Picture of Homemade Spot Welder
Anyone can build this simple to use light duty spot welder. I found all the parts I needed in my shop. I looked at some other plans on the Internet but they all seemed to require something I didn't already have.
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Step 1: The transformer (core)

Picture of The transformer (core)
I had three MOTs (Microwave Oven Transformers) collecting dust in a box with a bunch of adapters and transformers. The one was exceptional for the fact that the primary and secondary were separated by a shunt. Placed on it's base the secondary was on top and the primary on the bottom (pictured).

I used a pipe cutting blade (hacksaw blade) on a reciprocating saw to cut the secondary off the transformer core. Near the end of the cut I had to use extreme caution as I did not want to damage the primary coil. The primary and secondary can be identified by the number of turns and the gauge of wire. The secondary has many thousands of turns and uses hair thin wire. The primary is more like 18 gauge.

Once the secondary coil was cut away I used a short metal bar and a hammer to pound the trapped portion from the core. It was wedged in there pretty good. I cleaned out everything except the shunt and the primary.

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JuanitaL5 months ago

That's so cool! What would you use a spot welder for? My husband-strange as it may sound-has recently taken up welding as a sort of hobby. I bet he'd love to make his own spot welder. How thick was the wire you used for this?


JSM_436 months ago

A monumental lack of electric knowledge displayed here!

1) The key to understanding transformers is that, excluding losses which can be made rather small, the power on the primary side equals the power on the secondary side. So if 110 V is applied on primary and 10 amps flows then the power IN is 110 X 10 = 1,100 watts. Say now you measure the voltage on the secondary side and you find 5 volts then the current on the secondary side MUST BE (1,100 divided by 5 =) 220 amps. If the gauge of primary and/or secondary coils are insufficient to carry their respective currents then you have a fire situation!

2) A rule for your welder might be that the "waste" in the wiring should be less than 5% ie 50 watts in the above case. From this and the resistivity of copper the gauges can be estimated. One can take more loss in a welder because by its nature its use at high current is short term.

3) Whenever the secondary is open circuit the current drops to zero and the power also drops to zero. The primary current is them zero (or close to).

4) Another useful measure of transformers is the term "turns per volt". Hence if the primary has say 220 turns and 110 volts is applied the turns per volt is 2. This applies equally to the secondary so if you now want say 5 volts on the secondary of that same transformer you need 10 turns on secondary coil..

5) To estimate the turns per volt just wind a few turns around primary or secondary, measure the voltage in your new temporary coil then divide your number of turns by the voltage measured. Typical for a tiny transformer could be 15 and for say a 2 KV power transformer could be say 4 or lower.

6) A whole new area is what are known as "current transformers" (essentially no different in principle, just in how they are used) which can be highly dangerous and are best avoided!

jds1969 (author)  JSM_436 months ago

Thanks for sharing your knowledge on transformers and your monumental lack of social graces.

ryanlimyx7 months ago

can we use thicker or thinner wire to tweak the amp?

jds1969 (author)  ryanlimyx7 months ago

Using thinner wire will likely just melt the wire. The magnetic shunts sitting between the two coils is used to limit the power. If you want to adjust the amperage the number of turns on the secondary needs to change or you need to control the amperage in the feed to the primary. Check power limiting circuits.

-CCCP-1 year ago


-CCCP-1 year ago


mstone103 years ago
I have a problem with mine, basically the work piece will get red hot almost melting but will not actually weld together.

Anyone experience this? or anyone know how to resolve this problem?
I think maybe the electrodes need to have an immense amount of clamping force to actually work. I could be wrong, I would appreciate some insight from anyone who knows whats happening here.

Thank You.
jds1969 (author)  mstone103 years ago
It does take a certain amount of pressure but usually just enough to make contact through the two pieces which is what it sounds like you have.

