On Tue, 20 Feb 2007 07:51:17 GMT, James Sweet
put finger to keyboard and composed:

Fpbear II wrote:
I have a 12V 0.8A Nidec TA450DC fan in some equipment and I need to slow the
fan down to about 75% of its speed. It has four leads: red, black, yellow,
and green. I guess that yellow is the RPM sensor and green is the high/low
voltage sensor.

I tried using a 5-ohm resistor as well as a zener diode on the red lead.
This did not work because the equipment senses something is wrong, and the
fan won't spin. To try another strategy, is there something I can place on
the green or yellow lead to trick the system into reducing the fan speed?




Pick up one of the little speed control boxes made for computers. They
plug in series with the fan and use PWM to control the speed. Any place
that sells parts for high performance PCs will have them.

Brushless DC fans use Hall effect sensors for electronic commutation.
If you must chop the supply to these types of motors, then you should
ensure that the PWM frequency is much lower than the fan's rotational
frequency, otherwise the speed control logic will be disturbed.

This old post illustrates the strange things that happen when external
PWM control is applied to a PC CPU fan that has its own internal speed
control:

http://tinyurl.com/2y9eqk

- Franc Zabkar
--
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