Transformer from a microwave oven

The magnetic shunts are there to limit the line current during a secondary overload, effectively adding leakage inductance between the two windings. This makes the voltage regulation worse, instead of better.

If you take the shunts out, the idling current will rise and the transformer will no longer be as overload proof. These transformers are also designed to use just about every bit of flux capability of the core, so they are quite lossy and so, rated for intermittent operation and/or high core temperature rise. The secondaries are also often not very well insulated on one end, since one end is grounded in most microwave oven designs.

If you remove the shunts and use the space to add some primary turns, to lower the flux swing, they may settle down and look more like a normal step up transformer (better regulation, with lower core losses, magnetizing current and audible hum. This will, of course, lower the output voltage, too.

Want to build a big tube amplifier?

This discussion comes up fairly often. I suggest start by doing an advanced google search in groups.google.com and use < transformer "microwave oven" > limited to only this group. I just did that and came up with lots of old posts.

(snip)

Some informative, if depressing, replies. Thanks for all.

Firstly, since posting I knocked out the magnetic shunts and rechecked the secondary voltage. It's now 2000 VAC (off load) for 120 VAC in, a ratio of nearly 1:17.

Of course, John (two of you, other one from "tubes" NG) is right about these shunts, regulation is worse. I was confusing them with that resonant CVT design that uses a capacitor and magnetic shunts in a weird way. Mea culpa.

That's a good point about the insulation at the frame end (Roger L.), it's not good for 2 KV but, in other uses, it may not have to be. Anyway, I'd decided not to push it very far but thought it would be quite safe at normal tube B+ (DC.) However, from the tube NG, Phil said the LF and HF response would be very bad. I could readily agree the HF would suffer from the crude winding layout but figured that, at a couple of watts, the LF would be OK - lot of iron! Anyway, impedance match is impossible and the matter of DC in the core is yet another problem for SE amps and P/S chokes.

The more I think about it the more it looks like a door stop. Too bad... Thanks again, Roger

HI, I've used them for making test gear. Flash tester (insulation tester), you can use 1 or 2 with a variac to give you a 2 or 4KV variable supply. Put a suitable resistor in the high voltage lines to limit the current to less than 1mA. Fast transient burst generator (emc test) if you make 1 of these you need a variable 2kv dc supply to drive the pulse generator, the transformer/diode/cap from the oven is perfect, you still need the variac, if you build them all together you can use the same 1.

The high voltage secondary must not work floating from the frame - the insulation from frame end of windings to frame is not rated for this.

Its a nasty transformer, designed for fan cooling and limited duty cycle.

Roger Lascelles

A resistor in the primary is safer than resistor in the secondary, since there is then no place where high current, >1kV is available from the circuit. Probably the magnetising current will mean that the resistor in the promary can't be as large as you would need to make the output current safe, so the best approach might be to put a resistor in the primary and also a string of resistors in the secondary such that several resistors have to fail before it becomes dangerous. A metal box suitably earthed is absolutely necessary. Also beware of the voltage rating of the resistor, and the possibility of arcing between any wiring associated with the resistors.

If at all possible, I would suggest avoiding using the high voltage from a MOT and instead use a flyback converter which is incapable of putting out so much current.

Chris

The only reasonably safe suggestion I can think of is to cut away the secondary and sell it to the scrap dealer as recyclable copper, and then thread a few turns of welding cable or copper bar through the core, to make a very high current low voltage transformer. You might be able to spot weld things with it, or at least melt nails, etc.

For spot welding, I have found that a relatively low current works if the electrodes are not very tight on the sheet metal that you are welding, but that you need much more current if the electrodes are clamped down hard like in a real spot welder. I am suspicious of the welds produced under light pressure. Recently, I have seen some websites about making spot welders from about a Farad of capacitors charged to 50V or so, and discharging this into some probes. These are supposed to work for attaching tabs to Lithium cells, but I don't know if they are very good or not.

Chris

I've once changed a transformer from a microwave oven which failed - a ball of sparks and flame, but I'd consider that pretty rare. This one was being used by my parents' in their caravan annex by the beach, so was in a pretty corrosive atmosphere. Not to say they don't fail (Sony had a spare on the shelf) but it isn't a terribly common occurrence to my knowledge. Anyway, yours isn't much of a spare now... :-)

Ken

Some ideas on those pages.

Ed

Done this, they pushed out easily.

I still have to measure the off-load magnetizing current.

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Thanks, Ed. Cheers, Roger

There's a number of websites detailing how to make a spot welder.

I built one and it works great. Hacksaw off the secondary, and wind 4 turns as a new secondary, using as many parallel bits of wire than will fit into the window. I control the primary using a 555 timer driving a solid state relay. I also switch a large WW resistor into the primary to give a high-low setting. With the resistor and the timer variable between 30 and 350 mSec, I spotweld pretty much any thickness of sheet metal, from foil to about 0.9 mm.

Contact pressure is important and you need to mechanically control that to get consistent results. However, I use it a lot, and, for one-off jobs, I can gauge it pretty well by hand.

Barry Lennox.

I'm interested in the Fast transient burst generator, can you supply more details of this?

Thanks,

Barry Lennox

I actually use a couple of these transformers, winding connections clipped short, on my general workbench. They provide a nice compact anvil like mass for hosts of little jobs that I don't want to walk or bounce when using percussive persuasion. One transformer in a cloth is also just right to hold in your hand for the occasional "biffing without bouncing" session on a wide variety of house projects.

Don't drop it off a ladder. It will make a hole in concrete! Don't ask :-(

JustMe

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Correct.

After taking them out I measured the no load magnetizing current. Very high: 3 amps at 120 VAC on the primary. But I think it is high with the shunts in anyway (but not measured.)

Correct.

Actually, I measured it before (1,925 VAC) and after (2,000 VAC) for

Er... no.

(snip)

Just did it: 3 amps at 120 volts input. Horrible! Did not measure the phase angle so I don't know the no load losses.

Cheers, Roger

Fast transient burst generator (emc

What do you want to know about it? Basicaly it provides a pulse (50nS) up to 1kV to simulate a spark. These are repeated at 5KHz for 15mS (burst) each burst is repeated every 300mS. The pulses are injected into the power lead of the equipment under test. See European standard EN 61000-4-4 .

I wish there was. It's bad enough trying to comply with them and they change them every so often. There was a time when I thought they would put me out of business but it seems the lead free directive (june 2006) will do that.

cut the secondary out (I used a chisel) and replace it with a low voltage winding, you'll be getting close to 1 volt-per-turn ...

The heart of an indestructable bench, or CB radio supply... etc

Bye. Jasen

Is there a cheap way to see it? Chris