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Tapping a main toroidal
Tapping a main toroidal
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Old 7th May 2011, 10:23 PM   #1
akis is offline akis  United Kingdom
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Join Date: Jul 2008
Default Tapping a main toroidal

I have an 80VA 2x25V / 1.6A mains toroidal which I am planning to use for a simple bench PSU. The panel instruments need 5V and so I thought of winding some enameled copper wire around the toroidal aiming for 5V DC under moderate load (big LED segment displays).

I pulled out a random length of wire and wound it around, did not count the turns, and measured approx 3.8V DC after rectification + filter. I unwound the wire, measured the length, and then cut a new piece twice as long. This gave me 4.2V DC. I cannot understand this. I pulled it out and re-wound it the other direction, much more carefully and more evenly than before, and measured 4.4V DC.

How can double the length of wire (hense double the turns) not give me double the voltage?
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Old 8th May 2011, 02:51 AM   #2
phaselag is offline phaselag  United States
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Join Date: Nov 2010
Location: N.E. PA
First measure the ac voltage, not dc, for comparison.

Second it is the number of turns, not the length. The circumference may not be constant.

paul
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Old 8th May 2011, 07:42 AM   #3
akis is offline akis  United Kingdom
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Join Date: Jul 2008
I added another length and I have gone over 5.5AC. Happy now.
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