Unwinding o-core trafo to lower voltage - what´s the trick ? - diyAudio
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Old 14th August 2005, 06:19 PM   #1
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Unhappy Unwinding o-core trafo to lower voltage - what´s the trick ?

I´m trying to make a 24 V trafo from my 30 V one. The principle is clear - unwind wire until matching required voltage. But I get 23,6 or 24,4 volts and nothing between; not able to match exactly 24,0 V . When wire ends on top of core = 23,6 V. 3/4 winding later = 24,4 V. Do I have to cut the wire inside of the core ? What´s the trick ?????

The trafo is a fat 1000 VA 230 prim. with 4 secondaries.

Thanks a lot ...
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Old 14th August 2005, 06:25 PM   #2
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I wouldn't really dispair about that 0.4 V. Fluctuations of voltage in your AC line (coming from the wall) may bring even higher error margin.
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Old 14th August 2005, 06:26 PM   #3
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And it will drop with loading, so aim a bit higher.
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Old 14th August 2005, 06:30 PM   #4
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The easy solution is to calculate the drop according to the regulation, and by all means use a true RMS meter.

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Old 14th August 2005, 06:30 PM   #5
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At least where I live, mains voltage can change enough every few minutes to vary that much! If 24V -*Exactly*- is needed may be better staying with 30V and building a regulator to bring it down to 24.

Anyway I don't think there is a "trick". If you were an manufacturer making thousands of these you would also have the option to change the number of primary turns so as to get more nearly the right ratio. In your case you only can change the primary.

I don't see any reason why fractional turns wouldn't work except that directing the wire away may result in an awkward configuration with respect to mounting.
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Old 14th August 2005, 06:31 PM   #6
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So my target voltage is 24,4 Volts.

THANKS !
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Old 14th August 2005, 06:32 PM   #7
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Quote:
Originally posted by Peter Daniel
I wouldn't really dispair about that 0.4 V. Fluctuations of voltage in your AC line (coming from the wall) may bring even higher error margin.

Sure measuring the 230V AC to see if you actually have 230V is a great beginning.

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Old 14th August 2005, 06:44 PM   #8
sam9 is offline sam9  United States
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Quote:
Sure measuring the 230V AC to see if you actually have 230V is a great beginning.
Measure several times over a period of hours and days. If it is always the same - lucky you!

Word of warning (apart from checking mains voltage in a safe manner) -- don't let yourself get too fascinating with exploring the nature and quality of mains voltage. Somethings are better not to know. I made this mistake -- discovered the mains DC offset, discovered the spectra, even looked at the current wave form. Now I'm majorly depressed. It's a small miracle that even the doorbell works!
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Old 14th August 2005, 06:57 PM   #9
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Quote:
Originally posted by sam9


Measure several times over a period of hours and days. If it is always the same - lucky you!


Now I'm majorly depressed. It's a small miracle that even the doorbell works!

Exactly, don't measure over longer periods.....it will only make you depressed, and what you really need to know is the voltage here and now, in order to calculate what the output should be in order to be 24V at 230V.

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