's very stiff. is there any way to connect thinner gauge wire to the thick leads? I want to make some sort of hand piece or probes to get into tighter spots (as opposed to having these fixed in one place and maneuvering the pieces to be welded under it). Leads to my next question…

2) The heat created is too intense and actually melts the metal too quickly (I am creating armatures out of relatively thin garment hanger wire). Is there any way to attach a dimmer switch/potentiometer to this? And if so, will a regular household dimmer switch work or do I need something else?

Any answers and suggestions are greatly appreciated.

kweixelman made it!3 months ago
I wired in the light and use pallet wood total if 15$ spent.

Looks awesome

harry.godwin39 months ago

Anyone noticed we're trying to fit 6 inches of 2x2 into a 4.5 inches of space?

2x2s are 1.5"x1.5"

vshljn059446 months ago

Sir can you please tell the rating of transformer you used in this project

that is the input and output voltage rating and current required to generate the melting at the desired spot to melt

pandamopeds137 months ago
Whatisthe thickest metal this will tolerate? Again, another fantastic write up sir.
Charles511 year ago

I put one of these contraptions together using your video as a guide and those of several others on YouTube. Melts wire amazingly. As an artist I need to spot weld wire together to form armatures. The wire gets red hot and will melt but it does not weld together and comes apart when additional pieces are added. Any idea why?

Sounds like too much resistance in the wire. Have you tried using bigger wire? Also keep the wire run from the transformer as short as possible.

Possum Living7 months ago

Great instructable, Grant!

RobertP78 months ago

Hi, enjoy seeing what you have made. On the spot welder what wire are you using leading to the welding tips? 2/0 seems too stiff to have the arms removable or did I just miss something here?


Lots of projects I see I'm going to start doing in my retirement years!

fredF19 months ago

Are the transformer shunts left in or taken out?

The shunts are removed.

pnosko9 months ago

Let me ask a dumb question. You are using 1/4" hex screws to mount the terminal lugs to the arms. Considering that your metal melter videos show you melting similar screws, what is preventing these mounting screws from melting?

pnosko pnosko9 months ago

Answering my own question below after watching some educational electricity videos, I think the reason the 1/4" bolts are not affected is because the leads direct contact to the terminals make that the shortest path for the current, so it does not actually pass through the bolts.

Fission Chips10 months ago

Cool! I know this is dangerous to some extent, but exactly how dangerous is it?

felippec1 made it!1 year ago

I made it and work fine. thank you for inspiration.

Felippe Cardelino


SIRJAMES091 year ago

Sir, you never cease to amze me!

this is just too cool!!! :)

TY for sharing Sir.

stamatelos1 year ago

Thank you it is very usuful i will try to bild sun

I_StarkGuy made it!1 year ago

Finished mine! I changed the design to adapt it to available materials and I added the microwave fan to cool the MOT.

Can you weld galvanized steel? Mine is not able to do it, though my math gives me above 950 amps.


I think you can weld galvanized BUT the fumes given off are very toxic and potentially lethal. If you have to do this use a respirator, work outside or better yet, just try to avoid this if at all possible.

It is not that toxic in small quantities. And I work outside and I have been exposed to the fumes when melting screws.

The toxicity of galvanized metal when welded was one of the first lessons taught me by a pro. That said, just be careful (really, wear a respirator no matter where you are)!

JestGold1 year ago

Really like these projects and the metal melter/spot welder appear to be exactly what I need. Was wondering a couple of things:

a) I create metal sculptures from thin gauge wire rod and is there any way to regulate the amount of power so the metal fuses instead of melting (the spot welder seems to do just that)? and…

b) can the arms be separated or made into some sort of hand piece to make it more practical/accessible to the work?

Many thanks!

