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Old 5th July 2013, 08:00 PM   #1
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Wink Question About Transformer Voltage Output

Good evening everyone,

I am putting together a small DC power supply with simple bridge rectifier and smoothing caps.

The traffo I got is rated at 100VA - 30V
this one from Farnell
RK100/30 - BLOCK - TRANSFORMER, 100VA, 2 X 30V | Farnell United Kingdom

When connected to the mains (230V in UK), instead of 30VDC at the output of the capacitor's bank, I get around 44VDC?


I have tried with the secondaries in parallel, and with just a single secondary...
Click the image to open in full size.
Same result...

Am I missing something?
(Please don't be too harsh in case I did, as it is my first serious foray into PSUs.)

Thanks for your enlightened help everyone!

Cheers

Francois
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Old 5th July 2013, 08:39 PM   #2
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30V is the RMS voltage of the transformer secondary when it is delivering 100VA.
The capacitor bank is charging to the peak voltage of the transformer secondary, minus losses (eg, in the rectifiers and connecting wires).
The circuit is functioning as it should.
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Old 5th July 2013, 08:46 PM   #3
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30volts is only 0.707 of the peak AC voltage.
So 45 volts is about right.
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Old 5th July 2013, 08:52 PM   #4
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30V is the AC voltage. You will get approximately 1.414 x 30 volts after rectification, you will lose the 0.7V of the diode drops but at 30V that's almost insignificant.
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Old 5th July 2013, 09:50 PM   #5
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Oh, I see...

I should have checked my theory books first!

I does make sense now.
Thanks everyone

F
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Old 5th July 2013, 10:33 PM   #6
AJT is offline AJT  Philippines
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load your psu and the voltage will sag...guaranteed....
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Old 6th July 2013, 09:42 AM   #7
AndrewT is offline AndrewT  Scotland
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Output voltage ~ Mains voltage / rated primary voltage * rated secondary voltage * [1+regulation].
For a 230:30Vac transformer of 7% regulation expect when used in the UK to give ~ 240/230*30*(1+7%) ~ 33.5Vac.

The peak of the sinewave will be sqrt(2) * Vac = 47.36.
From this you need to subtract the voltage drop (Vf) of the bridge diodes. say 1Vf for a no current load and the rectified output will be ~ 46.4Vdc from your 230:30 7% transformer.

The maximum voltage using the UK's 254Vac will be ~ 49.1Vdc. Just about the absolute limit for {Vdc + V interference} of a 50V capacitor.
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