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PEU Fri 30 July 2010 11:00

Microwave transformer > Power Supply
6 Attachment(s)
I got this nice transformer from an old microwave oven (circa 1980) its sizes are: with the windings facing you: width 120mm, heght 100mm and depth 70mm, the inner section is 40mm, so the core area is 40x70mm, on the sides of the winding windows is 20mm on each side.

First thing I did was carefully remove the secondary with a chisel and patience, used a piece of MDF when working near the primary, attached photos of the process.
Nice thing about this transformer is that the primary is winded with 110v, 220v and 240v, since I plan to use the 220v coil, if I ever need to lower the voltage a little I can use the 240V coil and have a 10% lower output.

Then I began looking for a transformer calculator to get an idea of what can I get from this transformer and found this site:

I want to have 3 secondaries, 35v, 9v and 4v to get 48,12 and 5v rectified. So entered these values in the calculator and best I got was 35v18A, 9v4A and 4v3A, very nice since I wanted 35V20A

Do you guys see anything that needs to be changed? Do the calculations made by the program look ok?


lumberjack_jeff Fri 30 July 2010 20:15

Cool. It would have been nice to have access to that program when I was doing mine.

I had a hard time getting all that wire in mine, but I was looking for 50-ish volts and 5v.

riesvantwisk Fri 30 July 2010 21:21

I don't want to discourage Pablo,
but a word of warning to everybody else, doing this can be dangerous if you don't know exactly what you are doing and know how to test such a transformer for it's power output, proper way of winding yada yada.. it can kill..

Besides that, a transformer is so cheap, compared to the overall build price of a MM, and expensive components do get connected to it that I don't see a reason to safe any $$ on it.

I d whish Pablo good luck with each conversation of this trafo though.

Pablo, if you didn't get your parts from the US yet, Antek makes good priced transformators.

PEU Fri 30 July 2010 21:24

1 Attachment(s)
I still need to probe that program correct, a lot of things can make the needed turns more or less, losses, permeability, etc etc.

I did this drawing in coreldraw to check how much of the winding window is used by the copper, read in many sites that the recommended maximum is 85% because more makes the winding complicated:

On Monday I will go to a shop near home that sells enameled copper wire by the meter or by the kilogram, I estimated around 1.2kg (worst case) of copper for all the windings, better purchasing a couple of meters more than falling short :)

Also calculated the maximum power this transformer can handle:

Max Power=(40mm x 69mm / 100/1.16)^2=566.1w (40x69mm is the area of the core)

The Core will be able to handle this:
34v 16A=544w
9v 2A=18w
4v 2A=8W

I don't know the density of the core, and that value helps determine the amount of turns, from different pages I visited my calculations range from 1 turn per volt to double-ish that, so I'm shooting at the higher value for the wire purchase just to be safe.

I guess if I push a couple of amps more on the 34v winding nothing will happen, but, I have 5 motors at 5A each, and the 5th motor will be barely used, that leaves 20A consumption, but per Mariss estimates you have to derate the consumption by 33% to get the real consumption, that leaves 13.2A for 4 motors or 16.5A for 5 motors at full load.

So it seems this transformer will do the job if winded properly :)

PS: again, if someone that really knows this stuff (not like me) wants to correct me, I'm more than happy :)

PEU Fri 30 July 2010 21:32

we posted together Ries :)

Thanks for the advise, yes I know, I'm used to work with electronics/electricity so I know how to handle it, if I weren't confident I would not go this route.
Also asked for the rewinding to be done by a professional, still did not received his quote, if its reasonable I will save myself the work.

When I made my 1st cnc router I did the same, took a transformer out of an old UPS and had it rewinded to my needs for pocket change, still works flawlessly to date!

riesvantwisk Fri 30 July 2010 21:47

it's rewording, that's for sure!

OT: I had one transformer made here in Ecuador for my Amplifier I brought van holland (van Medevoort) and it was of such a horrible quality, that I had to put a ventilator on it to cool it!
I have used that (see above) program myself, but just to confirm that what the guy made for me was right, and so it wasn't... obvious he made it to small and I had to return it. When I asked to lacquer it, he just did...... with a brush :S obviously I did mean to put it in a vacuum chamber, then impregnate it with lacquer.. ooo well.... Now it hums a bit and get's warm to hot.

Out of curiosity, are you be-able to get toroids in Argentinie?? Here in EC it's a no everywhere, unless they import but nothing in stock.

PEU Fri 30 July 2010 22:09

Yes, I was quoted 685 pesos for a toroid that fit my needs, thats USD170, my guess for the rewinding quote is no more than USD40 or if I do it mysilf less than USD20, labor is cheap here and I guess is the same in Ecuador.

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