MOT's stuff - Page 1
Forum section: electronics / projects

author: davelectronic
author_rank: Frequent Contributor
timestamp: August 05, 2020, 02:49:03 am
content: So i have had a dabble with MOT's (microwave oven transformers) In the past. I never throw stuff out if there might be the opportunity o use it at some point. What i found was rewinding a single MOT secondary winding, and powering a single primary winding its just to wasteful of power, and heat. where by you have to blow a hurricane over it to keep the heat down. My first MOT rewind gave me 14.70 Volts, which i was happy with. As of recently i have acquired another MOT, there are two scenarios i had in mind. The first was to remove the secondary winding, then replace with the same secondary MOT, so i would have two identical transformers to wire the primary winding's in series. Hopefully to get the primary winding heat down a bit, as each would share the 240 Volts AC we have here in the UK.The second scenario was to leave the secondary winding on the newly acquired MOT, and use it as ballast, again wiring it in series and shorting the high voltage secondary winding. Again to try and get a better primary wire up, with less heat. i would prefer the first scenario, as i would get a higher secondary voltage from two transformer's in series. Makes it a bit more usable in terms of options. As you can guess, i'm stuck on which route to take, and i know i can by an off the shelf transformer, i have a few already at different ratings. I just like the idea of of something for nothing in the microwave oven transformer's. Any thoughts on which of the above options would be better, and more efficient, or as efficient as it can be. Thanks for reading, ideas appreciated.

author: davelectronic
author_rank: Frequent Contributor
timestamp: August 05, 2020, 01:49:57 pm
content: Does anyone know which option is the most viable ? I know MOT's are not super efficient, but I know you get really good current output if you can get the wire diameter turns on there. I just wanted to improve some what, the primary efficiency.Please, if anyone could suggest the better primary scheme. Ballast shorting a second MOT wired in series, or second MOT, withers secondary rewound, and also wired in series. So ballast or second series wired with secondary rewound same as first MOT. Thanks again.

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 05, 2020, 01:58:40 pm
content: Quote from: davelectronic on August 05, 2020, 01:49:57 pmThat's the best option if all you have to work with are oven transformers. An alternative would be to look at switching power supplies, specifically server power supplies which go for cheap on the surplus market.

author: davelectronic
author_rank: Frequent Contributor
timestamp: August 05, 2020, 08:02:13 pm
content: Thanks for replying, I do have other toroidal transformers of different voltage and current ratings. It's just an idea to put together a power supply using MOT's. I did one ages ago, but wasn't happy with the heat the transformer generated. As it was just a single MOT, it got up to 85°C with a 12 Amp load. I don't think the secondary was producing the heat, and fairly certain it was the primary windings. The secondary could have produced much more power, but the circuit it was powering couldn't much over 12 Amps at 12.50 Volts.

author: Pawelr98
author_rank: Regular Contributor
timestamp: August 06, 2020, 12:17:25 am
content: Generally speaking MOT are fairly "bad" transformers.Core is often welded, way too small for the power it usually carries and the primary is aluminium.The idle current is also very high, my MOT used in a spot welder draws 4-ish Amps at 230V while idling.Core is fairly saturated with the magnetic flux.A normal transformer would have a far bigger core, properly isolated metal plates, as well as far more primary turns.

author: davelectronic
author_rank: Frequent Contributor
timestamp: August 06, 2020, 06:43:44 pm
content: Yes, I remember cutting the high voltage secondary winding from the first MOT. Cut one side and drifted out the windings, and thinking, this isn't copper.vaguly I did one years ago, that early one was copper.

author: coppercone2
author_rank: Super Contributor
timestamp: August 08, 2020, 04:13:26 am
content: Quote from: Pawelr98 on August 06, 2020, 12:17:25 amhow does a bolted core compare? I have a bolted transformer I wanted to reuse for a welder ballast mod (choice of bolted core vs MOT core). I see on the internet they already were successful with a MOT core but anyway. Should a bolted core use nylon bolts? Its Chinese but bigger then a MOT, I thought I could varnish the plates with transformer varnish and rebolt it together with my choice of wire.

author: davelectronic
author_rank: Frequent Contributor
timestamp: September 11, 2020, 09:21:30 pm
content: I didn't think it necessary to start a new thread, taken a while to rewind one of the MOT's secondary. I've used 12 AWG silicone insulated cable, and added a single layer of heat shrink to the cable just to be certain insulation is good. This time I have a voltage of 12.90 this seems a bit better. Although if I rewind the second MOT the same, running the primary windings in series, so 120 Volts to each primary winding, I should get 12.90 Volts total from the transformers running in series. I don't know if that is high enough voltage to get a final regulated 12 Volt output. With an LDR 2940T is it possible with a decent capacitance after rectification ? I've done it with 14.70 Volts RMS and an LDR regulator, and a huge 90000uf from a capacitor bank. But 12.90 Volts might be just to low. I'm unlikely to rewind again with 14 AWG, not only that, but the available current will start dropping off. Any thoughts on 12.90 Volts to obtain a regulated 12.00 Volt output.Thanks for reading.

