Introduction: How To: High Current Microwave Oven Transformer

Picture of How To: High Current Microwave Oven Transformer

 Hey! in this instructable i will be detailing the steps involved in modifying a microwave oven transformer for high current lower voltage use which shall be achieved by removing the transformers original secondary windings and replacing it with thicker ones of your own. Now the thing to take into mind before you do this is what kind of power you want out of it.
Examples, if you use thicker wire and less of it you will get higher current but lower voltage, if you use thick wire (not as thick) and a few more turns of it you will get high current and slightly higher voltage. or if you keep the original windings you will get one to two thousand volts at one to two amps.

Now to jump right in here's what you will need!
1) A microwave oven transformer
2) An angle grinder or if you don't one and your very keen.. a hacksaw.
3) 1-5 Gauge insulated copper wire.
4) A grounded 3 pin wall plug

Onto the next step!

Step 1: Removing the Secondary Windings

Picture of Removing the Secondary Windings

 Now for the fun part! (not really it kind of sucks)

Step 1) Take you microwave oven transformer and secure it tightly into a vice or onto a solid bench with clamps. Then plug in your angle grinder or pick up your hacksaw (with proper safety gear of course)

Step 2) Cut away the secondary windings, the large amount of thinner wire. and use whatever means necessary to just remove all of the secondary windings. 

Step 3) there will be a small length of usually red wire in the middle below where the primary was this is the filament windings, remove these as well. then there is the magnetic shunts, you can also remove these, just tapping them out with a flat head screw driver will work. once all this is removed leaving only the primary left,continue to the next page!

also pictured below is what you should have, but in the pictures I have not yet removed the shunts. (the primary connections are on the opposite side.)

Step 2: Adding in New Secondary and the Final Product. (VIDEO)

Picture of Adding in New Secondary and the Final Product. (VIDEO)

Now the last part! this was easy wasn't it..?

Step 1) Now that you have removed the secondary windings, shunts and filament windings and its as tidy as possible, you can take your new secondary wire (whatever gauge your using) and wind as many turns as possible on your transformer as you can fit, the more turns the more volts. and the thicker the wire the more current you will get. to wind the new secondary, just wrap your new wire round the core of the transformer where the old secondary was.

Step 2) Now you can attach whatever you want to your new secondary's two output leads, whether it be some big heavy duty alligator clamps or connecting it up for use as a spot welder or stick welder 

Step 3) Get your 3 pin grounded wall plug and connect the ground to the base of your transformer, and the hot wire and the neutral wire to your transformers primary inputs.

Step 4) Plug it in, turn it on and you have yourself a power supply that theoretically should be able to supply up too a kilowatt of raw output power! 

And of course a video. 2-3 turns approx 3v AC and around 500 amps of steel melting power...

Heres a snapshot of one rewound MOT i put a bit more effort into. I wound it into a power supply that has 5 voltage taps which once rectified the voltage on each is 0v (GND), 12v, 24v, 30v and 50v DC! all of the voltages besides the 50v can put out 30 amps+ and the 50v limited to around 20 amps.


GeoK (author)2015-07-25

I use 1/4" heavy wall soft copper tubing for the secondary when I make high current transformers out of old microwave transformers. You can buy high temp fiberglass insulation tubing on eBay to slip over the copper tubing to insulate it from the iron core. Using copper tubing allows you to pump coolant through the secondary allowing for continuous use. The coolant also helps cool the core as well as an added bonus. When making spot welders you can run the copper tubing all the way to your electrodes as the wiring as well with T fittings up near the electrode ends with nipples to connect the coolant lines. Then just add a 120mm radiator (of the liquid cooled PC variety) with a 120mm fan and a small pump from eBay as well for a nice compact liquid cooled unit. I found without active cooling the intentionally inefficient (AKA cheap) microwave oven transformers over heat in short order when used for high current applications. They are designed to run a capacitive load through a half wave voltage doubler and they are also designed to be manufactured as cheap as possible without burning down too many homes in the Make sure to remove the shunts as well as they also add to the inductive losses. They are there to balance out the inductance of the half wave doubler circuit in their designed task of driving magnetrons.

dkbdarian (author)GeoK2017-05-24

you can wire a second MOT in series with the first, fully short the secondary on the second MOT, and you should run a lot cooler on any setup you use. I get 45 minutes using mine without risk of meltdown/smoke. Not sure EXACTLY why it works, found it while reading about transformers in material from the early 1900's. I think it is a sort of inductive resistor, which allows for the absorption of extra power on the circuit when your primary is not running, so you have a resistor that reacts to your use of power. That is at least how i think it works, the writer had a few theories of his own.

lmelnitsky (author)2011-07-29

can this power a tesla coil

JessV1 (author)lmelnitsky2015-05-11

Sorry people lied to you, yes you can use this to power a tesla coil, instead of wrapping the secondary coil with thick gauge and few turns, you use thin gauge and many turns. This will create high voltage and low amps. It'll take quite awhile to wrap since you'll probably want to wrap it a few hundred to over a thousand times.

