Introduction: DIY Spot Welder From Microwave - Now With OLED Display!

Picture of DIY Spot Welder From Microwave - Now With OLED Display!

This project is an outline of how to build a resistance spot welder using salvaged parts from an old microwave. Im using it to weld nickel tabs onto 18650 battery cells but depending on how you position the arms it can be used to weld sheet metal and other metals objects. Lets get welding!

Step 1: Salvage the Microwave

Picture of Salvage the Microwave

Quick note of caution!

The inside of the microwave is quite dangerous. The large capacitor maybe charged and can deliver a nasty or even fatal shock so ensure you discharge as soon as possible by touching a metal rod such as a screwdriver across the terminals to discharge it.

Okay so open up the microwave casing to reveal the electronic bits. Discharge that capacitor and get to work on removing the parts. You should find the transformer which should look very similar to the one in the photos. Remove the nuts and it should slide out pretty easily. I salvaged a few limit switches that we will use later and some of the cables are handy for power connections.

Step 2: Remove the Secondary Coil

Picture of Remove the Secondary Coil

We are going to rewind the secondary coil of the transformer as we want more amps and less volts. The primary is where the mains power is attached and the secondary has finer wire windings with the red wires attached.

The fewer turns of wire increase the amps but lowers the voltage, and more turns increase volts while reducing the amps. We don't need the secondary coil so it can be removed either by cutting off or by grinding the weld on the transformer body and sliding it out. Be careful not to damage the primary coil as we will be keeping this.

Step 3: Add the New Winding

Picture of Add the New Winding

The new winding will provide the current needed to weld stuff. By using really thick cable we can reduce the thousands of windings to a couple which will provide loads of amps. The thick cable is needed as the resistance will cause it to heat up and melt the insulation if its too thin. Not good!

The primary coil is put in first followed by the 2 shunts either side and finally the low gauge wire (blue cable) is wrapped a couple of turns. Remember to leave a decent length of cable that will attach to the welding electrodes.

Step 4: Finish the Transformer

Picture of Finish the Transformer

Our super high powered transformer is nearly complete. We just need to weld the top back on to seal it up. You could alternatively use 2 part epoxy to stick it on. Options are good, pick whichever one is easiest for you. :)

Step 5: Electrode Terminals

Picture of Electrode Terminals

We now need to attach our cable ends to the copper pins which we will use to weld. I machined some copper terminals but you could use some copper clamps from the hardware store. I've also attached the CAD file for the electrode clamp that I made. Here's the Fusion360 link too.

Step 6: Welding Arm

Picture of Welding Arm

I'm using this to weld nickel tabbing to battery cells so I've positioned the two welding electrodes side by side although you can easily mount them opposing like a traditional machine. I designed and laser cut a simple MDF case for the arm which houses the switch for operation and holds the electrode terminals.

Step 7: Package It Up

Picture of Package It Up

There is 230v of dangerous electricity coming into the transformer so its important that its covered. This laser cut case should do the job pretty well. Just make sure everything is contained as it will also look much better as well as being safe. Bonus!

As for wiring its pretty simple. Just connect the live and neutral to the transformer primary using the existing spade terminals, I would recommend adding a switch in between one of the power cables to make it easy to turn on and off. I salvaged this one from the microwave.

Thats it we're done! Have fun with your new spot welder!

Step 8: Add an OLED Display & Pulse Control

Picture of Add an OLED Display & Pulse Control

Add this circuit for precise control of the duration of the weld pulse. Very handy for welding 18650 cells and other thin metals.

The duration is varied using a potentiometer and the duration is displayed on the OLED display in milliseconds the coil of the transformer is connected inline with the SSR.

Build up the circuit on a breadboard and then transfer it to some perfboard when you've got it working. The big red button can be replaced with a simple foot switch if preferred.

I installed my board and SSR on top of the transformer, remember to add some insulation between the metal transformer body and the circuit board to prevent shorts.

You may need to add a 5v source for the Arduino as the SSR can draw a modest amount of power, I just cracked open an old 5v phone charger and connected the 5v output to VIN and GND on the Arduino.


leeturner1000 (author)2018-01-13

Hi, Love the laser cut case, cheeky i know but is there any chance of the design.? Its the best one ive seen, and ive seen a lot!

tete_sergio (author)2016-02-16

Hi! Someone tried to solder a copper wire with galvanized sheet metal by this method ?

in the image below it looks like is welded with tin.
JackDavies (author)tete_sergio2016-02-16


That isn't spot welded its soldered.

Cheers, Jack

jamesm854 (author)JackDavies2017-12-02

haha lol

tete_sergio (author)JackDavies2016-02-19


5ov1k (author)2017-04-18

Hello, may I ask what type of cable do you used as your secondary? Can't find similar in our hobby market (only CYKY cables). Thanks in advance.

douwe123 (author)5ov1k2017-11-24

i just use car jumper cabels but if you are buying you should get at least get 10mm2

HalimK (author)2017-07-10

Hello jack

Where to connect the potentiometer center pin

Jack Strapp (author)2017-04-26

Hello Jack, I am currently building this project, Im a bit stuck on step 8, what are the values of the potentiometer and resistor please.


