Basic question about LM317 - diyAudio
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 28th January 2005, 04:03 AM #1 diyAudio Member     Join Date: Dec 2002 Location: Calgary, Alberta Basic question about LM317 When using the LM317 with a voltage divider for the R1 (OUT to ADJ) and R2 (ADJ to GND), I understand pretty well that Vout = 1.25 (1 +R2/R1) and that current through R1 must be at least 3.5 mA, and that is how you select R1 and R2. However, when using a zener to float the regulator for higher voltage applications, how does one select a value for R1 (OUT to ADJ)? Is it a simple V=I*R where I is 3.5 mA and the diode is in forward bias and so its R is negligible? Just want to be sure.... __________________ The whole problem with the world is that fools and fanatics are always so certain of themselves, and wiser people so full of doubts. Bertrand Russell
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Join Date: Jan 2005
Location: Hong Kong
Quote:
 However, when using a zener to float the regulator for higher voltage applications, how does one select a value for R1 (OUT to ADJ)? Is it a simple V=I*R where I is 3.5 mA and the diode is in forward bias and so its R is negligible? Just want to be sure....
If you want to use zener diode for high voltage application, I suggest you first calculate the R1 and R2 base on normal method. Afterward, you connect one end of zener to ground (OF course), the other end of zener to R2 that is originally grounded(refer to national datasheet). Connect a new added resister (R3) between Vout of LM317 and the zener.

To make sure the zener work, passing 20mA to it is a good starting point, which tells you the value of R3.

Hope this help
Scottie

 4th February 2005, 06:53 AM #3 diyAudio Member   Join Date: Sep 2004 Location: Ohio Check out the data sheets from National, On Semi, etc. There's plenty of info about this at their sites.

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