Yet another MOT Spot Welder

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Joined
Dec 2, 2017
Messages
79
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.


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


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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.


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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

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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.



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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.


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


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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.


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