Transformer secondary winding is heating up a lot! - Page 1
Forum section: electronics / beginners

author: hummusdude
author_rank: Regular Contributor
timestamp: March 17, 2021, 05:21:33 am
content: I have a salvaged microwave transformer that I was thinking of using for a power supply for an audio amp. The amp is a circuit based on a TDA 7264 gainclone chip. The amp works great but it really needs a dedicated power supply...my bench supply just isn't designed for this kind of thing. So I carefully cut the secondary windings off and used some 16g stranded wire with good insulation to do a test. I wound it 10 times and measured about 8 volts on the secondary with 120 vac into the primary (I'm using an isolation transformer into a variac for safety). The test went fine with no odd behaviour, no heating or humming.But the wire I used for the secondary is much too thick for the space that there is for the secondary windings. I need closer to 45 turns on the secondary to get the desired voltage out that I need to drive the audio amp to 20W out. So I redid my experiment with some 18 guage magnet wire...copper coated in varnish. But when I powered up the primary it was buzzing right away at only 10 v in. And as I slowly increased the voltage I started smelling the magic smoke at around 40 v in.It turns out the secondary winding was getting very hot. I read online that saturation of the core can cause excessive current in the secondary, but it's a burly core. I put an ammeter on the secondary and sure enough it was reading 2 amps with only 15 or so volts into the primary. So I'm getting a lot of current through the secondary with the magnet wire, but I don't see how it could have been doing that with the insulated, slightly lower gauge wire. So it seems like something is different but I'm not sure what it could be. I've read on other threads on this forum that microwave transformers are poor choices for repurposing on account of the design constraints for microwaves are rather unique. But even if the efficiency is in the tank it shouldn't be heating up like this I would think.

author: Weston
author_rank: Regular Contributor
timestamp: March 17, 2021, 05:36:04 am
content: Microwave oven transformers run the core close to saturation, which leads to high loss at the full line voltage (loss in the core goes up linearly with the AC voltage). Some people suggest adding a few more primary tuns to reduce the loss in the core. You should also remove the magnetic shunts, they reduce the coupling between the windings and will give you worse load regulation.In your case though it sounds like you scraped the insulation off a patch of wire during winding and it is now shorting out.

author: Gyro
author_rank: Super Contributor
timestamp: March 17, 2021, 12:32:03 pm
content: How have you insulated your new secondary from the core? A photo would help.From all the buzzing and general secondary heating, it sounds as if you may havemultipleshorted turns. Is it possible that you scraped the magnet wire against the outside of the laminations as you threaded every turn?

author: CaptDon
author_rank: Super Contributor
timestamp: March 17, 2021, 01:18:20 pm
content: Only a shorted secondary would cause the secondary to heat up if there is nothing connected to it. M.O.T. transformers will run hot because they are nearly saturated on the primary at rated voltage and were never designed for constant use. If you removed the entire secondary you should have lots of room for a winding of that low a voltage. You should also add about 10 or 15 turns up in that vacated space to be placed in series with the primary to lower saturation. I use one of these M.O.T. transformers for a ham radio linear amplifier and having added 10 turns to the primary makes the transformer a happy camper for continued run time. I had to unwind about 1/3 of the secondary since I only wanted about 2200 volts after rectification and filtering. The extra turns for the primary fit nicely into the space where the magnetic shunts were removed from.

author: jmelson
author_rank: Super Contributor
timestamp: March 17, 2021, 07:19:19 pm
content: Your trial run was with stranded wire, then you replaced that with solid wire. The solid wire might develop enough eddy currents to heat up. The flux will be the same in the primary and secondary, so what kind of wire did they use for the primary?In cases where eddy currents cause a problem, there is "Litz wire" wihch is a bunch of parallel strands of thin magnet wire. The separate wires are only connected at the far ends of the windings. Stranded wire is a poor substitute for real Litz wire, but probably does defeat the eddy currents to some extent.Jon

author: Gyro
author_rank: Super Contributor
timestamp: March 17, 2021, 07:23:26 pm
content: If it's a standard 50/60Hz MOT then the primary will be single core copper or CCA. You need to go higher in frequency before skin effect becomes significant.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 17, 2021, 09:49:34 pm
content: Yeah, you shorted it out. Just looping wire on the welded-shut core? Likely to scratch something doing that.Note that the shunts should be removed, and 10-20% more turns should be added to the primary to get it out of saturation.A 20W amp is a rather poor match to a >1kVA (albeit more like 500VA continuous, after modification) transformer. Are you sure you wouldn't rather buy a proper one, and sell this thing for scrap? The copper alone is worth about as much...Tim

author: hummusdude
author_rank: Regular Contributor
timestamp: March 18, 2021, 02:24:59 am
content: Quote from: T3sl4co1l on March 17, 2021, 09:49:34 pmYou know, considering all the excellent feedback I've gotten to my post I'm inclined to take your suggestion and find an alternative. Thanks for the input!

author: Gyro
author_rank: Super Contributor
timestamp: March 18, 2021, 09:58:25 am
content: A very sensible decision, a mechanically quiet transformer is best for an audio amp anyway (not to mention lighter).Now that you've gone to the trouble of removing the original secondary of the MOT, hang on to it in case you get the urge to build a spot welder or degausser sometime.

author: hummusdude
author_rank: Regular Contributor
timestamp: March 18, 2021, 02:36:30 pm
content: Quote from: Gyro on March 18, 2021, 09:58:25 am...and it makes a stylish paper weight, too!