reddrexx1 year ago
I made one! But changed up the arms a bit. It works great.
14, 12:02 AM.jpg14, 12:02 AM.jpg
Are you sure you don't have AWG 2/0 wire ? It looks thicker than my AWG 2 wire. (2/0 is really 00 awg)
ccrow21 year ago
I have transformer from a die hard 6 and 12 volt battery charger I'm wondering how that would work, several input leads on the primary, the charger also had a 75 amp jump start setting also. Hmmm anybody wanna help me with where to start 6v 12v? I'd better do some math.
Pecheck1 year ago
late reply, but very awesome project. I'm definitely building one. I'll just ad a computer fan and some vent holes to by transformer housing.
kade1992 years ago
hey i was just wonder if for the MOT i should use the 2 AWG (since the 4 AWG melted) or if i should do the 8 gauge wire wrapped 16 times like in your arc welder video? which would be better overall for the spot welder?
I would think that if 8AWG wire wrapped 16 times worked better for the spot welder, then he probably would have said so, since they are both HIS videos ;-) But maybe he learned since making this one, and hasn't updated...
At any rate, 16 turns on the same core would result in higher voltage -- haven't watched the arc welder video yet -- guessing about 20V. Along with V increase, I (current, and resulting heat) goes down. I bet that the 2AWG (same number of turns) is the way to go here, since heating will occur in the secondary AND the work. I think the idea is to get the high current for the heat, and just make sure not to try melting anything that melts easier than the secondary's insulation! Assuming insulation was no issue, you'd simply have to keep the work easier to melt than the secondary itself, which is probably a complex relationship between relative masses and melting points.

But now I'm gonna have to go watch the arc welder video... are those capacitors I smell? :-)
GraphixS62 years ago
Do you know any place that you can reliably get dead microwaves. It seems my neighbors don't have a habit of just throwing them out.
f.masoni2 years ago
I love this project, i want to make one of my own but have a couple of questions.

Where i live the voltage of a power outlet is 220v does this change anything?

Also is it possible to make one that welds two separate spots at once with the same transformer.

Best Regards
calebwang2 years ago
I love your project. Your video editing is top notch as well.
The King of Random (author)  calebwang2 years ago
Thank you! Thank you!
Hi there. Thanks for this amazing project! I was amazed how easy it was to make. Luckily when I discovered this project, I knew were an old microwave was. I live in a small village in Guyana and most people here don't have microwaves. It was from a small restaurant. I successfully got the transformer out which was a little scary even for a 16 year old. I built a deluxe version of your design with the microwave light and fan incorporated in, along with four switches. My problem was that I didn't have any solid copper rod to use for the contacts. I had a few bronze welding rods so I used a small piece of that, instead. My welder is putting out about 1v. It melts nails and about anything else. I have not gotten it to actually weld. The metal to weld gets red hot but doesn't weld together. Any idea's whats wrong? I would appreciate any help. It's fun to melt metal, but it doesn't weld. Thanks.
Thanks for your comment! I'd love to see a little video of your welder as a video response to my video on YouTube, if you get a chance to do that?

It may help to press the contacts together harder when welding. You may also need more power. (Eg. Bigger transformer)
swolff12342 years ago
Yes, I am actually a ham. I got my license in Jan. No radio yet, but I have Echolink. I am getting my radio in March. Are you a ham? Thanks for all your help! BTW, the reply feature isn't working for me, I can only post new comments.
Sam Wolff KK4NVJ
The King of Random (author)  swolff12342 years ago
Awesome! I did get licensed but haven't ever done anything with it. I'm hoping that one day I can explore that world a little more :)
swolff12342 years ago
Found it! It was hiding from me..... Lol. Thanks again! Oh, how long does the trasformer last until the insulation on the 4 awg melts?
Best of luck,

Sam Wolff
The King of Random (author)  swolff12342 years ago
Hey, are you a Ham by chance?

