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Old 28th January 2005, 04:03 AM   #1
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Default 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....
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Old 28th January 2005, 06:47 AM   #2
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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
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Old 4th February 2005, 06:53 AM   #3
d3imlay is offline d3imlay  United States
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Check out the data sheets from National, On Semi, etc. There's plenty of info about this at their sites.
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