Transformer winding direction

** 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 le akage 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 an d use of strip conductors for the secondary.

Getting 3V at 200Amps is non trivial.

... Phil

Even using it in reverse, swapping primary and secondary, you will not get the low volts and high amps for welding. Did u-tubber just use the core and existing primary and rewind the secondary secondary with 1 to 2mm diameter wire? should be obvious in the vide, those sorts of dimensions

May be he is winding an autotransformer, although it's not very safe for welding :) .

--
Saludos

I think he was confused. Look at a transformer-style soldering gun, the hairpin secondary doesn't have ANY helical orientation at all, and it transforms down to high current at low volts just fine. Ditto for feedthrough AC current meter transformers.

Output windings on such high-ratio transformers don't couple capacitively to the primary (the low resistance and inductance completely swamp tiny capacitive currents), and it's hard to imagine any importance of flux coupling defects at low (50 to 60 Hz) frequency with soft-iron cores.

** Huh ?

See hi-res pic of soldering gun tranny.

Note use of copper strip and close coupling of the secondary.

** Do they ever ?

** Mick Faraday used soft iron in his toroidal job, but it was judged no much good. Silicon steel laminations have been the norm for over a century.

Also, many power transformers have separate windings on adjacent limbs of a U or C core. Invariably, there are primary and secondary coils wound on each limb that are later coupled in series or parallel.

Never seen the primary on one and the secondary the other - cos that results in a tranny with very poor regulation due to high leakage reactance.

.... Phil

Problems I discovered with these welders are:

600W ain't nearly enough power to weld anything substantial.

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.

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.

I gave up trying to weld with a MOT.

CD welders deliver a known energy to the weld and are very much less dependent on contact resistance.

That's what he was doing.

Isaac

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

Mine does a fine job on things like coathanger wire and the stainless steel strips from windshield wipers.

NP; just use a good SSR for switching.

Isaac

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

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.

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.

** Zero crossing switching is the worst possible for creating large in-rush surges and hence fully magnetising the core of an AC supply transformer.

Switching on at a voltage peak, while under load, is the best option.

A triac switch will always go off near zero current, which will be nowhere near zero volts.

... Phil

That's NOT a hairpin secondary! Look instead at this

Even if the transformer manufacturer works on good flux coupling, the external loop of the iron's tip will leak lots of flux; you should expect every CRT in the vicinity to shimmy while you hold down the trigger. There's no particular reason to care, in this case.

** So it's a one turn secondary on a toroidal core.

Magnetic coupling will be poor, but still good enough for a soldering tool.

** Any external flux will come from the iron core, not the loop. Removing the soldering tip will not eliminate external flux or even reduce it much.

... Phil

Well, I don't recall mentioning volts at all. Yes, triacs switch off at the zero crossing of the current. Not much you can do about that.

I question not turning it on at zero volts??? Please explain the large in-rush current when there's zero volts on the transformer primary?

** You did, cos "zero crossing" means at zero voltage crossings. ** Then why did you say it was needed ??

** Really ? Did you try Googling the topic? ** The surge current peak comes a little after the first voltage peak.

Consider that it takes half the time for the applied voltage wave to first average zero if you switch at a peak - half a cycle later instead of a whole cycle - minimising the low frequency component of the wave.

... Phil

if you say so. As long as you stay out of saturation, the main component of the primary current is due to the shorted secondary. I think I'll give my SCR the benefit of switching on when there's zero voltage.

Not really the best idea; it is then possible that the previous turn-off happened after a half-cycle (+), and if this ON state starts with a half-cycle (+) as well, that's two half-cycles of the same polarity. The likelihood of saturation is very high. Turn-on at peak V is a strategy that minimizes saturation risk.

** Wot a smug prick you are.

** No chance - you are all wet right now. ** The secondary is not shorted. ** More fool you.

... Phil

Well, I'll repeat what I wrote earlier in the thread:

I set the weld time by an integral number of cycles of 60 Hz.

The key word there is "integral" as in complete as in full as in you don't get two half cycles of the same polarity.

Designing your welder to avoid saturation is a strategy that minimizes saturation risk.

Seems to be your opinion of everyone.

Once again, your signature condescending tone declares that the other guy is always wrong. Once again, you nitpick instead of attempting to understand. Communication is difficult enough, even when you try. When you try NOT to understand, you're just being you. I'm sure you win a lot of arguments when the other guy just gives up trying to influence your thinking.

My presence in this thread is to help others get the most out of their MOT welder. I gave up trying to influence you long ago. Just trying to give some balance to the view.

There it is again.