cooling fan controller for chipamp

[AndrewT]

Hi,
never mind about the power output for 99% of the time.
Two things are MUCH more important.
1.) what is the quiescent power that your sinks must dissipate?
2.) what happens to the signal/output power in the other 1% of the time?

[cpemma]

Originally posted by oldfatbubba
Can anyone comment on using this fan control circuit in an audio environment? Will it introduce noise?

http://www.rason.org/Projects/fancont/fancont.htm

One thing I don't like about it; it's a thermal switch from off to on. The step changes in audible fan noise level are more distracting than a gradual change with temperature. Which is easy to implement with an L200 regulator

And linear control will give less risk of electrical noise than switching/PWM control.

[AndrewT]

If you can hear the fan coming on over all that music, it's an audible warning that the amp is getting hot and maybe the user should check something!!

[oldfatbubba]

Originally posted by cpemma
One thing I don't like about it; it's a thermal switch from off to on.

I like the concept of a variable speed speed and love the way you've incorporated the termistor into the regulator circuit. I will definately use that circuit in something!

Keep in mind this is a bedroom amp that will be pumping out a half a watt 99% of the time. The fan noise would be distracting at that level.

I could 'merge' the two circuits so the fan initially turned on slowly and then ramped up as the sink temperature rises. That may be overkill for this application but I love the concept.

Oh, I don't think electrical noise will be a problem as the fan circuit is behind regulators.

[cpemma]

A few findings from the quest for silent forced-air cooling

• Fan noise is a mix of motor/bearing noise and wind noise; the first can be reduced by choice of fan model, the second by choice of speed;
  
  
• The fan blade's tip speed rather than the quantity of air moved has most effect on wind noise, so a large slow-revving fan is much quieter than a small high-speed fan blowing the same cubic feet/minute.
  
  To quote this SPCR article,
  

  For example, a typical 120x25mm fan spinning at 1,000 rpm can move ~40 cfm. The best quiet 120x25mm fans can do this at <22 dBA@1m. In comparison, an 80x25mm fan has to spin at some 3,000 rpm to achieve the same airflow, and its noise will be a minimum of 30 dBA@1m. Subjectively, the higher pitched tonal aspects of the 80mm fan sound at this speed will make it seem more than twice as loud. In order to reach ~22 dBA@1m level, the 80x25mm fan speed has to be reduced to ~1,500 rpm, at which point, the airflow drops to about half of the 120mm fan at 1,000 rpm.

• The 120mm fans most common in low-noise PC cooling are effectively silent (no worse than a very low ambient noise level) below about 950RPM.
  
  
• Life is compromise...

[davidlzimmer]

I have to agree with AndrewT.

Forget the numbers for a moment. If you are listening to music low in volume enough that you can hear the fans, something is wrong!

I would seriously consider larger sinks with verticle fins.

[BWRX]

Originally posted by cpemma

• Fan noise is a mix of motor/bearing noise and wind noise; the first can be reduced by choice of fan model, the second by choice of speed;
  
• The fan blade's tip speed rather than the quantity of air moved has most effect on wind noise, so a large slow-revving fan is much quieter than a small high-speed fan blowing the same cubic feet/minute.

Three main things that have an effect on fan noise are the speed of operation, number of blades, and flow obstructions. The blade passage frequency (BPF) is the frequency at which you will have a peak in noise from the fan. BPF=(RPM/60)*(# of blades) That equation basically shows that the higher the rotational speed and the more blades there are the higher the frequency (and its harmonics) will be. Our ears are more sensitive to higher frequencies, so it's best to try and keep the BPF as low as possible if you want it to be quiet.

Fan noise (particularly the BPF and its harmonics) is also enhanced by unsteady/uneven loading on the fan. This basically means you want to avoid flow obstructions at the inlet and outlet of the fan for the quietest operation. Normally a minimum desired distance for obstructions from the fan blades is about one half the overall diameter of the fan.