DIY Spot Welder From Microwave - Now With OLED Display!

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About: Hey, my names Jack and I love technology and building stuff! Visit my site for more. jackdavies.co LinkedIn - http://linkd.in/1KzLd7Y

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!

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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. http://a360.co/1Loyh1j

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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    64 Discussions

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    ChrisW545

    2 years ago

    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

    2 replies
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    sdtrentChrisW545

    Reply 2 months ago

    Pinging an old thread here. AC output SSRs are available with zero-cross and non-zero-cross outputs. The one shown in the images above, the SSR-25 DA, appears to be a non-zero-cross type that has the ability to provide less than full half-wave power to the load. This is done by turning detecting the zero-crossing of the input power and turning the SSR on after a precise delay. Of course driving an inductive load, particularly with a steel core, can make the relationship between the turn-on delay and power delivered to the load. Some SSR are specifically rated to handle inductive loads (SSRs not rated for inductive loads can fail to self-commutate meaning they might turn on and never turn off because they begin self triggering).

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    garzoChrisW545

    Reply 6 months ago

    Maybe it's quite late for an answer but yes, the timer can't be regulated in smaller step than the 20mS for 50Hz and 16.7mS for 60Hz (check your numbers). Anyway i don't feel this a an issue. Can't answer question nr. 2.

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    adybulborea

    Question 4 months ago on Step 8

    Thank you very much . Very nice progect. Can you post schematic for connections?

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    wrsexton

    3 years ago

    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.

    1 reply
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    DuckTurdwrsexton

    Reply 5 months ago

    I have provided a photo of one of my electrodes, it consists of #6 bare copper ground wire as the actual electrode, a medium sized grounding clamp with a
    3/8 bolt hole, a brass battery bolt extender and a #1-2 cable ring loop connector. I'm using two pieces of 3/4 EMT conduit 18 inch long for my electrode arms. The #2 wire will travel up the center of each arm connecting to the ring loop connector either via crimp or soldered on, most likely crimped. Then I'll use a piece of silicone impregnated shrink tube over the wire & ring loop connector and brass battery bolt extender, slide it all into the pipe and J B weld into place. That's how I'm making my electrode set up. They're all different, it just depends how much trouble
    you want to go to as far detail.
    I want my wire conductors to be hidden so I figured out a way to run them in the center of the pipes. Good luck with yours.

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    valveman

    3 years ago

    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.

    3 replies
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    DuckTurdvalveman

    Reply 5 months ago

    Valveman is correct, in most cases its not necessary to do any cutting on the transformer as they can most of the time be successfully wound intact.

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    JackDaviesvalveman

    Reply 3 years ago

    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

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    valvemanJackDavies

    Reply 3 years ago

    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?

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    5ov1k

    2 years ago

    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.

    2 replies
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    DuckTurd5ov1k

    Reply 5 months ago

    Depending on the size spot welder you are building you want to use #4 smaller, or #2 larger, battery cable available at Napa Auto parts, ask for bulk cable purchased by the foot.
    OR, go to your local electrical supply house and ask for welding cable same size I listed above. Good luck!

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    douwe1235ov1k

    Reply 1 year ago

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

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    JOHNJ392

    Question 1 year ago

    Thanks for the project. Except for 1 part I followed it to the letter. The difference being my use of 0 guage welding cable. First practice runs using pieces of aluminum can(soda) resulted in burn through on 1 side. I dont have a meter capable of checking the # of amps. Did I use too large of a cable? Or is aluminum just too weak to test on. I made new electrodes from 6 guage solid copper grounding and increased the surface area of the electrodes contact but I ended up with the same result. Any insight would be greatly appreciated.
    John

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    DuckTurdJOHNJ392

    Answer 5 months ago

    Unless you are using a control circuit to control the weld time and or out put of the spot welder you may have continuous trouble welding thin aluminum. The melting point of aluminum is very low, when spot welding thin pieces of aluminum
    burn through will occur very rapidly. Not sure if you'll be able to successfully spot weld thin aluminum. Good luck!

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    lopdert

    8 months ago

    Hey, hoping someone can answer this. A transformer has big time inductance in the primary main line that goes into the wall and through the switch, due to the many coils of wire around an iron core ( a textbook inductor). When you flip the switch off, do you get arcing in the switch?? Do you need to acount for this somehow?? Maybe a capacitor in parallel, a rhetostat to turn it off slowly?

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    DuckTurdlopdert

    Reply 5 months ago

    In something like this you can actually use a small motor rated contactor to do the switching for you, instead of the wall switch. Contactors are better suited for switching medium to high ampacity loads, thus saving other switches and circuitry from damage.

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    Leao93

    Question 5 months ago on Step 6

    Hey Jack, Great project and thank you for sharing!
    Is there any place we can get the laser cut design files?

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    RenatoC2

    1 year ago on Step 2

    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

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    RenatoC2RenatoC2

    Reply 1 year ago

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