Microwave transformer, too good to throw away?

[BrevaParticipant @breva]

My old 850W microwave died last week and has been replaced. This transformer is a big heavy lump. Other than using it as a doorstop in the shed or a source for copper wire, What else can i usefully do with it?

The idea of making a spot welder out of it is beyond my self imposed safety limits and indeed probably my needs. So, what else could I  do with it? Which of the two windings A or B, is the primary?

[John HaineParticipant @johnhaine32865]

Pretty useless as they are very specific to magnetron drive.  Best regarded as a source of scrap.

[GmanParticipant @thomasgman46877]

You can make a powerful magnet with it. I did that, covered the entire thing in epoxy and use it as a mag chuck for the drill press. Works great.

[JohnFParticipant @johnf59703]

This may be interesting ?

 

[Frances IoMParticipant @francesiom58905]

the secondary is the top winding – it can be lethal with a very high voltage – easy to cut through the winding without disturbing the primary (the lower winding ) the aux secondary is just a couple of turns of wire between the primary and secondary windings used to feed the low voltage cathode heater of the magnetron; once the secondary removed quite easy to insert a new secondary of a couple of turns of very thick wire  (insulated) which will produce 100A or so but at low voltage – this makes your spot welder.

[SillyOldDufferModerator @sillyoldduffer]

Elf and Safety Note first: these are I think by far the most dangerous item commonly available for a hobbyist to tinker with.  Being high voltage and high power puts them into Electric Chair territory!

Breva has survived the first risk, which is the capacitor might still have been fully charged.   They can deliver a nasty jolt, so best to assume they’re still hot.    The usual procedure is to earth them with a suitable insulated cable, with one hand secure in a trouser pocket to avoid an across the heart shock.

The transformer outputs a couple of kilovolts at about an amp, much more when shorted by flesh because the core is designed not to saturate.   If you get across a microwave oven transformer, it’s delivers beyond maximum power until the 13A fuse blows, or the transformer melts, inflicting horrific life changing burns, even if the victim survives.  Don’t have to actually touch it because the high voltage can jump a few millimetres of air, and may not be slowed down much by a plastic screwdriver handle.

Also, don’t break into the Magnetron to see what’s inside; they often contain toxic chemicals.

None of these horrors occur when the transformer is safely installed inside a microwave oven.  Otherwise, approach with extreme caution!

Most common use for old microwave transformers is making Tesla Coils, though for these it’s safer to use a Neon Sign Transformer.

 

Neon transformers are designed conventionally with cores that saturate to limit the power output, making them only ordinarily dangerous rather than exceptionally vicious!

However, it’s usually easy to remove the dangerous high-voltage secondary and rewind the transformer with something more civilised, though the unrestrained core is still a concern.  The HV secondary is the top winding in the photo, visibly many thousands of turns of fine wire.    The other secondary is a few turns of thick red wire, providing a few amps at perhaps 6Vac to the Magnetron’s heater.

 

Remembering the core doesn’t limit current, possibilities:

• 1 – 3V at hundreds of amps for electrolysis or electroplating (secondary a few turns of hefty Copper rod)
• 10 – 50V at 30A-ish for welding (secondary several turns of thick wire)
• A powerful demagnetiser

I don’t think its worth making a home-made welder – bit underpowered, and by the time you’ve sourced a box, terminals, and controls, it’s likely to cost as much as a better made commercial model.

A few other components may be useful.    The magnetron is fitted with a very powerful permanent magnet: not ideal because they break easily, but I use one inside a disposable plastic bag to pick up swarf.  Not as good a commercial swarf magnet where the magnet can be disengaged by pulling a rod.   The HV rectifier and capacitor may be useful for certain electronics products and there are various microswitches, connectors, and wires that are generally useful.   I’m sure the tiny motor and gearbox that drive the turnable at a few RPM must be good for something, but I can’t think what!

Dave

[Nick WheelerParticipant @nickwheeler]

Dave clearly hasn’t priced a commercial spot welder recently! Even a weedy one that struggles to join two pieces of car bodywork is several hundred quid. Used ones aren’t cheap, and potentially have all the mentioned safety issues of mains transformers.

[bernard towersParticipant @bernardtowers37738]

Brilliant for a home spot welder there is some info on GTBA site and more

[Martin ConnellyParticipant @martinconnelly55370]

In my youth I found an ex-crt television transformer. Connecting and disconnecting a 1.5V battery across the primary it would produce a spark about 20mm long tracking across the windings. Can’t remember what I was doing this for so don’t ask. Found this out by accident and never got a jolt from it as I didn’t try to do anything further with it after this observation.

Martin C

[Grindstone CowboyParticipant @grindstonecowboy]

Discovered at an early age you could get quite a tingle from the old style electric bell running off 4.5 volts if you got your fingers in the right (wrong?) places, presumably on the same principle as coil ignition.

Rob

[john fletcher 1Participant @johnfletcher1]

Several years ago, using an ex micro wave transformer I made a spot welder. I  removed the HT winding and for the new secondary I used a piece of insulated MIMS (Pyro) cable outer, NOT the inner, one complete turn was enough. It was a large size Pyro as it took some bending. For control I made foot operated power ON switch. Some time later after I’d used it a bit on very thin sheet steel sort of can size, space was required so out it went.

An interesting thing to do, much spoken about  but waste of time really and for those not into or understanding the dangers of Electricity leave it alone. John

PS. ALWAYS SHORT OUT ANY CAPACITOR TERMINALS (DISCHARGE THEM USING AN OLD SCREW DRIVER) WHEN ON A BENCH, before touching them. Just reinforcing, as some one has already mentioned this.

