MOT spot welder tutorial

Picture of MOT spot welder tutorial

This instructable explains how to make a spot welder from a MOT (microwave oven transformer).

The primary coil (thick wire) will still be used as primary windings. The secondary coil (thin wire) will be replaced with very thick wire of less windings.

Do NOT plug in this transformer before it's ready. Especially not when the original secondary windings are still in there! This device outputs an extremely deadly high voltage. You have been warned!

Also, as in all my other instructables, read all warning notes! I'm not responsible for any accidents.

Test video:

Another video:
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Step 1: Requirements

- a MOT (microwave oven transformer)
- 3 female slide connectors (not required, you can also solder the wires)
- wall plug with ground terminal
- pliers
- cutting pliers
- drill with thick iron drill bit (10mm or so)
- iron handsaw
- wood saw or jig-saw
- 1m thick 2 gauge flexible wire (1 meter) (if you don't have this, check step 5)
- a volt-meter (or multimeter)
- a wooden shelf (see step 7)
- some other small wooden pieces (see step 6 and 9)

Step 2: Removing the secondary windings

I first tried to remove the coil with cutting pliers but I soon realised that it was totally not possible that way.
This seems to be the best way to do it:

- recover any slide connectors (picture 1)
- saw off the windings as far as you can without damaging the primary windings (picture 2)
- take a 10mm iron drill bit and drill a few holes trough the windings (picture 2)
- do not damage the iron core of the transformer
- use long nose pliers, a hammer and a screwdriver, or whatever it takes to remove the secondary windings
- remove the two magnetic shunts (picture 3 and 4)
- clean up the hole you've made

The other steps will be easier, don't worry.
RMHayes19542 years ago
What size wires are these?

If you get 2 Vrms open circuit when plugged into 120 Vrms primary then your turns ratio is 60:1; if the wall outlet will allow 15 amps (before the circuit breaker operates), then you could have 15*60 = 900 amps on the secondary. To carry 900 amps for any length of time you should have (3) sized 0000 conductors (0.46" diameter each). These look more like #12 or so.

Intermittent ratings will be higher, but you need to watch that the secondary is not getting almost as hot as the screw and breaking down its insulation. Any idea how much current is flowing when you burn the screw? You could estimate secondary current by measuring primary current and multiplying by 60.
That is incorrect for two reasons.

1) The transformer VA is based on core magnetic field and primary winding resistance (to oversimplify), not on the limit of the AC mains circuit breaker. Another way to look at it is that if your microwave was a 1000W model, this ballparks the transformer VA rating around 1000/110 = 9A.

2) Even if we used your 15A figure to get 15 x 60 = 900A, there are two problems still. The secondary area would not fit wire of sufficient gauge to have 60:1 ration and not have substantial resistance at 900A (actually it would melt the wire), AND, transformers (especially E core steel laminations at 60Hz) are nowhere close to 100% efficiency.

I do not know the efficiency of a *typical* microwave transformer but suffice to say it is unlikely you'd get half the usable amps suggested.
1) The 900A guess was based on a possible secondary current since the circuit breaker did not trip. Transformer rating (e.g. 1000 VA) is based on its intended duty cycle (microwave is practically continuous), which is not the case in this project (several seconds before the screw melts). A transformer will deliver much greater than its load rated current in a fault condition (this project puts a bolted fault across the secondary).

2) (a) I'm not sure what you're getting at. The 60:1 ratio is determined by turns, not by wire size. Turns out it was a 230 VAC primary anyway.
(b) Wire resistance does not change with current flow.
(c) we apparently had enough current to melt the drywall screw in several seconds. I don't think anyone knows what current that would be, but it seems likely that the wire would fail after a somewhat longer time.

If you like half the current, I'll take it. As I said, 900A was just a guess - a starting point. Whatever the secondary current was, it was enough to melt a steel screw in a few seconds, and would likely melt the three 14 AWG wires if the screw did not fail quickly. Such a bolted secondary fault might also fail the primary winding from overheating.
Electorials (author)  RMHayes19542 years ago
Ok, so this is what I can tell about it:

From the test in the video (melting the screw):
After this test, the End of the wires were 'warm' but that was just because the screw must have been at +- 1000°C.
The rest of the wire (the piece of secondary windings, around the iron core, far away from the hot screw) wasn't even feeling a little warm. It was almost at room temperature.

I'm not saying the windings wouldn't get hot, but I can definately melt 5 more screws before it would get hot.

