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transformer help needed

hi guys, i'm a newbie on here so i would like to take this opportunity to say hello :)

i have a salvaged transformer that i'm working on at the mo. It is the primary that is blown, but the secondary is fine. I am in the uk, so it is designed to run on 240. Because i am looking to produce high amps, one solution would be to use the secondary as a primary, otherwise i am looking at doing a full rewind. i'm kinda new to transformers, but have enough knowledge for the job in hand. my question is how many turns does the primary need to have? i know that you cant give me an exact figure without the dimensions of the transformer in question (700w microwave transformer if that helps) but i seem to remember from school physics that there is an equation to work out the number of turns a primary must have at a given voltage, i have googled for days to no avail, i'm guessing it is out there somewhere, but i cant find it :(

and the last question is that if the transformer was to have too few turns on the primary, what would happen when it was plugged in? my guess would be that it would short out the mains and the coil would rupture, possibly tripping the circuit breaker?

many thanks to anyone who replies :) and merry xmas to all :) 

Toga_Dan3 years ago
do you know the original output v of this secondary?
Toga_Dan3 years ago
i think that the guage of the wire will also have an effect on current. a combination of resistance and inductance.
gmoon3 years ago
Try searching here on ibles.. Oh, here's one.

I'm not sure what the minimum number of primary turns would be be...With fewer primary turns the lower the flux linkage, and the less efficient the transformer. At one point the "coil" (at zero turns) would would loose any quantifiable inductive quality and simply be a (low) resistive element and would certainly qualify as a short. 

Unless of course when you write "primary" you mean "old primary now used as the secondary"

In that case it's pretty easy to calculate the primary/secondary current ratio, and work from that, since the voltage and the amperage of the breaker are known.

Note that the number of turns in the secondary is NOT the load, and by itself meaningless. No load, and there's no appreciable current draw.