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Old 28th April 2004, 07:50 PM   #1
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Default Ac contents in dc PS?

Hi!
once again I humbly ask U nice guy´s here...:

I´ve heard that there is a certain amount of AC in a fully regulated dc-powersupply...how does that work? and what should one do to minimize it?

Hope my question isn´t to weard this time...

Thanks again!

/Peder
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Old 28th April 2004, 11:07 PM   #2
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Your question is too broad to answer easily....it is basically like asking "How does a DC power supply work?".

Start by reading:

http://sound.westhost.com/power-supplies.htm
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Old 28th April 2004, 11:37 PM   #3
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Well, something has gone wrong here...That´s not my question...how a ps work I do know, but when one want to get real serious about it we come to another level...and that´s where my q gets in...

Hope I didn´t offend anyone here...


/Peder
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Old 29th April 2004, 02:08 AM   #4
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peter

this is my opinion (and i may be wrong) BUT, i think i understand what you are asking about..it is called ripple..you are right that there is a very small amount of AC superimposed on PS DC outputs...this is usually twice the rectified frequency (120Hz) in the case of linear PS...in the case of switchmode, this will be much higher...linear PSU's are much less susceptable to this than SMPS...usually the ripple should be less than 1% of output voltage
...filters are the only way to minimise this...i dont think you can completely eliminate this without the use of sophisticated filter topologies
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Old 29th April 2004, 02:34 AM   #5
HDTVman is offline HDTVman  United States
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Default Re: Ac contents in dc PS?

Quote:
Originally posted by PederBass
I´ve heard that there is a certain amount of AC in a fully regulated dc-powersupply...how does that work? and what should one do to minimize it?

If a regulated DC power supply is properly designed and constructed there will be no AC ripple worth worrying about on the output. A poorly designed or overloaded regulated power supply may have AC ripple at the output.

Later BZ
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Old 29th April 2004, 08:09 AM   #6
Eva is offline Eva  Spain
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There is no analog circuit able to provide infinite attenuation to any finite frequency present on any of its inputs/outputs

This means that if you power a DC regulator from anything other than DC, then the frequencies present on the input will be also present on the output with a certain level of attenuation

This is true for every analog circuit, including opamps, power amplifiers, regulators, etc... They all show on their outputs a limited amount of the AC present on their power supplies

The relationship between the AC components present on the power supply and the same AC components present on the output is called 'Power Supply Rejection Ratio' and expressed in dB

Anyway, careful design may provide a PSRR high enough to reduce the AC residuals present on the output below the noise floor [1V of AC in a power supply may be troublesome, but after 100dB of attenuation it's no longer a problem]
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Old 29th April 2004, 10:25 AM   #7
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Default Re: Re: Ac contents in dc PS?

Quote:
Originally posted by HDTVman

If a regulated DC power supply is properly designed and constructed there will be no AC ripple worth worrying about on the output. A poorly designed or overloaded regulated power supply may have AC ripple at the output.

Later BZ

unless you have a perfect power supply, you are going to have ripple in the output.

but has anyone seen a perfect power supply?

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Old 29th April 2004, 03:45 PM   #8
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Quote:
Originally posted by PederBass
Well, something has gone wrong here...That´s not my question...how a ps work I do know
Sorry, I still have to disagree, you cannot say you know how a PS works and then discuss this shocking discovery that there is finite AC present in the DC. I am not trying to be negative here, just that my advice is you do some more reading before trying to decide specifics of how to tweak a PS.
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Old 29th April 2004, 07:15 PM   #9
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Well this was nice!

At last I got my suspicion confirmed, that it was a question of superimposed ripple of some kind...

Thank´s a lot to U who could understand my q....and gave me something to think about!

and to Leadbelly...keep up the work....maybe someone will be impressed...


/Peder
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Old 29th April 2004, 08:10 PM   #10
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Quote:
Originally posted by PederBass
and to Leadbelly...keep up the work....maybe someone will be impressed...
LOL
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