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 Power Supply B+ voltage?
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 6th December 2010, 08:41 AM #1 diyAudio Member   Join Date: Dec 2008 Power Supply B+ voltage? Are you able to check power supply B+ voltage without connection to the rest of the circuit? I need B+ to be 250v but getting 385v measured at he last PS capacitor. I think a resistor needs to be added but not sure where and what value.
 6th December 2010, 08:50 AM #2 diyAudio Member     Join Date: Feb 2002 Location: Lindau Hi! The higher the output impedance of your PSU, the more the voltage will rise if you test it without load. If your PSU has a choke input filter, there is an even higher voltage rise if the current draw is below critical value. In general it is good practice to have a bleed resistor across the PSU to discharge it when you turn it off. If you want to test it under normal load condittions but without the circuit attached. Use a dummy resistor to simulate your load. Calculate the resistor using Ohm's law so that it draws the same current as the circuit which will be attached. Best regards Thomas
 6th December 2010, 09:23 AM #3 diyAudio Member   Join Date: Jan 2002 Location: nowhere +1 to what Thomas said. Ideally u should load the PSU as if intended ciruit is hooked up and drawing power, though drawing some 50% of intended is probably close enough. That is still a lot of power for a resistor, so use several in series/parallel. You could perhaps use a lightbulb or two as well.
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Quote:
 Originally Posted by Vinylsavor Calculate the resistor using Ohm's law so that it draws the same current as the circuit which will be attached.
So if my amp should draw 200mA when running, and I have an unloaded B+ of 350, do I take the unloaded value or the B+ value I want to end up with?

Thanks

Charlie

 6th December 2010, 02:59 PM #5 diyAudio Member   Join Date: May 2007 The loaded value, because that is what it should be when loaded. However, except for a choke input PSU or one with large smoothing resistors, the difference won't be that great. Fortunately most valve circuits aren't too fussy about exact supply voltage.
 6th December 2010, 08:08 PM #7 diyAudio Member     Join Date: Mar 2007 Location: Auckland, NZ or just use PSUDII __________________ "It may not be easy for some to not hear differences, even if they are not there." - Vacuphile,
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Quote:
 Originally Posted by aardvarkash10 or just use PSUDII
isnt supported on macs unfortunately...

... maybe a cheap pc should be on the cards

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Quote:
 Originally Posted by losacco Are you able to check power supply B+ voltage without connection to the rest of the circuit? I need B+ to be 250v but getting 385v measured at he last PS capacitor. I think a resistor needs to be added but not sure where and what value.
Doanworrydouddit. Any PS (including batteries) will show a higher voltage unloaded than it will under load. The only reason to worry about the unloaded voltage is to avoid poofing the filter capacitors while waiting for the rest of the heaters to warm up.

VT circuits are a good deal more "forgiving" than most solid state, and so deviations from design nominal values have relatively less effect. If your loaded output voltage is within +/- 10% of design nominal, that's good enough.

If you need that 250Vdc to be accurate, then the answer is active regulation, either series pass or parallel, depending on the current range and PS capabilities.
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 6th December 2010, 09:11 PM #10 diyAudio Member   Join Date: May 2007 Power supplies often have non-linear output impedance, so measure at three points just to check whether they lie on a straight line. Typically you will find that the output impedance reduces slightly with extra load i.e. the voltage drops more slowly than you expect. This is because the charging pulse occupies a greater proportion of the cycle.

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