If you use the transformer in short bursts of 10 seconds or less, you should be able to use it indefinitely. Allow time to cool in-between. You'll be able to tell the wires are starting to melt because they will be smoking. So stop when you see smoke (or before) and it should last forever.
swolff12342 years ago
Thank you for the quick response. I have asked questions on other instructables on my old account about 3 years ago and still have not received an answer.
Do you remember what hardware store? I went to Home Depot and could not find it.
The King of Random (author)  swolff12342 years ago
It was Home Depot in the wire cutting area. All the best!
swolff12342 years ago
I also have a suggestion. When i built mine, I could not find a nail long enough for the pivot point. So I took a coat-hanger sized steel rod, and drilled a hole straight through the box and arm. I then bent the sides of the rod after I had fed the rod through the holes. I then found the nails. I compared the two pivot systems. I was amazed at how well mine worked.
The King of Random (author)  swolff12342 years ago
Excellent suggestion. Thank you!
swolff12342 years ago
First off, THANK YOU FOR THIS! I have been looking for something like this forever! Also, I don't have the solid copper wire, but I do have the same sized aluminum wire and I have stranded copper. Can I use the stranded and just twist it? And can i use the aluminum? Lastly, how much is the solid copper wire you are using and where can I get it?
The King of Random (author)  swolff12342 years ago
Thanks for asking. I got mine at the hardware store. They sell on rolls and you can buy it by the foot. Try to get 4 AWG solid copper wire. It's about $1.00 for 1 foot. The other wires you have can work, but it's going to be less effective.
jockywheel2 years ago
great design just check the 2x2 measurment. I have almost finnished mine cant wait to wire it up thanks for the easy to understand plans keep up the good work
The King of Random (author)  jockywheel2 years ago
You're welcome! I'm glad to hear you're having fun with your own project, and best of luck with it!
airbox10112 years ago
in my country it is hard to find solid copper to be used as electrodes, is it possible to use brass instead of copper
The King of Random (author)  airbox10112 years ago
I imagine those will work, but just not as well. Brass is an alloy of copper and zinc, and zinc has a much lower melting point (419.5 C). Copper is 1085 C so you'll probably have problems with your electrodes melting quickly. You can probably use it in small bursts, or find a way to keep the electrodes cool. That should help preserve them longer. Good luck!
airbox10112 years ago
i've made the spot weld similar to what was shown in the video using an 800 watts mot. my problem is when i tested it by joining two common nails or two galvanized iron wires it didn't joined together, the point of contact just turned into a red glowing and when it cools a white powder is produce.do galvanized iron wires can be spot welded? thanks for your help
Zephyr6552 years ago
I did mean an induction heater. I have heard about the stick welder video you are making (which i know can be used to make a basic arc furnace) and was curious if something similar could be made with this.
jdtrd052 years ago
hi i made a spot welder the same way you explain is it normal the transformer make a loud noise and it dont melt it only making spark thank you
The King of Random (author)  jdtrd052 years ago
It is normal to hear a loud buzzing noise. Sparks should not be in the transformer, only on the ends of the cables.

I hope that helps!
thank you :)
Zephyr6552 years ago
Is it at all possible to make this into some sort of furnace? Thank you.
The King of Random (author)  Zephyr6552 years ago
Are you implying an induction heater? Or a furnace for heat in the winter?
kenk122 years ago
Great video! Thanks for taking time to record and share.
The King of Random (author)  kenk122 years ago
You're welcome. Thank you for taking time to appreciate and comment! :)
freazzer12 years ago
this is awesome!! i voted for you on both entries!!
I'm seriously thinking on making one of this.
The King of Random (author)  freazzer12 years ago
Thank you very much for your comment, and for your votes!!
Hubiewan2 years ago
Didn't see the wire guage of the heavy wire used for the transformer secondary, in the instructable. I'm seriously considering making one of these, with slight modification.
Thanks for your reply..............
The King of Random (author)  Hubiewan2 years ago
You're welcome :)
The King of Random (author)  Hubiewan2 years ago
It's 2AWG. Thanks for asking!
sir you are a genius!!!
thanks for the project,
I watched your videos a few times and got straight to work.
a few things to mention as help to others....
1) use the round holes in the corners of the laminate core to keep the laminates together when dismanling the core. a 6mm threaded rod and 2 bolts does the trick .
2) if you weld up the core when its wound and back together, shield the primary from the welding sparks.
3) one of your followers asked about the secondary coils size, non imp size is 16mm.
excellent project, i made mine in no time flat, i will be using it for every bit of tinkering I do in the future, (always soldered everything)
Thank you thank you thank you
I'm really glad you liked it, and thanks for your feedback. Did you weld your transformer back together? I'd like some info on how you did that?
Jip ,the laminates are welded to begin with.
when i deconstructed the core, the laminates came unstuck so i used the threaded rod through the alignment holes in the corners to keep the whole lot together. so when it came to construction i had everything square. Clamped the whole lot in the bench vice and used my ac arc welder. Its an old brute and i cant get the amps low enough so using a 2mm rod burnt the core a bit. rather went for a 3mm which lowered the amps. i merely welded along the grinding wheel lines.
the spot welder, (or metal melter as it is now coz i got too inquisitive before finishing the welder) works great.

PS, on the subject of the old oil bath ac welder.
i want to start a bit of TIG welding and wondered if this would work....can i use a diode bridge rectifier to convert my ac arc welder into a dc welder and create a tig set up?????
here's where i saw it, http://www.weldingtipsandtricks.com/homemade-tig-welder.html can you shed any light on it for me?
thanks once again.
Thanks for the link.

