Tweakers' Asylum

You may want to experiment with...

189.60.53.29

casouza

In Reply to: RE: replacing zener diode question pls posted by kalido on February 22, 2009 at 17:03:46

...5Watt zeners and lower voltage zeners in series, I have found out that these usually are much less noisy than 500 mW high0-voltage zeners.
For example, look up the 1N4100 data sheet. A 5.1 V zener has 20 times lower noise than a 15 V zener.
Also, bypassing a zener with a very fast cap will help, like Panasonic FM, Rubycon ZL or Black gate.

Follow Ups:

RE: You may want to experiment with... - kalido 04:45:03 02/23/09 (2)
hi casouza,
thanks for the suggestion.
Will it be allrite to replace the zener with something of a different voltage ? Will that not affect the regulator's output ?
ALso what cap value would u recommend ? would a 0.1u BG Hi-Q be a good choice ?
- RE: You may want to experiment with... - casouza 08:57:32 02/23/09 (1)
  Hi
  Sorry I was not more specific.
  You must stick to the 15 V spec, otherwise as you said the regulator output will fall, however it is OK to "make"a 15V zener by placing three 5V zeners in series (totem pole style).
  Also, upping the zener spec to 5W will decrease zener self-noise.
  
  Zener diodes generate a lot of noise at audio and RF frequencies. A good bypass cap is a combo of a large electrolytic (100 uF 25 V) plus some smaller cas to eat high frequency noise
  Some tweakers like a 100: 1 ratio on bypass caps, so 100 uF lytic + 1 uF film + 0.01 uF film would be great. Remember to solder the small caps right across the zener diode string, so that the caps act as a very short path to ground for the high-frequency noise.
  I hope this helps.
  - RE: You may want to experiment with... - kalido 17:09:20 02/25/09 (0)
    thanx casouza,
    this is gonna be a good reference for me all along the project !at the moment the circuit has a 470u/25v generic cap across it ....:(
    will work on that too ....
    thanks again for ur suggestions..

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