About the current: *ac-dc* is also somewhat correct that the secondary current is greatly limited because of the small wire diameter. This makes the resistance of the secondary circuit "larger" (just several milli-ohms, but when working at such low voltage (2V) and high current (+-500A), it really matters a Lot!)

It would easily increase the current with 100A if I would use thicker wire.
Until you get a handle on the secondary current there's not a lot you can decide. The screw obviously had a lot higher resistance than the wire (since it burned up when it had the same current as the wire - I2R heating was high in the screw). Wire resistance (and reactance - this is AC) - is very small - ~ 1 mOhm for the loop (#14 is about 2.5 mOhm/ft) - so even 1.5 Vrms secondary could result in a very large current flow.

Anyway, thanks for the stimulating conversation.
Reactance only is in place when there is inductance or capacitance, and has nothing to do with the mode of current flow. The transformer being an inductor exihibits inductance and as such the proper term for the resulting impedance is reactance. However, wire, unless it is wound in a coil is either resistive or the inverse, conductive, but not reactive. An inductor is reactive in that it blocks the flow of AC, and a capacitor is reactive as it blocks DC...
I must have missed this comment back in 2011. Let me address it now if anyone is still listening.
Reactance certainly has to do with mode of current flow. Inductive impedance (=iwL) is zero for DC flow, but is freq dependent for AC flow. A length of wire most definitely has self-inductance - this is elementary electricity. It measures about 1 uH per meter for a single conductor in space. Wire does NOT have to be wound in a coil to act like an inductor. The AC magnetic field resulting from AC current flow in the wire causes the EMF in the wire that creates the reactance.
"1) The 900A guess was based on a possible secondary current since the circuit breaker did not trip. Transformer rating (e.g. 1000 VA) is based on its intended duty cycle (microwave is practically continuous), which is not the case in this project (several seconds before the screw melts). A transformer will deliver much greater than its load rated current in a fault condition (this project puts a bolted fault across the secondary)"

This is an incorrect assumption. No transformer can run at a 100% duty cycle, that is a physical impossibility. For a 100% duty cycle ( a term often misused), The input would have to be DC, since an AC sine wave has less than a 100% cycle due to the wave crossing zero voltage potential. Further As the voltage level changes constantly at any given point in time the current in the load changes (ohms law,,, voltage divided by resistance equals current) since the voltage level is constantly changing,,, This is why transformers which would act like a short at DC act as an inductive load at AC, the constantly changing voltage is creating a constantly changing magnetic field, the inductive reactance which in turn also limits the ability of a transformer to reach peak efficency, and as such also prohibits 100% duty cycle. Even switching power supplies with their high frequency torriod transformers don't reach 100% duty cycles.
Electorials (author)  RMHayes19542 years ago
The wires are 4x 2.5mm² wires. (European norm)
So the diameter is 1.7mm (0.066inch)

The wires don't get hot at all, just a little warm.

I don't know how many amps this thing can deliver, and I don't have a current clamp, but I could measure the primary current yes.

I'm not really sure if the turns ratio is 60:1? I can't count the primary windings.
(I could measure prim and sec voltage, and then calculate the turns ratio, but is this possible on real transformers? I thought it was only correct in theory).
it's relatively simple to calculate the current next time you burn a screw. First measure the static resistance,end to end, of the screw. Then measure the voltage across it when you start the welder. With Ohms Law you can then accurately calculate the current.

Voltage divided by resistance equals current.

I think you will find that you are not dealing with as many amps as you believe. Even Stick type Arc welders do not use more than 50 amps max. and they melt considerably heavier amounts of metal substantially faster.
Electorials (author)  Electorials2 years ago
oh, and it's 230V primary here so if I get 2V secondary, the turns ratio should be 115:1
BTW - nice job documenting your work. I should have mentioned that earlier.
Electorials (author)  RMHayes19542 years ago
Thank you very much :D
Turns ratio can be determined from unloaded voltage ratio. This falls apart when the XFMR is loaded and the transformer impedance comes into play. The ratio may be rough since you have only 2 turns in the secondary, but I would bet that 60:1 is not too far off.

The wire size you mentioned is about 14 AWG (32 amps)( I'd guess they survive because of the small duty cycle. Even commercial arc welders have limited duty cycle because of transformer heating.