I'm curious to know what you mean by the oil bath ac welder? I've actually been thinking about putting a transformer in an oil bath to help with the duty cycle on my stick welder.

Are there actually ac welders that do that?

Thanks for the updates on doing your weld on the transformer core. I'll have to give that a try.

As for rectifying the current, of course you can rectify the current, as long as you find diodes rated for that amount of current and heat sinks to help dissipate all the heat generated.

Thanks again for your feedback, comments, and best of luck!
Hi again,
Jip the old oil bath welders use transformer oil to cool the windings.
I say old but they are still available (and produced in the thousands)
They are rather heavy so are mainly used at small factories or home. I just find that the welding current can't be set low enough for thin material.
DC inverters have taken over, being light and portable.

I notice on the video (welding tips and tricks) that the heat sinks are large aluminium channels that the diode bridge is built on. this should dissipate the heat accordingly, I would just use a lot more vanes when i build one.
another thing to think about (hint hint hint) is a way to reduce the welding amperage, so when using the TIG set up, i can weld very thin material.
I'm about to start building a chopper that has no transmission, or chain, belt or shaft drive...... so will be fabricating gas tanks, oil accumulators, etc. and need low amps for that. watch this space....
Just want to thank you again, magic talking to like-minded inventors.

I'm working on ideas to adjust amperage as well. Aside from buying an expensive variac.

I enjoy your comments. I don't have much experience welding personally, so I pick up ideas from your suggestions :)
Hi again,
I spoke to a few people about the bridge rectifier, heat sink and reducing the amperage. seems like the solution is a potentiometer, (or POT as its referred to) this will allow us to reduce or increase the amps on demand for each job.
I'll check it out and tell you.
keep inventing....... its a human thing!
A Pot will work for DC current. I don't believe it works for AC. For that you need an inductive ballast or choke. I'm interested to hear what you learn. Thanks for the updates!
Jip, you're right, the POT will only be installed to bring the amps down AFTER the diode bridge, so i will be using it on the dc current.
Keep on keeping on.....
one other thing...
the whole thing gets a bit hot, will air cooling with a fan help?
Is it the cables that get hot, or the transformer as well? Cooling fans will help, but I don't think they are necessary.
its the cables. I see that industrial ones have a water cooling system
er that was a 6mm rod and 2 NUTS, does the trick
cketterer2 years ago
i've just two questions :
which kind of cable section do you use for the new secondary ?
Is this the same kind of cable section for the secondary with a primary of a french transformer in 230V ac ?

I have a 25mm2 cable.

thanks a lot.
The King of Random (author)  cketterer2 years ago
Thanks for your question. I'm not sure I fully understand what you're asking, but I used a 5 foot length of 2 AWG (American Wire Gauge) wire for the secondary.
cketterer2 years ago
i've just two questions :
which kind of cable section do you use for the new secondary ?
Is this the same kind of cable section for the secondary with a primary of a french transformer in 230V ac ?

I have a 25mm2 cable.

thanks a lot.
Fake_Name2 years ago
Please stop calling the transformer a metal melter. It's a transformer. Melting metal may be something it does, but it's not what it is.

Calling it anything else makes you sound silly.
I think he's referring to a previous project that used the MOT.
"Measuring the base of my Metal Melter, I found it was about 4-1/4"."

Sure doesn't sound like he's referring to a whole apparatus, rather then just the transformer.
I have no problem with the term metal melter.
The King of Random (author)  arsha721282 years ago
Thank you :)
By the way I really enjoy all your videos. I would like to make some rings that you have created such as the candy launcher.
The King of Random (author)  arsha721282 years ago
Thank you again, and I wish you luck with your projects!
The King of Random (author)  odiekokee2 years ago
Yes you are right. Thanks for your support!
The King of Random (author)  Fake_Name2 years ago
It is the Metal melter from a previous project
No, it's the transformer that you *used* to make something that melts metal in a previous project.

I'm sorry, but you don't get to rename electronic components just because you thing it sounds cool.
The King of Random (author)  Fake_Name2 years ago
I find your username interesting for someone who is so seemingly concerned about using proper terms.

Apparently this is a real concern for you, and I'm sorry you're finding a way to be offended by my article, when none was intended.
When you are new to a concept you work with the vocabulary you have. I think Grant does a wonderful job. Remember not all of us are purists. I know much of the time I am exploring outside of my field.
When you're at the point that you're writing a whole how-to article about something, you're not "new" to a concept.

