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MOT battery spot welder help

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shipto:
I am trying without any success to make a battery spot welder using a microwave oven transformer and wondered if anyone had any input.
I built a standard spot welder before and it worked reasonably well so thought this would be easy.
I have it controlled by an Arduino Uno supplying output to a SSR and even though it kinda works and gets the strip hot it is not welding the strip to the batteries which I suspect is the power not going deep enough and staying in the strip.
I have tried with more windings to increase the voltage but still no joy.

Another thing I have wondered about is the what I thought was a thermal switch but maybe not, should this be removed or kept in place?

S. Heslop:
I didn't have any success trying a similar thing. I burned a hole in a couple lipos before deciding i'm just going to cause a fire. My assumption, and I could definitely be wrong, is that this style of spot welder goes too slow compared to the capacitive discharge type - and lets the heat spread out alot before the weld is made. Which makes it too easy to burn a hole into the battery casing.

shipto:
Yes I am thinking your right even though youtube has many who claim success.  :doh:

ddmckee54:
How many turns did you have on your secondary side, and what size wire was it?  The less turns, the lower the voltage, but the higher the current you will get out of the winding.  Assuming that the transformer is 100% efficient, it isn't but assuming this will make it easy, then the VA (volts*amps) going into the primary side will equal the VA available on the secondary side.

When welding, you don't so much care about the voltage as you do the current.  It's the current that will do the welding, and the current will also determine how fast you can weld.  The faster you can weld, the less heat build up you will get.   The videos I've seen only use a few turns at most on the secondary side and those turns are done with some really BIG wire.

Don

Will_D:
I have 2 turns of 80mm2 It will just about spot weld 0.9 mm stainless. That is just manually squeezing the tips (I use big Mig welding tips).

Two improvements I think may be:

1. Lever system like on the pro jobs to squeeeze the metal more

2. I assume the pro jobs use a capacitor bank and a contactor and timer to up the weld current

Can someone confirm the capacitor theorey and recomend how many Farads would be needed

Another solution is to use more than one transformer connected in parallel - but get the phasing correct!!

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