From: John Woodgate
Subject: Re: LM317 or 7808 is better performance for 9V output ?
Date: Tue, 10 Dec 2002 18:43:54 +0000
Organization: JMWA Electronics Consultancy
Reply-To: John Woodgate
NNTP-Posting-Date: Tue, 10 Dec 2002 19:24:26 +0000 (UTC)
X-Newsreader: Turnpike (32) Version 4.01 <5Z8C9wtxbnpWyFnyfFzqmVF739>
I read in sci.electronics.design that Winfield Hill
wrote (in ) about 'LM317 or 7808 is better
performance for 9V output ?', on Tue, 10 Dec 2002:
>Anyone using a LM317 who's concerned with noise will certainly add the
> suggested filter cap from the ADJ pin to ground; You left this important
> part out of your drawings and analysis. The same advice goes for Bob's
> comments. Adding this cap lowers the 10Hz to 10kHz noise from the 317's
> spec'd 30ppm of Vout, to 30ppm of 1.2V = 36uV.
>By comparison a 7812 is
> spec'd with a massive 75uV of noise.
'Massive'? Just over 6 dB more.
> So there, all you LM317 nay-sayers!
> Mike, further to your various ripple-reducing schemes, you have failed to
> include my favorite highly-effective filter, which is a simple FET or BJT
> 2nd-order filter on the INPUT of the regulator. Add it just after the
> rectifier energy-storage filter capacitor, like this:
>. ,--||----------, _____
>. | | | |
>. ---+--------|---- D S --+--+---| |-----+--- out
>. | | G | |_____| |
>. | | | === | ===
>. '--/\/\--+--/\/\--+ | gnd |
>. | gnd gnd
> It can be much more effective to eliminate the problem at its source,
> rather than to struggle with it after the regulator.
What cut-off frequency do you have in mind? Something below the third
harmonic in the ripple waveform, 300 Hz (360 Hz for you)?
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
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