Microwave transformer - is it worth keeping??? - Page 1
Forum section: electronics / beginners

author: MarvinTheMartian
author_rank: Frequent Contributor
timestamp: February 23, 2016, 09:39:28 am
content: Does anyone have any suggestions for re-tasking a microwave transformer?I can't think of a use for one at this stage (I'm only only just starting out in electronics - not too much experience as yet).Heavy paperweight?Is it worth keeping?Thanks

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: February 23, 2016, 10:02:18 am
content: handy for winding custom high-power secondaries, e.g. for spot-welding

author: MarvinTheMartian
author_rank: Frequent Contributor
timestamp: February 23, 2016, 10:52:56 am
content: Thanks Mike.I might keep it for the future then.PS Thanks for your great videos - I've been watching yours and Dave's (among others) for a couple of months now.They are great for picking up lots of little tips/terminology and the repair/tear downs are very interesting too. I feel I've learned quite a bit just by watching the vidsMost of all they are great entertainment too!

author: prawncrackers
author_rank: Contributor
timestamp: February 23, 2016, 11:14:43 am
content: if you have the patience id wind up a diy isolation trafo

author: Gyro
author_rank: Super Contributor
timestamp: February 23, 2016, 01:19:52 pm
content: I doubt if you could do that (isolation transformer) safely given the amount of 'butchery' needed to remove the secondary winding. It's ok for spot welding etc because they use a couple of turns of fully insulated cable for the secondary. They are also lossy and get hot in continuous operation.

author: kc2skg
author_rank: Newbie
timestamp: February 23, 2016, 02:16:39 pm
content: I am keeping one just to play with. I just acquired a variac and will slowly bring the microwave transformer primary voltage up while monitoring the secondary voltage. I hope to use it with some electron tubes eventually.

author: Renegade
author_rank: Regular Contributor
timestamp: February 23, 2016, 02:39:25 pm
content: Quote from: Gyro on February 23, 2016, 01:19:52 pmCan I ask what you mean by "butchery"? I took apart 1 MOT and was able to remove it without significant damage. Is that unusual? I've seen videos of people hacking them out, but it didn't seem necessary to me.

author: Cliff Matthews
author_rank: Unknown
timestamp: February 23, 2016, 02:50:20 pm
content: Quote from: Gyro on February 23, 2016, 01:19:52 pmUse a big vice to hold and a hack-saw to cut one side of the secondary off. Then turn it in the vice (so the remaining side faces down) and tap down so the rest falls out. If it's wedged-in tightly, you can loosen it by drilling down holes in the winding on each side and tap downward again. Afterward, carefully tap out the shunts. 5 minutes maximum?

author: HoBoPanda
author_rank: Unknown
timestamp: February 23, 2016, 03:04:22 pm
content: I have 3 or 4 of them around. I used one in my power supply for my LED bench lights. pulled the secondary apart and rewound it to get about 11vac out of it using 14awg house wire, then add in a cheep ebay voltage regulator and some ikea LED desk lights reflector bridge and a big CAP. You now have a high current bench power source 10-13v and great lighting for your work space.

author: Gyro
author_rank: Super Contributor
timestamp: February 23, 2016, 06:47:42 pm
content: Quote from: Renegade on February 23, 2016, 02:39:25 pmI was thinking of (probably) the same videos you've seen and the several threads about people damaging the insulation (or the wire itself) of the primary during dismantling. The term 'isolation transformer' to me indicates some degree of safety isolation, which I can't see being guaranteed by home modification. As I said, low voltage step-down transformers where the secondary winding wire (or cable) can be wound with the insulation intact is a different case to trying to safely isolate a full (1:1?) secondary winding. Practical winding of a large number of turns would imply cutting the welded core laminations too. Many useful things can be done, but the word 'isolation' worries me, particularly in the Beginners section.Edit: Just to be clear, I was responding specifically to:Quote

author: MarvinTheMartian
author_rank: Frequent Contributor
timestamp: February 23, 2016, 10:33:07 pm
content: Quote from: Gyro on February 23, 2016, 06:47:42 pmI agree Gyro, as a beginner I think I would feel more comfortable purchasing an isolation transformer for my own peace of mind, knowing that it is truly isolated (whereas there is a possibility I do something wrong in my ignorance)Thanks guys for the information. I might look up those YouTube videos to satisfy my curiosityThe LEDS power supply is interesting but I already have leds set up on my workbench - I'll keep it in mind for a possible project in the future.

author: Seekonk
author_rank: Super Contributor
timestamp: February 24, 2016, 03:36:02 am
content: Use it to make a nice low frequency buck converter. They don't have to run at a hundred khz. Just gap the core with a thin piece of cardboard.

author: MarvinTheMartian
author_rank: Frequent Contributor
timestamp: February 24, 2016, 04:10:59 am
content: Quote from: Seekonk on February 24, 2016, 03:36:02 amHi Seekonk. I'm familiar with the term (buck converter) from the forum/videos but have yet to research what it actually is/does.I'll keep that one in mind for the future too, thanks.

