I wish I'd known how difficult controlling a 12VDC fan was;
I wouldn't have done it. I have two fans, fed by a self
designed and built controller. An IC sensor (output volatge
linearly related to temperature) feeds a servo circuit
(separate one for each fan) that keeps it running at the
lowest speed that will produce 50 degrees C at the power
amplifier heat sinks (I have two Aragon Pwier amps, 2 and 3
channels respectively, that sit in an enclosed cabinet).
Cogging is prevented by starting each fan only when its
drive voltage is greater then 6V. The circuit is simp.icity
itself, (3 ICs).
This have been working reliably for a few years now and
shows no signs of difficulty. The amps never overheated. The
fans are standard 12VDC fans, chosen for lowest noise. They
don't know how difficult what they are doing is, either.
-- Ron
On Mon, 11 Aug 2003 19:34:02 GMT, "Trevor Wilson"
wrote:
"Arny Krueger" wrote in message
...
"Trevor Wilson" wrote in message
**12 Volt DC fans are difficult to control.
Simply not true.
**I'll amend that to: MOST 12 VDC fans are difficult to control.
They have internal circuitry which regulates the speed.
Nope. Some have tachometers, but any speed regulating circuitry involved
is
not in the fan.
**Oh, yes it is. Modern 12VDC fans use an internal oscillator, driving a 3
phase, brushless motor. The only proper way to control the speed is to vary
the frequency of the internal oscillator. That is a difficult thing to do,
given that the circuitry is embedded.
Varying the input Voltage, is not very successful.
It's very successful. There are even inexpensive products that make
controlling them literally plug-and-play. This is an example:
**I did say: "Not very successful." It is possible to vary the speed within
certain limits. Cogging is common with primitive Voltage controls on such
fans.
http://www.computergate.com/products...?prodcd=BCFZFM
It's just a variable voltage regulator with support circuitry and heat
sink
in an insulating case, with plug-and-play wires attached. Often available
for as little as $4.95 $US. One of many products that are available for
the
purpose. IME very effective.
**I call "effective" a speed control which can vary the speed from (say) 10%
of max RPM to 100% in smooth controllability. You can't do it with a Voltage
regulator.
The rule of thumb is to keep the applied voltage above 7 volts. This
allows
almost a 2:1 adjustment range which is more than enough for a vast number
of
purposes. Take a fan that spins at 5,000 rpm and knock it down to less
than
3,000, and the noise decrease is impressive. Another rule of thumb is to
avoid using series resistors.
**That ain't decent speed control. I use 240VAC fans, which I can control
down to around 120RPM, all the way up to several thousand. You can't do that
with simple Voltage regulators and 12VDC fans.
I covered most of these points (except the Fanmate) in a post yesterday
morning. Did it get lost in the bowels of Usenet?
**Probly. My current newserver is brutal. 24 hours retention, if I am lucky.
Often it is less than that.