It would be interesting to see the primary current and guess at the secondary. The microwave was probably only rated about 900 watts, so the XFMR would be similarly rated for more that a few seconds of use.
Electorials (author)  RMHayes19542 years ago
Ok thanks
Mueed6 months ago
Umm before building this spot welder I wanted to ask can this thingy spot weld a 1mm Stainless Steel Sheet??? Plz Reply Thanks!
Electorials (author)  Mueed6 months ago
Yes, it will be able to weld that ;)
mattbeowulf8 months ago
Sorry I know you made this 'ible long ago (nice job and thanks, BTW!) but hoping you might notate pics 3 & 4, or perhaps just clarify.... what exactly are the magnetic shunts to be removed? My MOT has the secondary winding removed, and it now looks just like pics 3 & 4 -- there was also a thicker (about 14ga) wire turned only a few times at the bottom of the secondary, which I have not seen mentioned here. Before removal, it output 3V... I assumed it to be a tertiary winding used for powering the electronics, after rectification, but was THAT, in fact, the shunt?
Electorials (author)  mattbeowulf8 months ago
No, that is just a feedback wire. The shunts are the two laminated metal plates just beneath that 3V wire.
In step 2, the last two pictures, you can see the difference With and Without shunts :)
jasshopper10 months ago
what is the gauge of the primary winding?
rschechter1 year ago
For those harassing the author about safety, the spot welder is a common piece of shop equipment. If you don't understand how to use electricity don't build it. You can hold the output from this in your hand or even one in each hand. There is not enough voltage to overcome the resistance in your body. If you are electrically illiterate, don't blame the author.
Electorials (author)  rschechter1 year ago
Thank you rschechter :)
EVERYONE this will not kill you the voltage is not enogh to travel though your body!
codongolev2 years ago
also, if you left a wire poking out from either end and it pierced your skin, that high of amperage would kill you if it hit blood.
Electorials (author)  codongolev2 years ago
I think not. It's only 2V.

I think that you may even put 2v directly on to your hart without any problems.
It's just a little more than an AA battery.
Hy, in this case the voltage is not dangerous,high amperage is very,very danger.
Codongolev told you this,important thing is that you NEVER touch wires without insolation.
Electorials,nice job.Thank you
Electorials (author)  teramagica2 years ago
This case is totally safe since it works on 2V
Now don't come tell me 'high current kills you' because with 2V it's impossible to get high current trough your body!!!

This thing *can* produce high current, but because the resistance of our body is too high for that, you get extremely minimal current when touching the leads.
Please ask someone who understands Ohm's Law clearly.
Trust me, I am in Electrical Engineering, but I find so many colleagues that have no idea of basic principles.
As codongolev said in the first post, THIS WILL KILL YOU.

Here is how:
Skin resistance on the order of 2-3 megaohms (this is 2 to 3 million ohms).
2 volts divided by this high resistance, you won't even feel it (yes, it is just slightly higher than an AA cell).

However, the internal resistance, that is through blood, due to iron on our blood, is in the order of milliohms (we're talking something small times 0.001 ohms)
Now, 2 volts divided by that is a lot of current.

The ability of the supply high current at a specific voltage is what determines its power.

But trust me, a single alkaline AA cell can put out (when new) about 7 amps of current.

This does not mean you'll be in any way hurt if you touch it. However, if you have a wound, don't you dare come close to that...

So the moral is, if you do not need to touch the leads - don't.
If absolutely needed, make sure your hands are dry and have no major scratches... Maybe wear thick rubber gloves (electrician's gloves or something)

Hope I made things clear and maybe (just maybe) saved someone's life :)
You can thank me later
Electorials (author)  nbeched2 years ago
And you say that you understand stuff about electrical engineering??

If what you say is true, we would all better use tubes filled with blood as conductors for all our electrical applications because this conducts a lot better than copper wiring!

So you say 0.001ohm as resistance for our blood. I'll take you mean something like 1m distance for that.

The resistance of copper wires, 2.5mm² (used in home installation wiring), is 7.4mOhm/m (0.0074ohm/m)

The actual resistance of our internal body is 200-1000ohms. That doesn't really look like 0.001ohm right?

"Please ask someone who understands Ohm's Law clearly."
That was funny.

"a single alkaline AA cell can put out (when new) about 7 amps of current."
That would be a fast 15minute chargable NiMH battery.

Now, back to the point:
MOT welder voltage: 2V
Internal body resistance: 200ohm
Ohms law: Current = 10mA
-> You'll feel that, but you'll survive.
Clearly you do not understand Ohm's Law...

In order to cause 10mA of current externally, you need 10mA*2MOhm=20kV

Yes, I have exaggerated some numbers to make the while thing look impressive, but I know of a guy who died because he wanted to measure his internal resistance.
He poked his fingers (thumbs, I think) with Ohmmeter probes - died almost instantly.

And if you don't believe me, take a brand new AA cell, and conect a 0.1Ohm power resistor to it (looks like a plaster brick) and measure voltage across.
If you measure anything above 0.5V, that's your *at least* 5A of current.
(An Ampere-meter applied directly to the battery leads might not give you the right answer since the Am-meter usually has higher internal resistance)

60mA (0.06A that is) of AC @ 60Hz can cause fibrillation. 20mA can get painful.
Someone said that voltage causes pain, but current kills, VOLTAGE AND CURRENT ARE RELATED!!!!!

When a teaser sends 50kV to someone's body, first, it is pulsed 50kV, so the average voltage still has to be under whatever level to keep average current within safe level. The teaser continuously measures the output current and makes sure of this. That's why there are relatively few deaths from teasers (usually people with weak hearts).

Works like this: say you pulse something at 1% duty. this means that 1% of the time the 50kV pulse is on and 99% it's off.
If you need higher voltage, you just pulse a higher duty cycle, say 1.5% or 2%, etc. The period stays the same, but the time on and off changes.
Please read more about PWM (Pulse Width Modulation) and where it's used.

We certainly do die from current, but the two are related and nothing changes that.
If you want to go in-depth, during transient, Ohm's law still applies, but on an infinitesimal scale. Eventually it all stabilises and Ohm's law still works.

Understand this or not, please do not touch the leads unless you have to, in which case, practice extreme care.

Never meant to insult anyone. Sorry if I did.
"please do not touch the leads unless you have to, in which case, practice extreme care."

how about we stick to this and be done with it?
Right here: "Now don't come tell me 'high current kills you' because with 2V it's impossible to get high current trough your body!!!"

Poke your fingers and you if not die, will at least suffer severe internal burns.
Just as much as with one or two AA batteries.
This current INTERNALLY, will not just kill, but most likely will fry a human.

You may try it yourself, but I will not accept any responsibility for your actions (please don't). :)

And for real, I am sorry for insulting you. I did get carried away and said a few things I now regret.
What I wrote was meant to be educational for general public, so let's just agree with codongolev and keep it at not touching the leads unless absolutely required...

Electorials (author)  nbeched2 years ago
Ok, what you say now is correct :)
so well yes, you will indeed get shocked if you poke the connections in your fingers.

And it's no problem :) (I mean the discussion)
Beautiful tutorial. At least, i got something out of it as a person who hardly solders and understands. Today, i was trying to solder my Sub C type of batteries and "failed" some how and read many comments - watched videos online. Many people were talking about "not soldering" them because of heat damage etc.(Count me with my Lvl 0 electrical skills).. after a long and heavy work, I soldered them (yes i did) and now charging them. Wishing to a spot welder for BOTH thing-metal binding\welding and point welding(for batteries) i will be looking forward to find an old microwave...

Few Questions;
1- What type of microwave i need? Do all of them work?
2- Can i solder Sub C type of batteries?
3- Is there more "rookie" or "starter" type of information for people like me? (I used to have billion questions like why did you make 2 turns, why this, what that , what is that red cable , where em i etc... (= )

and finally returning back to basics, it is 4 am now and i must still master my soldering skills (yes..)

If you want to spotweld tabs on battery cells DON'T build a M.O.T. spot welder.
Trust me, I have already built one for the same purpose and its not usable.
The current is too high and the voltage too low, plus its uncontrollable.
If you are serious about making a spot welder for batteries look up
"capacitor discharge spot welder" or "CD welder".

I ended up soldering the cells.
You will need a big soldering iron (80W or more) and some skills,
but its not impossible at all.
Electorials (author)  halfmumi2 years ago
all types of microwaves will work.
I wouldn't solder to any type of batteries if I was you, but if you can do it quickly, it shouldn't be a problem.

There is no 'rookie' information here, but you can ask anything you like here in the comments.

I made two turns because the less turns you have, the more current you'll get. Using just One turn didn't work well any more because then, the output voltage got too low.

What red cable are you talking about?

The red cables on the first pictures, they were there from the beginning. I removed them.
i work under high voltage every day(electric power plant-350MW power)but this is not important,important thing here is that in any circumstances dont touch the wires.only naked thing what me touching are women. ;-))
Electorials (author)  teramagica2 years ago
I agree
Two thumbs up :P

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