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How to determine minimum AWG for re-woud transformer

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boylesg

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I have removed the HV transformer from a microwave oven and intend to dismantle it, remove the HV secondary and add my own low voltage secondary with 8.4V and perhaps 84V (12VDC and 120VDC).

But I am a some what uncertain about AWG current capacity. If I wanted to be able to deliver up to 10A at 120VDC then how do I go about determining the maximum AWG (minimum wire diameter) I can use for the secondary?
 

Well you can do alteration with microwave oven transformer to a high output current transformer.
First we will see the power capability
Normal power of this transformer is 1200W @ 2300V
So the output current should be 1200W/2300V = 0.52A
If you turn to 84V/120V
Maximum current 1200W/120V =10A ok Go head !!!

For 10A current you can use 10AWG copper wire( minimum copper cross sectional area 5.5mm^2)
Or multiple wire
17AWG x 5 strands or 20AWG x 10 strands or 23AWG x 20 strands

See this link also. Be careful
https://mad-science.wonderhowto.com...rmer-into-high-amperage-metal-melter-0140772/
 

Just be aware that these transformers work well into magnetic saturation and have aluminium windings.
Absolute minimum iron and aluminium (which is a lot cheaper than copper).
These things are built right down to absolutely minimum price.

The result is a very lossy transformer that heats up very fast even under very light loads.
That is not a problem if you want to run it only for a minute or two to heat up your dinner.

If you are building a power supply and only expect it to run at full load for a minute or two it will also work fine.

Run it for a couple of hours and it will probably burst into flames with any decent load.
Try running just the primary with no secondary, and no load and see how hot it gets after an hour.
You might be surprised.
 

I put one of those safety switch like wire fuse replacement things (8A) into my primary coil circuit and it is drawing less than 8A with no load on the secondary, so that is comforting at least.

And I don't intend to run it for hours, more like 20 minutes or so.

I have noticed that many of the microwave ovens you get these days have Al windings. But all them from a decades or more back had Cu windings.

This one had Al windings but I put the Cu windings from another transformer on it and it seem to work OK at least in short bursts.

Would there be any merit to attaching some large Al heat sinks onto the core to help cool it?
 

What these really need are more turns of thinner copper wire (to fit into the available space) on the primary to reduce the flux level.

Just judge the size of the iron core. It looks more like a commercial 250 to 300 watt rated transformer, not a 1,200 watt rated transformer.

Its all about penny pinching, cost saving, and planned obsolescence.
If you can save fifty cents manufacturing each one, and you make a million of them......
 

What these really need are more turns of thinner copper wire (to fit into the available space) on the primary to reduce the flux level.

Just judge the size of the iron core. It looks more like a commercial 250 to 300 watt rated transformer, not a 1,200 watt rated transformer.

Its all about penny pinching, cost saving, and planned obsolescence.
If you can save fifty cents manufacturing each one, and you make a million of them......
Short of being able to do this, what if I was to create a twin modified MOTs and wire both the primaries and secondaries in series?

I should get the same output voltage but with the flux density and 250V shared between the two cores by roughly half.
 

That would certainly work.
The only disadvantage might be slightly degraded output voltage regulation because the total primary resistance has now doubled.
But if free stuff can be put to good use, its certainly worth investigating.
 

That would certainly work.
The only disadvantage might be slightly degraded output voltage regulation because the total primary resistance has now doubled.
But if free stuff can be put to good use, its certainly worth investigating.

A bit less than 120V DC wont matter.

I calculate such a setup as drawing about 1.7A at 250V AC, which is about 425W and sound about right after what you have previously posted.

- - - Updated - - -

By the way, what sort of effect do the weld lines have in these cores re eddy currents?
 

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