Odd Microwave Transformers - Page 1
Forum section: electronics / beginners

author: reed
author_rank: Contributor
timestamp: August 27, 2021, 02:46:07 pm
content: It's odd to me, probably obvious to you, but...I decided to try making a spot welder today, from an old microwave transformer.I cut out the secondary coil and the heating element wires, at that point I stuck my multimeter trodes on the primary coil, and got nothing... Thinking I'd cut a wire of the coil, I tested another transformer and again I got nothing... I tried a third one and got continuity, which is what I was expecting since it's supposed to just be a coil.Three transformers I have give no continuity, whereas a fourth one does...What is the difference? Why? How can I use?Thanks

author: Zero999
author_rank: Super Contributor
timestamp: August 27, 2021, 04:20:32 pm
content: The transformers are probably bad, which is why the microwaves were junked in the first place.

author: Ian.M
author_rank: Super Contributor
timestamp: August 27, 2021, 04:54:17 pm
content: +1Microwave transformers lead a hard life, right on the bleeding edge of saturation, relying on load current to stay out of saturation, and forced air cooling to prevent them cooking themselves to death. It's not particularly surprising that they often die from insulation breakdown.I would expect that the failed transformers have an open circuit thermal fuse in series with the primary winding. If you are lucky it may be accessible if you remove some of the tape insulation from the winding, in which case you can *TRY* replacing it with the same style and temperature rating, in the hope that the overload was due to a shorted turn in the now removed EHT secondary, then soak test it and see if it 'comes good', but its better than even odds the primary is either cooked as well (smell it) or the thermal fuse is buried under the winding with no possibility of replacing it.If you get lucky, you now have to fix the insulation you removed. It was probably anchoring the leadout wires so that needs to be done properly with tape that's low stretch and heat resistant.Don't be tempted to bypass the thermal fuse as that frequently releases large quantities of particularly evil 'magic smoke' and can result in an actual fire before the breaker trips.TLDR: You are too sensible/intelligent/lazy to fix it so scrounge up another one!

author: reed
author_rank: Contributor
timestamp: August 27, 2021, 06:21:07 pm
content: Quote from: Zero999 on August 27, 2021, 04:20:32 pmProbably, but a lot of good stuff is junked simply because it doesn't fit with the new decor or there was a sale on. I gave up on collecting TV's, just way too many thrown simply for being too small... As Leftfield said, people...

author: reed
author_rank: Contributor
timestamp: August 27, 2021, 06:29:23 pm
content: Quote from: Ian.M on August 27, 2021, 04:54:17 pmI did come across "something" sort of enclosed on one, I'll take a closer look, I remember it being like half a stamped box (5mm x 10mm x 3mm) with two pins / rivets joined by a thin strip of metal...?Already sourced another and that's giving me a good reading, so one for a spot welder and another for a mag vise... I'm sure I'll find more soon!I have a little electric furnass, so I'll still collect and drill these things out, another ingot for the shelf...Thanks all

author: Ian.M
author_rank: Super Contributor
timestamp: August 27, 2021, 06:53:27 pm
content: Well now you know to check the primary before spending time cutting out the secondaries carefully rather than quick hack and smash to remove all the windings.

author: Zero999
author_rank: Super Contributor
timestamp: August 27, 2021, 08:53:41 pm
content: There can be other problems with the primary, than it being open circuit.Check the resistance between the primary and the core, to check there's no earth fault. Ideally this should be done with an insulation tester at 1kV, but an ordinary multimeter is better than nothing.There might be a shorted turn, in which case it will measure a low resistance, but will smoke, if turned on. If you're brave, you could just connect it up to the mains and check it doesn't draw too much current or get excessively hot. A more cautious approach is to connect it to the mains, via a 60W incandescent light bulb, which shouldn't light. Obviously make sure the HV lead is disconnected and keep your fingers well away from the HV terminal, when it's on. The voltage and current is high enough to give you a lethal shock!

author: CaptDon
author_rank: Super Contributor
timestamp: August 30, 2021, 01:37:48 am
content: Connecting a 'GOOD' microwave transformer to the mains through a 60 watt bulb willalways cause the bulb to light a nearly full brightness!!!! The idle current on theseM.O.T. devices is horrendous!!! They are nearly saturated at no load!!! They will runvery hot very quickly with no load!! I guess to make your welder you will have to hammerout the magnetic shunts also?

author: Zero999
author_rank: Super Contributor
timestamp: August 30, 2021, 09:28:04 am
content: Quote from: CaptDon on August 30, 2021, 01:37:48 amThinking again. You have a point. The dim incandescent lamp trick will work with an ordinary transformer, but might not work with an MOT. Unfortunately I don't have one to test, otherwise I would.

author: Wolfram
author_rank: Frequent Contributor
timestamp: August 30, 2021, 09:46:34 am
content: Check if you're making a good connection with your meter probes. The last microwave oven transformer I was hooking up had some heavy laquer on the terminals, making it very difficult to make a solid connection to it. I ended up having to use a wire wheel to get the terminals to a point where they would even take solder. An open primary could be the issue, but it's not so common that I would expect three out of four random junked microwave ovens to suffer from it.