Yet another MOT Spot Welder

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Dec 2, 2017
I decided to try a MOT spot welder because I realized that I already had most of the parts lying around. I had the MOT, an old PC power supply case, a SSR, an old phone charger, various circuit components and other small stuff.

After reviewing a few MOT based spot welder builds, I am trying a 555 timer circuit that can pick a SSR for the AC mains to the MOT.


Remove the really fine winding and put in one big fat wire


Decide where to put supporting components inside the old power supply case. It will be obvious where you need to make holes for the big cable... make the holes a bit larger and as smooth as possible so you can put extra protection around the HV cable. The orange block is a DC actuated AC Solid State Relay. Also needed is a small 5v charger, a small circuit board for the 555 circuit and a potentiometer. All that should fit inside the case.


Diagram for a Monostable 555 Timer circuit.
I picked a capacitor from the bin and guessed that the time required for a weld could be less than 1 second. I plan to start with a short pulse and increase the resistor value to make the weld pulse last longer if more heat is needed


I used a 555 circuit calculator to get the ballpark values for the variable resistor (R) using the capacitor value (C) that set the time constant for the output pulse.


On the left front corner of the case, I put the potentiometer knob to control the duration of the zap with "one shot" from a pushbutton trigger. (not shown yet).

Here is a trial fit for the leads and how it may work with a pack of cells.

Had to go to the store for these connector blocks and the big wires. Used about 1 foot of #6 and 2 foot of #2.


First electrodes. Just guessing on the shape, to get the electrodes as close as possible.


After some deliberation, I decided to solder the copper blocks together so the big cables, the connector blocks and welding tips will not be blocking my view as I position it for a weld.


Basic assembly with the connectors and tips from the operator point of view.