On 2/13/2015 9:12 PM, isw wrote:
In article , mike
wrote:

On 2/12/2015 10:30 PM, Phil Allison wrote:
isw wrote:

I recently came across a YouTube video where a guy rewound a microwave
transformer to make a spot welder.

In talking about it, he stated -- and repeated -- that it was very
important for the secondary winding to have the same "sense" as the
primary -- that is, both windings had to go around the core in the same
direction.

I know that matters if the transformer is handling very asymmetric
waveforms such as in a flyback configuration, but I have never heard
that it matters for plain old 60 Hz. sinusoids.

Does it? Or was the guy just confused?


** A spot welding transformer is almost the direct opposite of a
microwave oven tranny - big step-down instead of step-up and the lowest
possible leakage reactance instead of heaps.

To help with the latter, it could easily be important to wind with the same
sense. Also winding multiple, identical secondaries and connecting them in
parallel is the way to go plus using other techniques like interleaving and
use of strip conductors for the secondary.

Getting 3V at 200Amps is non trivial.


... Phil

Problems I discovered with these welders a
600W ain't nearly enough power to weld anything substantial.

Mine easly pops the 15 Amp. breaker if I set it for too long a pulse.

Yep, if your objective is to weld your breaker, you're on the right track.

Very thin stuff like battery tabs require very accurate energy
delivery. The difference between no weld and blasting a hole
thru everything is a small pressure difference holding the weldment
together.

Mine does a fine job on things like coathanger wire and the stainless
steel strips from windshield wipers.
So does mine on things that aren't damaged by overheating.
Just hit it until it glows red.
Battery tabs aren't so forgiving.
I spring loaded the tips separately to give some repeatability.
I got about 90% good welds. When it's a 10-cell battery pack,
90% ain't nearly good enough.

If you don't turn it on/off at zero crossings, the saturation
state of the core can make a great difference in the next weld.

NP; just use a good SSR for switching.
Agree, but I expect most DIYers don't.
I set the weld time by an integral number of cycles of 60 Hz.
Took about 6 cycles to weld a battery tab.

Still claim that a controlled energy dump is far superior.
It's relatively insensitive to contact resistance.
My CD welder is rated for 7V peak across .001 ohms.
It's over before the cell case even gets warm.

Isaac