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Ac contents in dc PS?
Ac contents in dc PS?
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Old 29th April 2004, 09:17 PM   #11
jan.didden is offline jan.didden  Europe
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Originally posted by PederBass
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...

...which of course shows that Leadbelly was right on target, that you don't really understand power supplies. Missed chance for you, really. Pity.

Jan Didden
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Old 29th April 2004, 10:30 PM   #12
PederBass is offline PederBass  Sweden
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Yes, indeed do we both agree.....
But...In my humble opinion there is a certain difference in REALLY understand something to be adviced to read the basics where, IMHO there were no mention of the things I was asking about..

I do really try to read so many info I can BEFORE posting here..but if the answer here is supposed to be something like: "read that and that BASIC page before asking us...("Learn the alphabet before tryin to set together a sentence") then I will stop asking here at once...sorry for taking up ur precious time...

Most humbly....


BTW...We must understand that some times the language stops us from seeing that some post are just humourus...
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Old 29th April 2004, 10:53 PM   #13
roddyama is offline roddyama  United States
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There are members with all levels of knowledge here on our forum. When you ask a question and it is not necessarily clear what your level of knowledge is, than your going to get answers that reflect that lack of clarity. That doesn't mean that the member who was gracious enough to answer your question was trying to insult you. You should be more respectful to anyone who is generous enough to answer your questions.
Rodd Yamashita
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Old 29th April 2004, 11:01 PM   #14
PederBass is offline PederBass  Sweden
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Oh dear sir...

I have full respect for everyone here that gives me an answer, no doubt about that!

But...instead of giving an answer in a "general matter" and trying to guess my kind of knowledge, why not try to pinpoint the real question? I mean, it´s more interesting to read about some elaborating about power supplys than a question of my knowledge or lack of knowledge..right?


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Old 30th April 2004, 02:18 AM   #15
slowhands is offline slowhands  United States
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Default Re: Ac contents in dc PS?

Originally posted by PederBass
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!

There will always be a small measurable amount of line noise and random noise on the output. A regulator uses negative feedback to reduce the line noise by a large percentage. This only attenuates line noise, it does not perfectly cancel it (to cancel it, you would need positive feedback, and it would oscillate like a banshee). That's just reality.

You need to determine what level of line regulation your load needs, and design accordingly. This is easily done, so don't worry about it too much. Only front end audio circuits need regulation, and their need depends on the exact design. Typical opamps have high power supply rejection ratios, to make them easy to use. If you have a discrete design, you are responsible for power supply noise rejection. For instance, that's why differential amps and current sources are used so much --- they have inherent high noise rejection.
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Old 30th April 2004, 02:47 AM   #16
HDTVman is offline HDTVman  United States
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Default Re: Re: Re: Ac contents in dc PS?

Originally posted by Eva
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]

Originally posted by millwood

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

but has anyone seen a perfect power supply?

I think it would be a good idea if you two gentlemen would read more carefully before slamming someones post. If you had read carefully you would noticed the words (worth worrying about) used when describing the amount of ripple on the output of a well designed regulated power supply. I'm sorry I didn't put down a number for you but the design of the power supply and the load demands all come into play here. So you see an exact answer is not possible.

By the way I have used many Linear regulated power supplies that I would really like to see you measure the ripple on the output. It would be interesting to hear what you said after you tried. Some as low as a few uV at rated load. Like I said "not worth worrying about".

Later BZ
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