P.S. when you download as a PDF step 8 does not show.

Jack Strapp (author)Jack Strapp2017-04-26

If anyone else knows the values of the potentiometer and resistor please feel free to reply.


summat (author)Jack Strapp2017-06-01

The potentiometer is a simple potential divider - you can be pretty relaxed with the value. I'd recommend any one value anywhere between 1k and 100k linear. If you're ordering one specifically for this then chose 10k.

Jack Strapp (author)summat2017-06-06

Thank you Summat, I appreciate the advice. I purchased a 10k one.

ChrisW545 (author)2017-06-05

This looks great - I will build it. Thanks. Two questions about SSRs though:

1. SSRs do zero-cross switching, meaning they only turn on when the mains goes through zero volts. This means your timer will not be accurate because it will not come on when you close the switch, it will come on at the next mains zero crossing after you close the switch. It crosses zero twice per cycle - i.e once every 30mS if you run 60Hz or 25mS for 50Hz. Is this level of consistency an issue?

2. SSRs, I'm told, have a problem with highly reactive loads - which this transformer is. They are likely to fail early. Please tell me how many welds yours has done. If it's hundreds, then maybe it's not much of an issue after all.

Thanks again, Chris

Arty Marty (author)2017-03-05

May I ask what value the potentiometer is? I've already made a spot welder like this but that's an awesome add on with the timer! (and believe it or not I have all the parts except the oled).

BrianD64 (author)2017-01-14

Do yo have the arduino sketch for the pulse control?

JackDavies (author)BrianD642017-01-14

Hey Brian,

Sure thing, apologies I thought I had uploaded it. Its attached to the Step 8 just above!

Cheers, Jack

BrianD64 (author)JackDavies2017-01-14

Thanks Jack! On another note. I am thinking about using your idea to make a pin style cup welder for tacking on insulation to sheet metal. so basically it would tack a stud the size of a finishing nail with a washer on the end of it to a sheet of duct insulation to hold it against the sheet metal. Any ideas how I might go about something like that?
Thanks for your input!

animan (author)2016-02-18

240 volts on the houses main? In the USA, microwaves all use 120 V AC. I assume this project will still work with a USA microwave running at 120 Volts?

ninjanody (author)animan2016-02-18

It should work just fine but you must divine the second winding length to half if you want to keep the same input/output ratio as this instructable.

RogN (author)ninjanody2016-02-29

I would leave the same number of turns on the secondary. You actually want approximately the same output voltage. There almost certainly about half the number of turns on the 120V transformer. It would therefore have about the same number of turns per volt. Great Instructable!

animan (author)ninjanody2016-02-18


JackDavies (author)animan2016-02-18

Yeah we have 230v in the UK - its actually around 240v but stated as 230v now.

Cheers, Jack

PatricioE2 (author)2016-02-18

hola aqui en sudamerica se ocupa 220v, si ocupo para el embobinado un cable de corriente 320v que es industrial, cuantas vueltas tendría que darle para que durara unos 10 min. continuos para soldar.

hello here in South America is concerned 220V, if I hold for winding a power cord 320V which is industrial, few laps would have to give it to last about 10 min. continuous welding.

FelipeS46 (author)PatricioE22016-02-22

Hola, Ten en cuenta que es un soldador de punto, no una soldadora continua.

Voiding Warranties (author)2016-02-19

Attaching wires to batteries is a good use for this welder, however you can use spot welder for so much more. How would the wood housing hold up if you were to start welding bigger things or for longer periods of time? The electrodes and the components being welded would heat up much more.

taur561 (author)2016-02-18

Just out of curiosity, when you spot weld tabs onto button batteries ,does the high current not destroy the battery or cause it to explode ? . We run a business which entails replacing batteries with tabs and we would like to spot weld the tabs onto the batteries . Do you have a jig which makes sure pos and neg do not short out in the process ? . Thanks

JackDavies (author)taur5612016-02-18

Hey, spot welding creates a lot less heat put into the cells, as it it only engaged for a fraction of a second. It should be quite hard to short the cells if your just welding tabbing, I just taped two cells together when welding to keep them together then removed it when done. I wrapped the whole pack in heat shrink sleeving when finished.

Cheers, Jack

taur561 (author)JackDavies2016-02-18

Thanks Jack . I will try it out . Cheers

Irritable_Badger (author)2016-02-18

Although it's not terribly likely, discharging a capacitor via dead short can result in catastrophic failure of cap itself, and potentially your shorting bar.

Exploding caps blow really hot and nasty stuff in every direction and the slag melts right through most synthetic fabrics (including carpet and clothes). It's really not much fun.

More likely is your shorting bar ends up welded to the terminals on the cap and/or you burn the bejesus out of your hand with the now very hot screwdriver, wrench, etc..