I'll take a few stabs in the dark on this one:
1) clean and sand both side of the weld prior to starting
2) ensure the weld stays unstressed and clamped until cold
3) certain metals/alloys are notoriously hard to weld
4) sharpen the welding tips to concentrate the heat at a point
5) I always count to time my weld (experiment with longer times)
6) try thinner material (I couldn't make a weld in anything over 1/16 inch thickness)
fidgety23 years ago
i have a semi-unique issue i have built a spot welder and it occasionally works very well but the majority of the time what happens is when i place a piece of metal in between the electrodes and plug in the machine the secondary wire insulation begins smoking and getting very hot unfortunately the metal in-between the electrodes (contact points) stays cold could someone please help me or tell me what i am doing wrong and how i can remedy this problem i really hope that this is just some silly beginner mistake and not something more serious that will require a considerable amount of work and effort

thank you,
fidgety 2
In my opinion it sounds like you have used thin wire for your secondary winding what gauge wire did you use?
jds1969 (author)  fidgety23 years ago
Sorry for not getting back to you until now. This sounds very serious and dangerous. I would be extremely cautious about using this primary winding as it sounds like there is a short somewhere in it. With enough heat internal to the unit like this, the core can blow-apart. If it is possible, try to replace your transformer and core. A lot of your secondary can likely be re-used. At the very least I would ensure a metal box around the troublesome primary core for your own safety. One way to test for a short is to accurately measure the ohms of the primary coil and compare it to a good one of equivalent size.
Thank you I appreciate your insight
xXxFelipexD3 years ago
it works by giving short circuit? because I think it would be necessary to place a resistor
My multimeter is saying is that my primary resistance of 3.5Ohm
I think it strange
I would like to say thank you for your instructable - yours is one of the designs I looked at before coming up with my version, and gave me some inspiration!
jds1969 (author)  throbscottle3 years ago
Thank you for the compliment.
narpas4 years ago
So is there any electrical danger from the actual arms? Or is the voltage not high enough to be a problem. Is it just a danger from the parts that are running at 110 V?
wiseboy narpas3 years ago
Current (less than 1 Amp.) may kill . Be sure to ground and NEVER touch any parts when experimenting ...
Dry air and floor are vital as electricity resistant gloves ...
Electricity is no game ...
Be sure you don't bet your life !
jds1969 (author)  narpas4 years ago
If the device is working properly and everything is electrically isolated the only issue with the arms is an unintended short-circuit on the work piece that might prevent the weld.

The DANGER comes from unforeseen / unexpected paths for the electricity. In theory the voltage at the arms should be around 1.5V but if the transformer or any of the other circuit shorts out you may be exposed to the full voltage.

Sometimes safety devices are there to limit your exposure to situations that are rare but still possible. When it comes to AC power take no chances!
Doddity4 years ago
Having a problem with mine. I have two transformers with the input in parallel and the output in series. I get a great spark between the wires while testing but when i joined it all to my welder the secondary wires get really hot and it doesn't make the weld. I used non-copper contact screws by the way.
Any help would be appreciated
jds1969 (author)  Doddity4 years ago
The key is low voltage, high current. I've never tried two transformers at once but I think you might want to run your output in parallel rather than series. If you used steel screws/bolts then this is likely causing some of your current to be burned up in heating the screws. The difference in resistance is important when welding. I suggest you try finding something made out of mostly copper to make tips. You might want to try a single transformer and then move up to two if you still think it's needed. Good luck and stay safe.
Doddity jds19694 years ago
I got the Amperage over Voltage thing sorry that was badly said. The circuitry is all fine and me and my dad came to the same conclusion about the tips.
Thanks for all the help anyway I'll try and comment a picture when its working
You might have the secondaries  (output) of your transformers connected out of phase.  You may need to swap the wires on one side of one of your transformers.

The phase of a winding depends on which direction it is wound, clockwise or anticlockwise, in relation with the other windings.  Your  transformers should be setup like the diagram below.  The dots on the terminals represent the phase polarity, like positive on a battery.  If the primary is wired the opposite way the phase flips on the secondary

If one of the secondaries is wired the other way or it's phase is flipped then you will get little or no voltage from your welder and the secondaries will get hot. The 2 secondaries are fighting each other instead of helping (adding) each other.

If you connect one of your secondaries the other way your welder should work. But as jds said more current is better than more voltage.  Only time you may need more voltage is if you have a high resistance metal or joint.

Phase is also important for parallel connection as well.  Even more so beacuse as soon as you apply power the secondaries are "fighting" and you can blow a fuse or a winding.
Mine is set up as your first diagram shows. I really have to talk to my electrotechnology teacher about finding the phase better.

The way I found it was connect it one way, see how powerful the spark was, then switch and compare them. Connected one way I only got a small spark, the other way there was a bright glow, akin to a arc from an arc welder. I assumed this is because the power was adding on as opposed to cancelling each other out. Would I be right in assuming this?
I would say bigger spark would mean that you have the secondaries wired in phase.