[noel shelleyParticipant @noelshelley55608]

A yard of welding lead (25sqmm) a 555 timer and a few other bit will make a spot welder for thin sheet. So long as you have a basic idea about electrics and safety. BE VERY wary of the condenser, it may have a discharging resistor but a resistor, a bit of wire and a stick or plastic tube will work. Noel.

[Oily RagParticipant @oilyrag]

It could be the building block of a CDI (Capacitive Discharge Ignition system!). These are quite commonly used on motorcycles (Japanese origins) and especially two strokes (less common these days). They were also used on race engines and still are today. The DFV and derivative Cosworths used a Lucas CDI very effectively in the 1980’s. The difference between an Inductive ignition system (sometimes referred to as a Hall Effect ignition) and a CDI is the HE ignition gives a moderate Voltage peak output over a longer time (typically 40Kv for a duration of 300 microsecs) whereas a CDI will deliver a much higher voltage over a shorter time span (again, typically 120Kv over 50 microsecs). This was described to me by the ignition ‘guru’ at Lucas as ‘sheet lightening’ versus ‘forked lightening’.

I was once setting the rev limiter on a DFV engined F3000 car at 8,900rpm (to avoid the mandatory FIA 9,000 rpm ‘Monk’ limiter which deprived the driver for sparks for some 2 seconds). The setting was done using a hand held signal generator connected to the spark box dialled in at 8,900, and holding the coil’s king lead whilst looking for a suitable place to see the spark when near grounded by a 5mm gap, with my spare hand on the roll hoop, I saw the mechanic leaning into the cockpit to switch on the ignition – I got as far as NNNNoo! before I received the full force of the near 600Hz shocks. This threw me backwards in the garage and as a result, after the feeling came back into my left arm, it miraculously cured my arthritis in my wrist – not a very pleasent cure though.

[Grindstone CowboyParticipant @grindstonecowboy]

To be fair, the OP asked what he could do OTHER than making a spot-welder…

Rob

[BrevaParticipant @breva]

Yes, I knew to discharge the capacitor, carefully!. The fireworks show was a bit disappointing as the microwave had been sitting for over a week not plugged in and it seems to have discharged itself.

I learned a lot from the replies especially Dave’s explanation. The high voltage secondary will certainly be coming out!

There are several options then. I will read up on the spot welder idea, but the one that would potentially be most useful to me would be Gman’s idea of making a magnetic chuck. I have no idea as to how to do this or how effective the result could be. Some research needed there!

So, lots of ideas in your posts. Thank you all for your very informative replies.

John

[Robert Atkinson 2Participant @robertatkinson2]

A mag chuck may be difficult with a microwave oven transformer. This is bcause they have a weld run across the laminations making it very difficult to re-arrange them as all E facing the same way to make exposed poles.

 

[Andrew TinsleyParticipant @andrewtinsley63637]

The line of weld is hardly a problem. 5 minutes with a triangular file or 5 seconds with an angle grinder will fix that problem. I have done this when rewinding several transformers which have the line of weld.

Not much good for a decent Tesla coil. I work on a friends machine which uses a pig transformer. 240 volts AC into the secondary and 11,000 volts out of the primary. This goes to a rotary spark gap. Now that is a decent Tesla.

Before H&E fans complain. I am qualified to work on high voltages and have a lifetime’s experience in the business. The Tesla power supply is puny compared to some of the work I have been involved with.

Andrew.

[Andrew TinsleyParticipant @andrewtinsley63637]

Dave’s photo of a “Tesla” is in fact a Van der Graaf.

Andrew.

[FulmenParticipant @fulmen]

It’s not, you can see the flat primary in the photo. And while a VDG produce high voltages it cannot produce the continuous currents needed for those arcs. It’s true that most VDGs use a spherical top load while TCs use torroidals, but both styles will work on either machine.

[Nicholas FarrParticipant @nicholasfarr14254]

Hi, I agree with Andrew about the welds, which can be removed as he says, or by plonking on your milling machine and just milling them off, they are barely skin deep as no filler wire has been used.

But you would have to also remove the spot welds holding the baseplate on, which could also be done cleanly with a slot drill on your milling machine.

Regards Nick.

[Robert Atkinson 2]

On Robert Atkinson 2 Said:

A mag chuck may be difficult with a microwave oven transformer. This is bcause they have a weld run across the laminations making it very difficult to re-arrange them as all E facing the same way to make exposed poles.

 

Coming from complete ignorance, why can’t you just saw/machine the top away to leave an E shape?

[Nigel Graham 2Participant @nigelgraham2]

Don’t those welds defeat the object of the laminations? They are that so currents cannot circulate around the core.

[Nicholas FarrParticipant @nicholasfarr14254]

Hi Nigel Graham 2, well the one that I have worked OK when it was in the microwave oven that it came out of, and it doesn’t seem to effect the one in the Video that JohnF linked too.

Regards Nick.

[john fletcher 1Participant @johnfletcher1]

I have noticed that those cheap stick welder have a weld across laminations in similar way to which micro wave transformer have, must be a valid reason. John

[Nigel Graham 2]

On Nigel Graham 2 Said:

Don’t those welds defeat the object of the laminations? They are that so currents cannot circulate around the core.

No. If you consider each outer leg individually there would be no path for the Eddy currents to flow unless they were welded on the inside edge as well.

Russell

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