Furthermore, if you do want to explore outside your own field, you don't just *make up* terms for things that already have names, you *ask* what the things are called. That's how you learn.
oz936662 years ago
Very professional !!! The other way to go is a 3000F 2,7v super capacitor charged up by a small battery , light and portable , but only good for quick pulses, not the sustained melting power of a mains transformer....
The King of Random (author)  oz936662 years ago
Do you know where to get a good deal on one?
hard to get a good deal on these ...I buy everything on Ebay , at the moment they're ,expensive, about $50 , I bought a dozen (Maxwell, not new) a year ago $25 each
The King of Random (author)  oz936662 years ago
Thanks for your quick reply!
dsandds20032 years ago
I used to use carbon arc rods on the spotlight from school in the 70's.
The King of Random (author)  dsandds20032 years ago
Nice! I've been thinking about doing a project on carbon arc lighting as well! Any helpful hints you have are appreciated! :)
tmn8tr2 years ago
What about Carbon rods from AA/AAA batteries instead of copper rod? The only problem I've had is that they need to be slightly moistened to conduct current.
Gordyh tmn8tr2 years ago
I used to do carbon arc cutting with copper clad 1/4 inch round carbon rods. I can tell you that these were fragile, the ones from the batteries would be more fragile since they don't have the cladding on them, and are probably smaller in diameter.
The King of Random (author)  Gordyh2 years ago
Carbon arc cutting? I'm interested .. what do you cut with it, and do you have more info I can look into?
It's been 30 years since I did any of it. Carbon arc cutting is very old school tech, pretty much went by the side when modern small 90 degree angle grinders and cutoff wheels came out ;-) We used it for cutting out welds from pieces we wanted to salvage.

A hand held torch holds 2 carbon rods, about 2" apart at the torch and angled so that the rod tips just about touch. The torch also has a air nozzle in it hooked to an air compressor. The rods are +/- so the arc is between the rods and not between the rods and the work piece. You touch the work piece with both rods just to complete the circuit and get the arc started. Hold the rod tips and arc close to the work piece. When a molten puddle forms pull the trigger to give it a shot of air , this will blow the molten metal out of the way. Once started you can keep cutting, like with a oxy/ acct. torch. As the rods burn away you need to loosen the thumb screws that hold the rods in place and move the rods to close the gap back to the starting point. A good operator could remove the weld with little or no damage to the parts being separated. It's loud and very dirty also as I remember.
The King of Random (author)  Gordyh2 years ago
Wow, great description. I'd never heard of that but see how it could work! Thanks for the details Gordyh!
A memory popped into my head this mourning as I woke up ;-) I was told that lacking other cutting devices you can use a copper clad carbon rod in a stick welder to make a cut. I have not had the carbon rod on hand, but have used regular welding rods to make a cut. Simply make small forward and back motion along the line you want to cut with the rod/arc and the molten metal drops away as you go. As I mentioned before very dirty as the carbon burns away into very fine black soot in the smoke and settles on every thing.
tmn8tr Gordyh2 years ago
Good point, I didn't think of that!
tmn8tr Gordyh2 years ago
Good point, I didn't think of that!
Gordyh2 years ago
I already have a factory built spot welder like you have pictured. But I will build one like yours as it will be more portable, because it won't require a dedicated high Amp circuit to run on.

Thank You,
The King of Random (author)  Gordyh2 years ago
Good to know Gordy! I'd be interested to hear how the two compare as I haven't had any experience with the real deal :)
From your video yours is 3 to 4 times slower. But that does not matter cause your not aiming for factory production levels ;-) Yours also turns a lager area red with heat. The factory one might have a capacitor in it or just higher output voltage, I don't know which or both.
The King of Random (author)  Gordyh2 years ago
Thanks for your quick reply!
eyesee2 years ago
The King of Random (author)  eyesee2 years ago
dozer7892 years ago
Wow! Very nice project, Good luck in the contest, your work might pay off!
The King of Random (author)  dozer7892 years ago
Thanks! :D
doodlecraft2 years ago
Wow Awesome! I will have to show Mike this! :) Thanks for sharing your awesomeness! :)
The King of Random (author)  doodlecraft2 years ago
Thank you very much Natalie!!
rimar20002 years ago
All your instructables are genial.
The King of Random (author)  rimar20002 years ago
Thank you sir!
ray742 years ago
Very cool
The King of Random (author)  ray742 years ago
fretted2 years ago
BTW that was a pretty good goat do you do other animals as well ?

The King of Random (author)  fretted2 years ago
Hahaha no
Macflame2 years ago
Excellent instructions as usual. Great project.
The King of Random (author)  Macflame2 years ago
That reaction when i first saw one of your project's on YouTube , the project's keep getting better and better , can't wait for your next upload :D
lol that's awesome :)
friger2 years ago
Thank you for this awesome Instructable! I just happen to have an old micro wave and a week off, this is going to happen.
The King of Random (author)  friger2 years ago
Perfect! Thanks for your compliment!
ceanes2 years ago
awesome thanks for posting
The King of Random (author)  ceanes2 years ago
You're very welcome :)
tjk19392 years ago
First class ible, I'm on the lookout for a microwave. This is a must do project.
The King of Random (author)  tjk19392 years ago
I'm glad you liked it! Thanks!
mdeblasi12 years ago
Now THAT is an instructable!!!
The King of Random (author)  mdeblasi12 years ago
Thank you :)
=D my grandma just broke her microwave =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =)
Hanzo2 years ago
Nicely explained - Love it. Thanks.
The King of Random (author)  Hanzo2 years ago
I appreciate your feedback. Thank you!
Very nice video, Thanks for sharing!
You're very welcome!
Nutters2 years ago
Nice work and great instructions.
Apart from an alternative choice of materials used (wood), one thing that I would add to your project is a fuse for your mains in.
This will give you and your home some protection against an electrical fault and may prevent an electrical fire.
Definately an ideas man!
The King of Random (author)  Nutters2 years ago
It's a good suggestion! Thank you!
ngadhno2 years ago
Well done. I'll say this is the best guide for a spot welder with short and clear instructions and whith HD photos and video. Great job.
The King of Random (author)  ngadhno2 years ago
Thanks! I'm glad you appreciate the work that went into it :)
rikkiesix2 years ago
Amazingly cool stuff and a really great instructable
Greetings from Belgium
The King of Random (author)  rikkiesix2 years ago
Thank you Erik from Belgium!
aw19292 years ago
Hi Grant!

Interesting project and really useful. I am wondering if you wound the transformer yourself or if it is an existing unit? I'd like to know what the voltage and current are for each half (that is, I understand the primary is 120V AC house current) but it isn't clear what the secondary is doing. If it's an 800 A. secondary current, that implies that the voltage is quite low and the number of turns on the secondary small and using large gauge wire as well.

As the standard 20A., 120V AC circuit (outlet) is restricted to about 13A (about 1600W) of power for safety reasons, that would imply that in order to have 800A available at the electrodes on the welder side the secondary needs to be about 2 Volts. (The power at each side of the transformer must be the same because you can't get something for nothing, don'tchaknow).

Please understand I am not taking issue with you about any of this...I am a consulting engineer in the radio business, and I just have this penchant for understanding the nature of the beast. I also restore old airplanes and a spot welder would be really useful in that respect!

Anyway, it's a really cool project and I plan to build one!

Thanks a lot!

The King of Random (author)  aw19292 years ago
Your calculations match mine Michael! And the power assumptions match my measurements! It sounds like you've got the beast figured out! :)
paiwayne2 years ago
Wa-wu! I like it. I wish that I can make one.
The King of Random (author)  paiwayne2 years ago
bekk222 years ago
Awesome Dude !!!
The King of Random (author)  bekk222 years ago
Thank you!
robbadooz2 years ago
The King of Random (author)  robbadooz2 years ago
jalsene2 years ago
Way cool! I knew I was saving that old microwave for something.
The King of Random (author)  jalsene2 years ago
Awesome :D
danielcha2 years ago
Would this spot welder be ok to perform spot welds on rechargeable batteries?

I was trying to get a spot welder to perhaps rebuild my 18v power tool battery packs.