The easiest way to check phase is using an oscilliscope so you can see both AC waveforms at the same time.  Wire the mains to the primaries in parallel and then use a dual trace scope to connect to the secondary windings.  The windings should not be connected together.  If the waveforms are in phase, the peaks and troughs will happen at the same time.  If out of phase the peaks will line up with the troughs.  Once in phase the positive of each probe will be the dot.

But not everyone has a scope.

The other way would be to measure wire up the secondaries in series.  Then measure the voltages across each secondary, V1 and V2.  Then measure the total voltage VT.  If VT= V1 plus V2 then in phase.  If not equal, probably close to 0VAC, they are out of phase, flip the connections on one secondary.  Use an AC meter for the voltage measurement.  You also might need a resistor across the secondaries, say 100 ohm at 5 Watt for this situation, the resistor may get hot.

Note: this only checks the phase relationship between the secondaries.  It will not check the phase relationship between primary and secondary (dot convention) of each transformer.  To do that you need a different type of setup for safety.
Thanks for your help, I'll try get back to you once school starts as i may have access to an oscilloscope. Fingers crossed =).
Muy bueno gracias...
the.rollie4 years ago
those are the only two reasonings i ever needed to build something.... wanted one, and needed one for some miniscule task/problem
Where can I get some 4 Gauge wire? Will this do? http://dicksmith.co.nz/product/W1230/4-gauge-power-wire-transparent-red
jds1969 (author)  electroboy13374 years ago
That's very similar to what I used but it was 25 stands of 14 gauge rather than 1700 of 48 gauge. In the end, it's about current carrying capacity. Welding supply shops have this sort of wire if you are looking for another source.
ustech4 years ago
harlyquin4 years ago
I'm sure I'm restating general knowledge here; if you can get wire for the secondary new, it will make winding much easier. Copper work hardens and becomes ridged and brittle if any deformation occurs. If you find 10 g wire difficult to wind, it will only get harder if you try to redo it!
Udon5 years ago
Hey, great instructable.
I follow how it works, but I got a question.
I'm not sure if this has been asked and answered yet, but, what is the shunt in the MOT?
What does it do, and what affect does it have on the transformer?

jds1969 (author)  Udon4 years ago
Hey sorry. I just noticed that no one replied to you. I will try to answer but again I'm just a transformer theory novice.

The shunt is two pieces of iron that short out the flux between the primary and secondary coil. In the end it limits the amount of current that can flow through the transformer.

Removing the shunt could in fact cause a blown fuse if the total current exceeds 110V/15A. MOTs are sometimes referred to as unregulated transformers and there are lots of warnings because of it. Hope that helps.
maxpush995 years ago
It would help if you try to have the 2 windings in resonance. after you remove the secondaru windings you should weight it you use the same amount of copper by weight in your new winding,\ Just calculate lenght by thickness or weight the new windings good luck
But just think of all that fine guage wire that was wasted..... I have like 50 spools of wire from transformers, I just cannot let go of those little guys....
I use it for electromagnets, I wind my own guitar pickups, use it on pcbs, etc... Great stuff!
It would take FOREVER to unwind all that wire. It would be just easier to cut it off like he did. Maybe you could brng all that copper to a recycling center for some money.
rimar20005 years ago
This question is addressed to several authors of welding related instructables.
Some time ago I was excited with the possibility of constructing a spot welder, but here in my city there is no way to get an
used microwave transformer: nobody throws away something as that.
Then I tried with my 220-volt electric welder, but I could hardly weak solder some iron wires of 2 mm, even though I was a good time trying.
Today I decided to uncover my welding machine, and found that in the secondary winding there is no place to put
even a loop of thin wire. But in the primary, yes!
I think I can easily add several turns of thick wire, by way of a "bis" secondary winding. Now come the doubts, and related questions: whether the primary winding has 248 turns (more or less, it is what I could count), and get 220 volts, it is assumed that each round of "my" coil will produce 220 / 248 = 0,887 volts. What for me? Put only one or two turns, or try to reach five or six? A more laps, more volts but less amps. I suppose that losses play an important role in the case of small voltages, and who knows what is best.
Maybe you has an answer and save me the work of trial and error, which can become very tedious. Thanks in advance!
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