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Old 26th November 2011, 07:34 AM   #1
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Question Transformer power question

I have a tx dual secondary 9V 30VA (15VA per winding so total power 30VA) & I need to know if the total power configured in series to have 18V is 30VA or 15VA?
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Old 26th November 2011, 07:46 AM   #2
Mooly is online now Mooly  United Kingdom
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Both the total VA rating and the individual ratings must not be exceeded.

So 9 volts at 15VA is 1.6 amps per winding. (15VA).
In series you have 18 volts and can still only draw 1.6 amps (total 30VA)

In parallel you would have 9 volts at 3.2 amps available (30VA)
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Old 26th November 2011, 07:50 AM   #3
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Thank you Mooly
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Old 26th November 2011, 01:48 PM   #4
DF96 is offline DF96  England
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VA assumes resistive load. If you are rectifying and smoothing it you need to take into account RMS current and duty factor. In some cases you could end up back in the same place, but for high duty factor (e.g. Class A amp) you might need to derate by a factor of three or four.
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Old 26th November 2011, 01:59 PM   #5
AndrewT is online now AndrewT  Scotland
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yes, DF,
Many seem to be unaware or forget about the de-rating factor that transformer Manufacturers specify for a capacitor input filter in lieu of resistors passing AC current.

The easiest way I remember the approximate DC current available from a rectified and smoothed supply after applying the de-rating factor is:
Maximum Continuous DC output current = 50% of the rated AC current.
This takes account of the extra resistive heating due to IsquaredR losses being much greater for high crest value currents, cf resistive loaded AC windings.
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