JohnW32 (author)JessV12016-05-21

More data: You get voltage drop if you have too many winds. If your input voltage doesn't meet the ohms of reactance then you will have wasted your time winding. In short, yes you can power a tesla coil. Just reverse the primary/secondary. But there are a few pitfalls to winding your own transformer.

M2aestro (author)JessV12015-07-14

I'm not certain from your brief description that you are describing anything like a Tesla coil, which would not have the laminations, but would have LC-coupled tuned coils. You seem to be describing a rather conventional high frequency laminated transformer, and one that might be of high loss design, for the laminations might not be matched to your input frequency. I'm not an LC sort of engineer, but I don't see what you briefly described as resembling a Tesla Coil in any way.

lmelnitsky (author)lmelnitsky2016-01-04

this was years ago and i ment the MOT

alexhalford (author)lmelnitsky2011-09-20

No... Tesla coils require a high voltage input (not nearly as high voltage as the output of the TC mind, but we're still talking thousands of volts; most use about 10 - 15 kV). This voltage of the output of this transformer is probably about 2V (yes, two) the current is massive, that's the interesting bit.

So to summarise, no. Try looking into neon sign transformers. Though to be honest, with respect, a tesla coil may be a little beyond your (safe) reach, given your question.

JeannovyB (author)alexhalford2016-01-04

no that is not true, there're also Tesla Coils that work on only 12VDC!

jei01121307 (author)lmelnitsky2012-06-25

can i use that in dc motor?

rsafa (author)2013-05-28

Hello,I live in France,and I am from Africa,I would like to change the life of my mom by creating something capable to supply 220v and wich can make a microwave work.back where I come from, the electricity is extremely expensive, few(I can even count them)people have a microwave into their homes.what I need is just something to help my mon cooking without using timberwood.if anyone have an idea I will be thankfull

JessV1 (author)rsafa2015-05-11

Hey rsafa, you should get her a Fresnel lens and build her a nice frame for it. Google "solar death ray" or "frensel lens". I don't know about the availability in France, but old TV's in America had them. They refocus the sun's light, you can use it to cook food, light fires, if you have a good one you can melt metal. All FREE!!! Just make sure it's sunny ;)

M2aestro (author)JessV12015-07-14

During good weather (i.e., sunny weather) a collapsible linear/near parabolic reflector could be more practical; even a set of flat surfaces with aluminum foil reflectors could be more practical than a Fresnel lens of the circular type, but a linear Fresnel could be OK, of used with the same caution you would with a really good reflector design. Either the Fresnel lens or the reflector device could be used to preheat something along the type of a Dutch oven with heat-storage rocks inside or outside, much like using a pizza stone, but thicker, so that actual cooking could be done when the sun is at a lower angle, so that one would not need to cook at peak power of the day. Useless, perhaps in early morning.

double_g (author)rsafa2014-01-29

Hi rsafa,

What electricity is available at your mother's house (i.e. AC 110v, DC 30v etc.)? Stepping up or stepping down the voltage won't decrease the amount of power you're using so you'll still have to buy the expensive electricity back home, however it should be possible to get the right voltage levels to run a microwave if you have a source that can provide the power for it.

gbgeorge (author)2014-03-18

Your MOT 1200W? (240V 5A) .
2.4V 500A (or 3V 400A)
50V 24A

"in this instructable i will be detailing the steps involved in modifying a microwave oven transformer"

So what's the wire gauge of your secondary re 12V,24V,30V,50V?

indy1970 (author)2013-09-06

What is the stepped down voltage with 2.5 windings on the secondary

stuffdone (author)2013-05-19


charlieb000 (author)2010-10-16

does that work (heating of nails and screwdrivers) if the secondary was left at 2000v?

if not then why? the same input power is consumed....

charlieb000 (author)charlieb0002010-10-16

i guess the secondary would burn....

Jimmy Proton (author)charlieb0002010-12-19

as well as the primary, when i shorted out my mot the primary got hotter than the secondary.

