DIY Spot Welder From Microwave - Now With OLED Display!





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

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

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

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

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

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

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

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

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

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

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.



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    Hello, Where does on buy the Arduino and then the wiring diagram?
    Regards, Colin


    Are you sure about this? Primary coil is always thinner wire, as secondary has to put out much more amps.. So you are removing the primary coil, and replacing it with few coils of wire that will act as a new secondary, whilst old secondary coil will be the new primary coil

    1 reply

    Never mind, this is a high voltage transformer, and it rises the voltage for the magnetron...

    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!

    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.
    3 replies


    That isn't spot welded its soldered.

    Cheers, Jack

    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.

    1 reply

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

    Hello jack

    Where to connect the potentiometer center pin

    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.

    3 replies

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


    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.

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


    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

    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).

    Do yo have the arduino sketch for the pulse control?

    2 replies

    Hey Brian,

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

    Cheers, Jack

    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!

    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?