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Hi A friend just gave lots of transformers some very big
How do I rewind them
I once saw a site "Roll your own power transformers" with some formulas
where is it? I want to know how to calculate the power rating starting from the core area in square inch or square centimeter
I've already stripped one transformer but what is its maximum power transfer?
mewasures 5.2cm by 5.9cm

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Why would you ever want to do that ?

Basics?  Well The ratio of turns determines the voltage.
10:1 will convert 10volts to 100v.

The number of turns will determine the Power. The more turns the more power.
(It has to do with a measure of the coupling between the coils. The whole thing is very complex.)

If a transformer is rated at 12 @ at 25ma, if you draw more than 25ma the internal resistance of the wire in the transformer will start to drop the voltage output.  In effect you could simply measure the transformer in VoltAmps (watts)
and then a 12v @ 25ma transformer would be the same as a 6v @50ma unit.

The down side is that while the magnetic part of the formula is solid, the thermal part is out of whack.  Because now the windings are handling more current than before and they will heat up more.  If the physical size and shape of the transformer can't handle the heat it will melt down.

And then there are the mind killers.  Flux leakage, Q Factors, Harmonics, Phase shift, and even Inductance mismatch (Depending on the frequency you will be working with if the two coils Pri & Sec are not matched to the frequency you would loose a lot of power even with everything else just perfect!)

Let us know what your goal is and maybe we can guide you better.


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  • 2 weeks later...

Not true...

For iron core transformers...

The voltage equation is


V is the voltage across the winding
F is form factor (1.11 for AC)
f is the frequency
a is the cross sectional area (cm^2)
N is the number of turns
B is the flux density (gauss)

The power equation is


P is power (VA)
J is current density of the transformer (amp/cm^2)
W is area of core window (cm^2)

What this tells you is that the size does matter, and the variables are the cross sectional area and the window area.

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  • 4 weeks later...

Hi Jean-Noel

Welcome to our forum

10 years ago when the electricity company suplied us only 170vac instead of 220, i had a step up transformer. One day it burned and emitt abig amount of smoke

i decide to rewound it and simply did the following:
got a paper and a pen to write notices
open its cover and take a long look
disconnect all connections and make any sign to help u in the reconstruct process
unwound the wire noticing the direction of wound and the number of turns
take it to shop and ask him the same wieght and diameter

when i wound it by hand and because i was harry the turns were not side to side so it take more size than it must, so i reduce the number of turns by about 10% for each step

finally it work and i had a very good results and i believed from that time that there is no critical

Now I ask you to share with us your formulas

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