author: IanB
author_rank: Super Contributor
timestamp: February 24, 2016, 04:15:31 am
content: Quote from: MarvinTheMartian on February 24, 2016, 04:10:59 amThere are two ways to use a magnetic element in a circuit. One way is to use it like a traditional transformer, but the other way is to use it more as an energy story device (somewhat like a capacitor). With the right design, you can charge up the magnetic field and store energy inside it, and then discharge it to get the energy back out again while changing the voltage (or current). In a flyback converter you arrange to get a much higher voltage than you started with, and in a buck converter you arrange to get a lower voltage, possibly with a current boost.

author: mtdoc
author_rank: Super Contributor
timestamp: February 24, 2016, 04:16:57 am
content: Quote from: MarvinTheMartian on February 24, 2016, 04:10:59 amA buck converter is simply a type of circuit used to decrease or step down a DC voltage to a lower level.There is a nice tutorialhere.

author: MarvinTheMartian
author_rank: Frequent Contributor
timestamp: February 24, 2016, 04:30:05 am
content: Quote from: mtdoc on February 24, 2016, 04:16:57 amThanks mtdoc for the explanation and link.Looks like a good site to delve into, I'll save that as part of my reference library.Much appreciated!

author: bills
author_rank: Frequent Contributor
timestamp: February 24, 2016, 05:01:57 am
content: MarvinDo yourself a favor, Don't plug it in as it is.They have Lethal voltages .I am sure you knew that but be careful.bill

author: jh15
author_rank: Frequent Contributor
timestamp: February 24, 2016, 05:34:04 am
content: I don't understand the 'safety' problem of taking 2 microwave oven transformers and making an isolation transformer.unless you are making tesla coils or hv stuff, they are fine to use for hot chassis antique radios, switching supplies, etc.

author: bills
author_rank: Frequent Contributor
timestamp: February 24, 2016, 05:40:03 am
content: As I said be careful the secondary's have up to 3kv. more or less.I would think that requires some caution.bill

author: MarvinTheMartian
author_rank: Frequent Contributor
timestamp: February 24, 2016, 06:56:25 am
content: Quote from: bills on February 24, 2016, 05:01:57 amToo bloody rightI'm not applying any power to it as is - just looking for any useful things I may be able to do with it (once reworked?)Quote from: bills on February 24, 2016, 05:01:57 amThanks Bill - well aware of that!

author: Gyro
author_rank: Super Contributor
timestamp: February 24, 2016, 10:11:14 am
content: Quote from: jh15 on February 24, 2016, 05:34:04 amYou missed the 'Smiley off your post!..... err, you weren't serious were you?How would you connect two MOTs back to back safely? If you connect the two secondaries together they you would have a connection running between them at several kV. If you tried to connect them primary-primary then the thin secondary windings wouldn't be capable of any significant mains current (and one side of the secondary is grounded).Edit: Ok you could do it in theory, but without appropriate HT insulation it would be very dangerous. Not something to suggest in this section.

author: DmitryL
author_rank: Regular Contributor
timestamp: February 24, 2016, 11:03:10 am
content: I made a nice isolation transformer from a micrewave oven one.. But it required quite a lot of work.- need to disassemble the existing transformer (cutting welded joints with dremel, separating plates, removing magnetic shunt etc)- need to remove all windings. They are made of copper cladded _aluminium_ and have no use. Primary also has too few turns, microvawe oven transformers are designed to work in saturation when not loaded- need to make a coil bobbin- make first winding with a proper copper wire (about 1 turn per volt)- make screen between windings- make second winding- make this bloody thing not humming by soacking windings with hot epoxy- glue and bind iron core together..

author: Gyro
author_rank: Super Contributor
timestamp: February 24, 2016, 11:09:19 am
content: Sounds like you had some fun with that one Dmitry!Just another thought on the back to back MOT idea. Both transformers would contain magnetic shunts, designed to limit the current to the Magnetron. I'm not sure how much current would make it through two connected ones.... Maybe a bad thing, maybe not.

author: Halvmand
author_rank: Regular Contributor
timestamp: February 24, 2016, 02:45:44 pm
content: Imo they are not useful for anything else other than intended. Maybe a spot welder.You can't run them without a fan for extender periods of time. Even at no load they draw huge amounts of current, and thus heats up.I have one laying around. With no load it consumes 150 W, not VA. After an hour with fan cooling the outer layers of the primary winding reaches 80 deg C.

author: Seekonk
author_rank: Super Contributor
timestamp: February 24, 2016, 02:48:22 pm
content: By definition, a MOT is not a very good transformer. The core is not interleaved, reversed E's. But where else are you going to get a core that is easy to work with that has that much metal. I have built low frequency buck converters with transformers for my solar arrays. These are cheap and easy to build. Layout doesn't matter at all and easy to build with uno type controllers.