Re PWM, ideally choose a circuit where you can vary the
PWM frequency - Pulse Width Modulation frequency.
PAPST make 2 PWM controllers:
o PCM001
o System 3000
They are expensive, and neither have the facility to
vary frequency - which in some cases is *vital*.
Why?
Well, the frequency relies on slippage of the fan, to keep
turning whilst power is removed. That is fine, but obviously
this is a characteristic which will vary between fans & makes.
As the fan gets slower, on some the PWM frequency ends up
creating a "growl" which completely negates fan noise reduction.
For example the PCM001 does this with PAPST 6212NM fans,
a 174mm 12V fan of 220-235cfm. It growls horrendously at
lower speeds completely negating the benefit of PWM.
Similarly, the PCM001 whilst ideal for 120mm papst fans is
utterly useless with panaflo 120mm - they growl terribly.
Different bearing & weight, different slippage etc.
Many DIY circuits DO allow you to vary the PWM frequency.
Sadly a 8-pin chip did exist to do most of this, for ~2.50$US
but it was discontinued at least in Europe if not USA also.
Could have made a very good unit with variable frequency.
Just something to consider in achieving real silence.
--
Dorothy Bradbury
http://homepage.ntlworld.com/dorothy...ry/panaflo.htm (Direct Prices)