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Voltage puzzlement
Voltage puzzlement
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Old 4th February 2018, 02:44 PM   #1
bluerooster is offline bluerooster
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Join Date: Dec 2017
Default Voltage puzzlement

Interesting finding in my HV supply. A 400-0-400 transformer, 83 full wave rectifier, 100mfd reserve cap, 8H input choke, 100mfd filter cap.
At the transformer I measure 413vac, at the rectifier, I get same. after the pi filter network, I get 585v DC A voltage increase of about 172v. I would have thought it should be closer to about 390, accounting for voltage drop accross the twin diode, and resistance of the choke..

Oh, never mind. It just dawned on me. After the pi network, I'm getting peak voltage, not RMS. Due to the filter caps. So under load, it should be about what I need.
413RMS*1.414=585peak. (just round figures) Or am I wrong?
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Old 4th February 2018, 04:17 PM   #2
DF96 is offline DF96  England
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A capacitor input PSU gives you the peak voltage. You will get some voltage drop when you draw DC current from it.
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Old 5th February 2018, 03:02 AM   #3
PRR is online now PRR  United States
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You won't get much drop from that heavy-metal rectifier.

As you have not said your load or PT or choke DCR, we can't guess what drop you will get.
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Old 5th February 2018, 10:16 AM   #4
AndrewT is offline AndrewT  Scotland
Join Date: Jul 2004
Location: Scottish Borders
A choke input PSU gives a loaded DC output voltage ~ 90% of the secondary Vac.
A capacitor input PSU gives an unloaded DC output voltage of ~141% of the unloaded secondary Vac.

If you reduce the output current of the choke input PSU you will find that voltage rises towards that you would get from a capacitor input filter.
i.e. the low current output voltage can be 60% higher than intended.
All the downstream components need to be selected to survive this overvoltage situation.
regards Andrew T.
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Old 5th February 2018, 12:07 PM   #5
bluerooster is offline bluerooster
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Join Date: Dec 2017
Thanx for all the input ya'll.

After running it through a PSU simulator, I think I have it sussed. I'll be making a few changes today, and will report back here later.
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