MOT power supply

Thread Starter

Hutch2793

Joined Jul 7, 2020
118
I’m new to the forum and I’m needing help with a mot power supply build I’m doing. I have pulled two transformers from microwaves and both of I have damaged the primary winding trying to remove the secondary. It’s a bear to remove them and I want to know if the damage is too bad to work with or if I need another transformer. I am trying to build a transformer with a pi filter capable of providing 7 volts and around 20-24 amps continuously without no ripple. Please help me determine the best course of action. I can post pics of the damage once someone replies and after maybe someone more experienced provides their input.
 

Irving

Joined Jan 30, 2016
814
mot?

If the primaries are damaged - even just a scratch across the winding - I'd have to err on the side of caution and say don't use them. I doubt a photo would be sufficient to advise safely, but put some up, a good well lit, focused close-up and a wider shot for context.
 

jpanhalt

Joined Jan 18, 2008
9,755
I have removed a couple of secondaries. Never a problem. They were in separate sections.

Maybe your microwave oven transformer (MOT) is different. A picture before and after damage might help us advise what you are doing wrong.
 

AlbertHall

Joined Jun 4, 2014
10,056
I think you need a new transformer.
This shows quite a good way to remove the secondary - note the putty knife, and take it slow.
 

Thread Starter

Hutch2793

Joined Jul 7, 2020
118
I have seen that video. I didn’t use a putty knife because I always think I can do something careful enough. Is there another way to make this power supply without repurposing a mot or, can I make my own primary and secondary with standard sheathed 10 gauge wire?

I think you need a new transformer.
This shows quite a good way to remove the secondary - note the putty knife, and take it slow.
 

Papabravo

Joined Feb 24, 2006
13,939
mot?

If the primaries are damaged - even just a scratch across the winding - I'd have to err on the side of caution and say don't use them. I doubt a photo would be sufficient to advise safely, but put some up, a good well lit, focused close-up and a wider shot for context.
mot = Microwave Oven Transformer
One of the more obscure references on this site.
If you're going to be obscure, please try to be obscure clearly.
 

Irving

Joined Jan 30, 2016
814
The insulation is scratched, but it doesn't look as if any turns are shorted. If you want to try it, get some conformal coating (for PCBs), or if you can't find that, some car paint clear lacquer (or maybe the stuff above, but I've never used it and not sure how thick it is). and give it a good dose. Wrap one or two turns on the secondary side and try powering it on. If there's no sparks or magic smoke and it remains cool (use IR thermometer not fingers) then measure AC volts across secondary & divide by number of turns to find volts per turn on sec side.
 

Thread Starter

Hutch2793

Joined Jul 7, 2020
118
None of the windings are cut all the way through but, it is about half of the diameter of wire deep gouges. Can anyone tell me the why the adhesive is so important?
 

Irving

Joined Jan 30, 2016
814
None of the windings are cut all the way through but, it is about half of the diameter of wire deep gouges. Can anyone tell me the why the adhesive is so important?
Then I'd say its toast, if its that badly damaged.

The adhesive is to stop the windings 'singing' or moving which could abrade the insulation and lead to shorted turns over time.
 

Tonyr1084

Joined Sep 24, 2015
4,719
The insulation is scratched, but it doesn't look as if any turns are shorted.
I don't see anything shorted either. My concern is how deeply those wires are nicked. Their current carrying capacity is compromised. That means those nicks will be the hot spots during operation. Covering them with lacquer won't help.
it is about half of the diameter of wire deep gouges.
Here is side view.
The picture doesn't look like it's half way through. Nevertheless, you're the hands on inspector on site. You have to determine the risk and decide whether to go forward or not. Drawing 24 amps at 7 volts - that's 168 watts. That's a fair amount of heat. Given the damage to the wire, and the hotter the wire is the less conductive it becomes - I'd personally opt for a new transformer. That's my 2¢.
 

Papabravo

Joined Feb 24, 2006
13,939
We have a saying in the QA end of the manufacturing business: "You can't test in reliability."
In short that means even if you complete the assembly and it seems to work, these areas of compromise will always weigh on your mind. this is why people who make products don't use second hand stuff. You just don't know the provenance. If you insist on engaging in this activity you should chalk this one up to experience, bin the trash, and be more careful with the next one. It's not like you paid for it. If you can't do that then remove all the wire and keep the core.
 

Thread Starter

Hutch2793

Joined Jul 7, 2020
118
Thanks for all the advice. It isn’t half way, I’m being hard on myself because they were nice Samsung transformers I completely trashed. I guess it would be a good learning experience to try them at least. Is there any way to make a transformer from scratch or one of those Jameco kits that can output that much power. I know the reason for using the microwave transformer is they Generate a lot of current.
 

AlbertHall

Joined Jun 4, 2014
10,056
Thanks for all the advice. It isn’t half way, I’m being hard on myself because they were nice Samsung transformers I completely trashed. I guess it would be a good learning experience to try them at least. Is there any way to make a transformer from scratch or one of those Jameco kits that can output that much power. I know the reason for using the microwave transformer is they Generate a lot of current.
They are also free, which new big transformers certainly are not.
 

Thread Starter

Hutch2793

Joined Jul 7, 2020
118
Have you done this before? Do you know how to calculate/size the winding?
I have only done research and modified transformers to measure outputs of primary windings. I know what size gauge to use and how many turns to yield the desired output on the secondary. I’m not concerned about figuring out the math and variables involved, if that’s what your asking?
 
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