DIY Spot Welder

Cremona, August 2018.
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Hi there,
this is a brief review with some notes on how to make a spot welder out of microwave transformers (MOT).
Nothing new with respect to other online tutorials, except that this has:
  1. Two MOTs in parallel for more power;
  2. A solid-state relay for timing, controlled by Arduino for simplicity.
Here is the gale:
ATTENTION! This project deals with high and medium voltage. This is dangerous and unsafe. I'm in no way responsible of any damage that you may cause (even to yourself).


There are many many many spot welder tutorials out there. Mostly, people use microwave ovens transformers (MOT), removing the High Voltage secondary and replacing it with just few turns. I use two MOT in parallel, for higher output current.
This is enough for making a basic spot welder, however something cool is the ability to control the welding time. Also, people say that a short pre-welding pulse softens the material and thus creates a better electrical contact (aka best welding). In my case, I use arduino to implement a simple time controller. Anyway, the controller can be bypassed with a switch, to operate the spot welder in direct mode.


Here's a pic of two microwave oven transformers (MOTs) obtained from a micro wave oven.
Be careful, microwave ovens are very dangerous objects. There's a high voltage capacitor retaining high voltage for long after the oven was disconnected and hazarduous chemicals in the magnetron. People say they will make you blind.

The MOT on top still needs to be modified. The high voltage secondary (tinier wire, many more turns) need to be sewered away and replaced by few turns of a thick wire. The magnetic shunts should be removed. Magnetic shunts reduce the effective power of the transformer, since they short circuit a part of the magnetic flux. Secondaries are replaced with 3 turns of a thick wire (in my case I used two wires in parallel to have a bigger section, so you will see 6 turns, in place of 3).


Electrodes are usually made of copper (not brass, people say it would stick while welding). I found some copper rivets with a big section, they work fine. Two short arms hold the electrodes in an improvised manner, and some springs keep the arms open.

Connect the electrodes, add a switch and mount everything. The transformers need to be connected in parallel (mind the phase!!), something like this:

At this point the basic spot-welder is ready and can already be operated. The measured voltage among electrodes is 2.72 VAC.
In the next section I create a controller for setting welding times.

Controller (electronics)

The transformers can be driven directly with the network 220V (direct mode) or via a timer. I decided to use Arduino as a timer, driving a cheap solid state relay (3 of them from ebay, for about 5 euros only). Solid state relays include (usually) a snubber, which is pretty important in high inductance applications (such as transformers!!) Normal relays would risk to stick when high currents are interrupted abruptly. So far the cheap solid state relay works fine.
The solid state relay idea is actually super simple: you provide a small voltage to the inlet (directly an Arduino output pin in my case) and the output becomes a closed circuit for the alternated (220V) voltage.

The scheme for Arduino looks something like that (sorry, I know it's rough..) Anyway, it's useful to add another switch, so that the solid state relay can be bypassed and the welder can be used in direct mode, as shown before. The MOT assembly was discussed in a previous picture.


The three trimmers are simply voltage dividers (470k should work great). The lever switch has three positions (center disconnected) and is discussed in the program. The other button starts the welding when pressed.

The simple Arduino program does the following in a loop:
  1. Check the state of a small lever-type switch;
  2. IF switch is on first position, then perform an automatic weld (times controlled) when a button is pressed;
  3. IF switch is on second position, set the pre-weld, pause and weld times using the three trimmers;
  4. IF switch is on third position (manual mode), then check the status of a button and if it's pushed, weld until it's released.

I heard what you said! Such an horrible bed sheet! Anyways..
In case you want some inspiration, you can find my script HERE. This also needs the LiquidCrystal_I2C library, for piloting the display.

Ok, at this point build the upper level of the spot welder, place a cooling fan (and a metallic grid on the back to let air exit passing through the MOTs) and add the electronics.

Finally, we need a panel for the trimmers, the display and some buttons.
panel-part1 panel-part2

That's all, folks!

Any comment? Feel free to write me!

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