Overriding fan speed with tach/voltage leads

I discovered that when I cut the green wire, the fan is blowing even faster. This may provide some clue but I'm not sure how to -lower- the speed.

"Fpbear II" wrote in news:bF9Ch.64207$ snipped-for-privacy@newssvr25.news.prodigy.net:

The usual way to slow down a DC fan is to drop the voltage below 12v until you get the speed you want. But it sounds like you've tried that and it didn't work. What kind of equipment is it hooked up to?

On Mon, 19 Feb 2007 04:25:43 GMT, "Fpbear II" put finger to keyboard and composed:

The datasheet

You may be lucky and have the PWM speed control option, in which case you could control the speed with a 555 timer running at a 75% or greater duty cycle. I don't know what the "alarm" is, but you could find out by monitoring the voltage on the wire(s) while heating the fan, stalling it, or varying its supply voltage. This assumes that the relevant signal is not OC, in which case you would need a pullup resistor.

- Franc Zabkar

Thanks Franc. I just discovered, I was able to use the resistor method on the positive wire -after- I cut the green wire. I had to disable the thermosister before it would accept the resistor.

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.

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

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:

- Franc Zabkar

On Tue, 20 Feb 2007 21:01:49 GMT, AZ Nomad put finger to keyboard and composed:

According to the following article, a BLDC motor is essentially an AC motor.

"BLDC motors have come to dominate many applications: Consumer devices such as computer hard drives, CD/DVD players, and PC cooling fans use BLDC motors almost exclusively."

"A brushless DC motor (BLDC) is an AC synchronous electric motor that from a modeling perspective looks very similar to a DC motor. Sometimes the difference is explained as an electronically-controlled commutation system, instead of a mechanical commutation system, although this is misleading, as physically the two motors are completely different."

"Because the controller must direct the rotor rotation, the controller needs some means of determining the rotor's orientation/position (relative to the stator coils.) Some designs use Hall effect sensors or a rotary encoder to directly measure the rotor's position. Others measure the back EMF in the undriven coils to infer the rotor position, eliminating the need for separate Hall effect sensors..."

"Although BLDC motors are practically identical to permanent magnet AC motors, the controller implementation is what makes them DC. While AC motors feed sinusoidal current simultaneously to each of the legs, (with an equal phase distribution), DC controllers only approximate this by feeding full positive and negative power to two of the legs at a time."

- Franc Zabkar