We have a piece of railroad track behind the shop and we push caps for discharge into the track by taping then to the fat end of a broken fishing rod (Anything metal will work the track was just here when we moved into the shop).

Like I said, the likelihood of an catastrophic cap failure and injury/property damage is low, but you can eliminate the risk all together by taking an extra two minutes to discharge the the capacitor in a safe way.

would be a lot safer, to use a power bleeder resistor with an insulated probe. with the same method, for discharging the old cathode ray tubes. which was even more dangerous, if a direct short could cause the tube to explode. and then put a shorting bar, across the cap. since high voltage capacitors, are notorious for rebuilding a charge after a single or second discharge. typically twice as much resistance, as the expected max voltage to keep the discharge no more than half an amp. keep in mind that, 3000v with a 1 ohm short can pull as much as 100 to 300 amps worse case. the capacitor, would not be safe to use, after that kind of discharge. and it, may not do you a lot of good either.

valveman (author)2016-02-18

I built a Spot-Welder also with the Microwave Transformer. I left the Transformer intact, removed the secondary and wrapped 3 windings of Welder Cable. So it is not necessary to cut the laminations of the transformer.

JackDavies (author)valveman2016-02-18

Yeah I did mention that, it was easier for me to just remove the top to simply knock the secondary out then re wrap, especially as my cable had pretty thick strands.

Cheers, Jack

valveman (author)JackDavies2016-02-18

Another thing i did was to make a timed relay to minimize the on time for batteries. I can go as low as 0.1s. Keeps the heat off the batteries. Have you tried that?

mkize410 (author)2016-02-15

and all i have to do is wait for someone to toss out a fully equipped shop! im on it!

JackDavies (author)mkize4102016-02-18


This is a pretty simple project, you don't need any specialised tools, it just makes it easier! Good luck :)


zerblatt007 (author)mkize4102016-02-15

The only thing you really need is a Vise and a metal saw. The rest can be made of/with junk you probably already have. Watch this video for more tips to do this "without a fully equipped shop" :

statebeak (author)2016-02-16

I like the rewound coil. That's what the other similar tutorials are missing. Nice clean crafting.

JackDavies (author)statebeak2016-02-18

Cool, thanks for your comment!

md_zoran (author)2016-02-16

Approximately how many volts / amps does this transformer put out in the secondary ? I would like to use something similar for electroplating, and I need about 1.5 - 3 V and 25-60 amps.

valveman (author)md_zoran2016-02-18

You will get about 2V and a ton of current. Not good for electroplating because there is no regulation and current will be extremely high given the low resistance of the electroplating solution.

ScottF43 (author)md_zoran2016-02-18

Transformers work using alternating current but electroplating requires direct current. It requires direct current because you are transferring metal based on the direction of the current. If you use alternating current, the metal will just flow back and forth to and away from your target. This complicates things a bit when dealing with high amperage because you need to rectify the high current. It could be done by partially rectifying the signal before the transformer into pulsating direct current. then the transformer would still work.

gddee (author)md_zoran2016-02-16

If it is a 600 to 1000W microwave transformer... P = I E (close enough)

1000 W = 5 amp x 200 v.

If you needed 3 v, 1000 W / 3v = 300 amp.... Holy smokes... (All rough guesses)

md_zoran (author)gddee2016-02-16

Yeah, that's what I guessed too, but... exactly ! Holly smokes and molten metal ! Sound too good to be true

Hectorg27 (author)2016-02-18

1)Hubiese considerado un secundario recubierto hasta los terminales, de espagueti de fibra de vidrio o termocontraible, 2)y el ventilador?,3)1000W es lo que consume el transformador,pero del secundario la potencia que podemos extraer es mucho menor,ya que parte de esta se transforma en calor. Muy buen proyecto,me gusto.

gddee (author)2016-02-17

Electroplating is a LONG term usage. Spot welding is for a fraction of a second. I would not subject the insulation on that wire to that heat. If the secondary was a much larger bar that was mechanically secured against shorts and large enough to continuously take 300 A, then it looks great.

JohnM90 (author)2016-02-16

A bit more information on how to set up the limit switches would be handy. They should be set up to give a brief pulse of power when the arm is brought in contact with the workpiece. The pulse should be as short as possible to prevent excessive heat from the weld damaging the batteries. Depending on circumstances, around .1 to .5 of a second.

wrsexton (author)2016-02-16

Excellent tutorial! I have finally learned why people change the windings in the transformer.

Could you provide a bit more detail about how you attached your cables and electrodes to the electrode terminals? I'm a little confused on that.

Newark CA Locksmith (author)2016-02-16


haltenfelder (author)2016-02-15

I like the rewound coil. That's what the other similar tutorials are missing. Nice clean crafting.

Chitlange Sahas (author)2016-02-14

Wow! That is a useful tool.

About This Instructable




Bio: Hey, my names Jack and I love technology and building stuff! Visit my site for more. LinkedIn -
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