HazzWold1993 (author)charlieb0002010-10-16


HazzWold1993 (author)charlieb0002010-10-16

When the output is at 2000vac the output current is around 400mA on a standard 1000watt MOT which isnt enough to heat resistive elements like a nail. even some batterys have up to 1200mA

HazzWold1993 (author)charlieb0002010-10-16

it wont work because the output on the secondary is a balance between voltage and current, the more voltage you want, the more turns you must use but the less current you get, and vice versa. and by lowering the amount of turns to 2 for example like shown above ^ you only get 2.4vac (1.2vac per turn in areas running on 240v) but you get about 500amps + (measure on a clamp meter) but the more turns you add the higher the voltage gets and the lower the current gets. hope this helps

Grooby (author)2010-03-16

Whats the Input Supply Voltage?

HazzWold1993 (author)Grooby2010-03-17

 Running off mains, 240 Volts AC 50Hz, It varies over the world

lemonie (author)2010-02-19

And you did this indoors?


HazzWold1993 (author)lemonie2010-02-19

 This was all done outdoors, thanks

lemonie (author)HazzWold19932010-02-19

Oh good, open-fronted space I guess. I must take my microwave apart...


HazzWold1993 (author)lemonie2010-02-19

 Sweet :) check my other ible on making a resonant arc setup

lemonie (author)HazzWold19932010-02-20

Yes I saw that, I'd been thinking about the plasmas, they seem to be partially-fueled by vapourised metal? I was wondering if a source of smoke in the gap might help sustain them.


HazzWold1993 (author)lemonie2010-02-20

 The more current you have the longer the arc can be stretched, as its fuelled by burning the oxygen out of the air in order to sustain a "plasma" flame arc 
there is no metal fueling, as ive used tungsten electrodes and copper, and they both hold up pretty well

lemonie (author)HazzWold19932010-02-20

You're starting with a touch on the electrodes, having some ion / particulate in the gap helps - you've seen microwave oven smoke plasmas I guess?


HazzWold1993 (author)lemonie2010-02-20

 Well they dont fully touch, the voltage is sufficient to jump about 8mm on my current setup using my two biggest mots a 3kv one and a 5kv one, good finds they were! lol

lemonie (author)HazzWold19932010-02-20

Oh yes, you've got a good setup there. Does it interfere with the TV?


HazzWold1993 (author)lemonie2010-02-20

i have had a small problem with it giving a little interference before, but it went away when i got rid of the big aluminium electrode heat sink i was using and added PFC, i dont know which one of those helped it though lol

rimar2000 (author)2010-02-19

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!

HazzWold1993 (author)rimar20002010-02-19

Well just to jump to the idea of microwaves again, if its totally impossible to find a dead one you can buy the transformers off ebay for around $20.
Question though, i may be misunderstanding here but you want to make another welder out of your welding machine???
if its not doing it for you could just take out its secondary entirely and put in a new one. the more turns of wire is better for a stick welder. but if your making a spot welder the few turns of really heavy gauge wire is best so as to obtain as much current as possible..
But to answer your actual question, you want if for spot welding yes? go for the biggest wire and just put in whatever amount of turns will fit. 

rimar2000 (author)HazzWold19932010-02-19

Thanks for the quick response.

I intend to add an additional secondary winding to my welder, without affecting the present one.
I noted that around primary winding there is enough room to do it.

But I don't knok how many turns I must add. For your response, I think I must add 5 or 6 if I can reach them.

HazzWold1993 (author)rimar20002010-02-19

 Yes, 5 would be about ideal, if using 3-4 AWG wire. what gauge wire are you planning on using?

rimar2000 (author)HazzWold19932010-02-19

I have a clearance around the primary coil of 31mm width, and a height of 11,5 mm on a side and 13 mm at the other (this is because the last layer is not complete). I think to buy a little piece of thick wire of two or three measures, and I will use the most appropriate.

If the diameter of the cable, including insulation, is 11 mm, that would allow me to barely 3 turns, a little tight. Maybe I can use a cable not so thick, say, 8 mm, but this will limit the amperage output.

HazzWold1993 (author)rimar20002010-02-19

 Let me know how it goes.
 used some 200 amp jumper cable which is like 9-10 mm and can handle the high amperage in excess of 800 amps for a short time. tried and tested lol

rimar2000 (author)HazzWold19932010-02-20

Today I went figuring out, and in my small town there are not many alternatives to choose cables. There was one of section 25 mm and another of 16 mm (5 mm diameter copper). I chose the latter, I will test it, and if it is insufficient, I will buy the other. Thanks, I will let know to you.

rimar2000 (author)rimar20002010-02-19


About This Instructable




More by HazzWold1993:How to: High current Microwave oven TransformerHow to: Resonant Microwave Oven Transformer High Voltage Supply.
Add instructable to: