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Old 27th September 2001, 04:24 PM   #1
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Join Date: Sep 2001
Location: Finland, Ostrobothnia
My question is: Does anyone know a rule of thumb how to determine number of turns v/s core area on toroid power transformers? I have found in a lot of places the diagram / formula for E-I cores, but toroids are a bit different. I have modified the secondary on a couple of toroids, but now I have to rewind one toroid completely. I know that you can measure/estimate the windings on a ready made toroid, but so far those I have measured have differed completetly, and far away from the E-I specs (much more number of turns). I know that manufacturers try to drive the cores to their limits in toroids, maybe that is why some toroid transformers have so many turns (for instance one 300 VA, 230V primary, 12 V secondary, toroid had about 930 primary turns)
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Old 27th September 2001, 11:33 PM   #2
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Join Date: Jul 2001
Location: Orleans, France

The answer is not simple at all. Generally speaking, the formula is N = A/S where N is the number of turns for 1 V, S is the area, and A is a parameter which depends on the core material (and on the geometry). A is given by the manufacturer of the core.

If the material is not known, you can try to rewind one toroid whith many turns (primary only) and test the temperature of the transformer after 1 hour. Then, you have to wind off the turns little by little until the temperature becomes excessive...

If you have at your disposal a variable transformer, it's better to make a winding, and test the maximum admissible voltage, and then calculate A.

Regards, P.Lacombe.

[Edited by P.Lacombe on 09-27-2001 at 06:40 PM]
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Old 28th September 2001, 08:44 AM   #3
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Join Date: Sep 2001
Location: Finland, Ostrobothnia
Thanks for the answer. Ok it seems not to be a simple procedure. Better to reuse a transformer's existing primary windings.
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