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Re: [APD] Cooling fans for hoods...

The PC fan controllers are generally one or the other of
two types and either will work on most DC fans. One type
simply inserts resistance and the amount of resistance is
controlled by a knob or a thermistor controlled circuit. As
you add resistance in series with the fan motor, the
voltage across the fan motor drops -- as the voltage drops,
so does the fan speed. 

The other type, also controlled either by knob or a
thermistor circuit is a pulse width modulator (pwm). The
pwm, basically, lops off the peaks from the halfwave
voltage or changes the wave shape of a DC signal to a
sqaure wave and widens or narrows the troughs and tops.
This type can be more energy efficient but it can also make
a fan sound chirpy.

In any event, whatever fan you use will have a minimum
voltage required for it to spin. If you drop below that,
the fan won't move but will still draw current.  A 12V fan
can usually operate down to at least 7.5V, some go lower
some need 8V or 9V.

--- Derek Parr <derekparr at earthlink_net> wrote:

> i'm pretty sure i've seen temp controllers that plug
> inline between the 
> psu and the fan sold at online computer overclocking
> shops.

* * * * * * * * * * *
Opening both barrels at once:

The Grand Opening 

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