author: davelectronic
author_rank: Frequent Contributor
timestamp: September 13, 2020, 10:04:04 pm
content: So I have some real time numbers now, these few tests where with AC and DC but not regulated yet. I wanted to see what kind of voltage drop I would get with varying loads. First I have two MOT transformers wired in series, each is getting approximately 120 Volts AC from the mains. Secondary winding's was 12 AWG silicone wire, and I added a single layer of heatshrink tube, just to certain the insulation was good. Average wire length was just short of 4 meters roughly, I purchased 10 meters to be sure I had enough wire and heatshrink tube.Each MOT is giving 12.90 Volts singly from the mains 240 volts supply. With secondarys in series I’m getting 14 volts AC. Next I tried loads up to 200 wats using halogen lamps, 50 watts each,No load, 14.00 Volts AC.50 watts load, 13.70 Volts AC.100 watts load, 13.30 Volts AC.150 watts load, 13.00 Volts AC.200 watts load, 12.70 Volts AC.Using a multimeter, the numbers for current added up, give or take a very small percentage, and I guess these where possible losses in cable and efficiency of the MOT transformers. Next I rectified the AC voltage with a 20 Amp full wave bridge rectifier attached to a substantial heatsink. I did measure the unloaded DC output at around 12.90 Volts to 13.00 Volts. Then I added a capacitor bank I made a while ago, its 9 x 10000uf capacitors in parallel, the big can snap in type, these are rated at 35 Volts each. So my DC filtered and smoothed voltage was now 18.77 Volts DC with no load. Now I was going to repeat the load test of the series wired MOT transformers under a DC load, but not regulated. The following was the outcome.No load, 18.77 Volts DC.50 watts load, 15.85 Volts DC.100 watts load, 14.43 Volts DC.150 watts load, 13.27 Volts DC.200 watts load, 12.28 Volts DC.I could have added more load beyond 200 watts, but the voltage would have just kept dropping away. I think because the transformers are only getting half there rated input voltage in series. On the plus side, nothing from the transformers got remotely hot. I would say they didn’t climb in temperature much more than my hand temperature was. With the 200 watt load applied to the filtered and smoothed DC voltage, I did measure the AC current directly from the transformers output, and almost at the limit of my multimeter, at 18.00 Amps AC.So i’m posting this as someone might find it useful, I am fairly certain I can get 10 Amps out of this MOT transformer configuration, i’m quietly optimistic I could make 15 Amps DC regulated just maybe, and it is a real maybe. If I wired the transformers for there full rated voltage, i’m 100% certain I would get 20 Amps with no problem at all. But then I would need forced air cooling equivalent to a hurricane. On a final note, something I found useful in maximizing the number of turns I could put on each transformer core was some silicone spray first, then remembered from years ago trying to pull a wet suit over a sweaty body lol, talcum powder made it so much easier to pull each turn through. I have tried the split core method, and found on reassembling the core, performance was much worse. I know its a bind trying to wind a closed iron core, but for me its what works best. This is all just a bit of fun, and a couple of old microwave oven transformers, repurposed for low voltage. I know they are not exactly power efficient, but for brute force power that’s free (hardware, not your electricity bill) they do work.

author: intabits
author_rank: Frequent Contributor
timestamp: September 16, 2020, 07:11:12 pm
content: Note that those wattages will actually be a little higher than stated, as the bulbs will presumably be 50W at 12V.So they will be drawing more than 50W at voltages higher than 12.

author: davelectronic
author_rank: Frequent Contributor
timestamp: September 17, 2020, 07:24:10 am
content: Yes I did think they would draw more current as voltage increased. But the meter readings looked ok when measuring current.

author: Psi
author_rank: Super Contributor
timestamp: September 17, 2020, 07:57:54 am
content: You will get better and cooler MOTs if you source them from really old microwave ovens.From before china realized they could cut corners if they added a fan.

author: davelectronic
author_rank: Frequent Contributor
timestamp: September 17, 2020, 03:34:53 pm
content: Yes they used copper windings in early microwave ovens. Today it's aluminium windings, noted as the secondary windings where removed. I'm getting 51 C with two transformers in series, at 13.87 Volts with a 200 watt load. I left it running monitoring temperature and voltage over around 90 minutes. I was quite happy with that. Hopefully I can use an LM2940CT 12 Volt regulator with multiple pass transistors, 20 Amps with a 50% duty cycle would be great. But realistically I think it's going to be a maximum of 15 Amps or so.

author: davelectronic
author_rank: Frequent Contributor
timestamp: September 17, 2020, 07:08:35 pm
content: I did try a single transformer with the same set up, 50 Amp bridge rectifier, 90000uf capacitor bank. See the image, that has been running for over 1 hour. I know it's wasteful of power in the primary winding, just wanted to see temperature and voltage over time. The fan is running at the meters voltage, the same output to the lamps. The purpose eventually is probably two transformers rectified filtered and regulated. The two heatsinks are just to stand the fan on, there not part of any cooling.