Cooling of overloaded transformer

P = E^2 / R

quick meltdown too.

With some counter you could pick every fourth pulse, maybe slosh it around and use a filter.

The FEL use to pick AC pulses,

10Hz popular, but you could get any AC fractional period.

George H.

If I ever get a new job description, I'll ask for a line about making mistakes... It's what I do best, well and making things work.

Making 'em work is the fun bit. Mistakes are par for the course.

George H.

Half price here?

Huh? Festool regulates the price very tightly. They'll pull the franchise from anyone selling under their MSRP.

Why wpuld they waste money on a capacitive dropper when for the price of a rivet they can have a resistive dropper?

--
This email has not been checked by half-arsed antivirus software

te:

lund:

in Europe

overloading it, but then keeping the heat down by forced convection, namel y a fan

er.

You may well be right.

NT

Depends on where they come from. Cheap ones may have no thermo-fuse at all, relying on a fire department not to be very far away :-)

Better ones have a self-resetting thermo-fuse but if this action happens a lot and under load that fuse can eventually fail. Medical grade transformers usually are not allowed to have a self-resetting thermo-fuse, it must be a one-time fuse and when that opens the transformer is, well, toast.

The main thing is that windings heat up way faster than a core. They have no real thermal mass and because they are "cocooned" the heat has nowhere to go.

--
Regards, Joerg 

http://www.analogconsultants.com/

This is very interesting. Microwave ovens have the transformer running very close to saturation. If you want to use it for something else, you might need to remove the shunt that provides a limit to the core saturation when firing a magnetron. Other applications might include a spot welder, which usually only runs for a few seconds.

The oven may allow cooking for 99 minutes. With the shunt in place, what happens to the winding temperature?

Of course, anything I can imagine cooking in a microwave would catch on fire long before it timed out:)

As little as they often weigh these days I assume many microwave ovens now have switching power supplies.

People in Europe also talked about "coreless transformers" for low voltage lighting, from the days when LV halogen bulbs were popular. Those things would take in 230VAC and deliver 24VAC or similar, via a small ferrite core. Of course not at the kW power level, there'd probably be no market for that.

--
Regards, Joerg 

http://www.analogconsultants.com/

Yes, you can buy switching microwave ovens in Walmart. They are usually much more expensive.

I womder if it would be possible to repurpose a switching power supply into a spot welder? Why not?

Simple microwave ovens have switchers as well, since a long time:

It's being done.

--
Regards, Joerg 

http://www.analogconsultants.com/

MOTs have a bimetal cutout on the core, and are normally able to cook for 15 minutes on full before the bimetal opens. Thereafter you only get low duty cycle cooking until the MOT cools down. That's why canning in a microwave is a no-no.

I'm surprised to hear someone say that a kilo or so of copper has no thermal mass or conductivity. Heat sealers run transformers at many times rated P_out for seconds at lowish duty cycle.

NT

--

John Larkin   Highland Technology, Inc   trk 

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

Yeah, cheaper ones are still iron cored. They use copper clad aluminum windings.

Matter of fact, CCA, and copper plated steel, is very common for cheap Chinese wiring. Even (especially?) piddly crap like breadboarding jumper wires.

No, V/turn is too high, and the control is different.

I mean, you could adapt the control, and put a weird winding on there that reduces the voltage further, but you might as well start with a new transformer.

I wonder how skin effect affects spot welding. Surely it would make the heating spot much larger (and stretched out along the direction of the electrodes), until it becomes hot enough (resistance becomes high enough) that it breaks through at the intended spot.

The inductance of the electrode arms would be a severe barrier above, say,

10kHz. It would look more like an induction heater (Q > 1), and you're wasting all your capacity on kVARs.

Tim

--
Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Contract Design 
Website: https://www.seventransistorlabs.com/

I doubt it'd have much mttf run like that, and there's fire risk to consider.

There are plenty of things one can do. Whether they're good ideas is another question. We all know it.

NT

The control circuit looks complicated.

I meant to convert a standard microwave oven switcher into a spot welder, like waht is done with a conventional microwave oven transformer. It looks like the control circuit might require considerable modification to handle the variable load. For example, there would be no requirement to monitor the load since there is no magnitron filament to keep warm. Rewinding the transformer to supply low voltage and high current might be a problem.

Probably easier to just find a junk microwave oven and go the conventional route.

Yeah, mine is a Panasonic 'Inverter' model... it's not notably lightweight, though.

Some pliers-type spot welders are, nowadays, switching-power input, but that's not the case for the cheapest Harbor Freight models... the problem is, you don't want or need DC for a spot welder, but that's what the inverter style has to offer. So, iron being cheap, the welders use heavy 60 Hz iron transformers.

I hear a lot about Cuk converters, but does that still have a DC power storage capacitor? Such storage is wasted for welding.

I think the inverters are AC. In any event, the question is can you convert a junk inverter microwave oven into a spot welder like you can with a MOT?

Probably handles short circuit loads better than an inverter. Also, as Tim points out, less problems with skin effect.