Thanks for such a great instructable.
The King of Random (author)  danielcha2 years ago
I haven't tried, but I don't see why it wouldn't?
TheGreatO2 years ago
This is an incredible instructable and a great video. I have a couple os questions which are probably very obvious, but how dangerous is the project to construct (provided of course that you dont make any stupid wiring errors) given that it involves high currents, and how come you are able to hold the objects that you are welding without getting shocked? Is it simply that the electrodes are so much lower resistance than your body that all the current flows through them and doesnt ground through you?
Many thanks
The King of Random (author)  TheGreatO2 years ago
Thanks for your compliment! If you're familiar and experienced with electricity and DIY projects, it should be fairly simple and safe. The electricity didn't hurt be because the voltage was too low to get to my heart.
skrieger2 years ago
Very Nice and very well done. I will have one by the weekend thank you
The King of Random (author)  skrieger2 years ago
Awesome! Did you get it done? .. I'd love to see a video response to my YouTube video.
Cool project! Dumb question: Do you need welding goggles or other equipment to use this safely?
Good question, These do not produce the high intensity light like a stick or wire feed welder, so no you do not need dark tinted welding goggles. That said DO wear safety glasses!!! Contaminates between the 3 contact points can and will shoot out when heated enough to make the weld.

The King of Random (author)  Gordyh2 years ago
Thanks for your comment!
Nazih2 years ago
Hi there, very nice video and clear instructions. I have built a similar unit, more or less with same components however I am not getting enough heat to melt 1mm metal sheets together. I used and old microwave transformer and an one coil of a thick electric welding wire. What would the cause of this low heat be?
The King of Random (author)  Nazih2 years ago
How many turns of wire did you put on your secondary coil?
mrsayao2 years ago
Will this trip a typical breaker circuit? 15-20A?
UniBot mrsayao2 years ago
it shouldn't, it has the same consumption of a microwave oven.
The King of Random (author)  UniBot2 years ago
The King of Random (author)  mrsayao2 years ago
No it shouldn't
Val-S2 years ago
Using wood parts in welder is not the best idea. Ask any fireman...
Sir I guess I disagree. Wood is much safer for the user especially a good hardwood. I come to the table with 30 + years experience and a degree in welding. This is a good amiture solution I would not want to use it to make my living everyday but for weekend projects it is a great solution.
The King of Random (author)  wolfgang642 years ago
Thank you for your comment!!
Gordyh Val-S2 years ago
Actually it has been done and work's. I have a very old (antique) Gambles Store stick welder that I use quite often. It has rough sawn 2x6's in it for the main frame.

crankyjew2 years ago
Really nice instructions. I wonder how tungsten would work for those electrodes? Pricey, but it probably last much longer...
What about the core of a "D" size flashlight battery. I witnessed an automotive electronic tech use one to solder on a battery cable end for my Kenworth. Got dead "D" size batteries laying around the house? Cost Zero $0.00 since they already gave their life.
I used the carbon rods of batteries to create a glass cutter circuit about 40years ago. The problem I had was that the carbon is very brittle and kept breaking on me. It just wont hold together for more than a couple tries.
The King of Random (author)  jwzumwalt2 years ago
That sounds interesting. How does the glass cutter work?
The King of Random (author)  jjlamberth2 years ago
Is the core a carbon rod?
Guys a quick word of caution. Tungsten electrodes are mildly radio active and when you grind them the dust is very fine and get's everywhere. For mild steel it's overkill and nobody need's the extra rads but if your working with stainless steel it's the only way to fly. Remember cleanlyness with this stuff,is the best way to a long life.
This is only true for thoriated tungsten electrodes.

You can get a whole variety of tungsten types:
Thoriated - Which is indeed mildly radioactive
Lanthanated - Non-radioactive, probably not too healthy.
Ceriated - Same as Lanthanated.
Zirconiated - Ditto
Pure Tungsten - Probably the best option here.

Realistically, you really don't want to use tungsten electrodes, as their resistance will be significantly more then similarly sized copper electrodes. If you look into production spot-welders, they all use copper electrodes, and just cool the tips to prevent damage.

Pretty cheap actually, just get some tungsten electrodes for a tig welder, about $20.
very smart. Not the idea only but the way you simplify the subject also...great job.
Thank you very much!
ironwind2 years ago
well done! thanks!
The King of Random (author)  ironwind2 years ago
I'm glad you liked it. Thanks!
Nevala2 years ago
Very cool! Please be careful when welding galvanized items (the washers looked galvanized in the video to me). Galvanized metal, when welded, produces toxic fumes.
Nevala is right Folks please be careful with galvanized material. Breathing Zinc oxide is very bad for you. Next it weakens the joint significantly(two disimalar metals) and you always need good ventalation for this kind of work.
wwaldok2 years ago
Right, Fake_name! For sure there!