Tempreature sensing fan - ideas?

[LektroiD]

I have two Mackie 2U rack mount power supplies for the mixing desk in my studio, and the fans inside are driving me insane.

I have reduced the speed of the fans by using a few diodes in series, the voltage drop of the diodes has been enough to quieten them, but they really only need to come on when the heatsinks are warm. What is the easiest (lowest component count), way of making a temperature sensing fan?

I was thinking of using a flatflex style CPU temperature sensor affixed to the heatsink with kapton tape in series with the positive feed to the fan. Would this work, or can someone think of a better idea?

[kripton2035]

something like this : http://www.ebay.com/itm/1pcs-Temperature-Controlled-Switch-Thermostat-80-C-N-C-KSD301-Normal-Close-10A-/111354130613?pt=LH_DefaultDomain_0&hash=item19ed3930b5

[mushroom]

You will find many example of such a controller (temp sensor + PWM or linear) with Google. Many have been built since late '90s. Google with "temperature based fan control" ; Google Images will shows you a huge amount of solutions. In the old times, it was analogic control. Now, it is very easy to build around an Arduino Pro Mini or Nano.

https://www.google.com/search?q=temperature+based+fan+control&source=lnms&tbm=isch&sa=X&ei=So9DVeSZApWwaeTsgNAK&ved=0CAcQ_AUoAQ&biw=1242&bih=742

There are also specialized ICs (used on mobos and graphics cards)

[dom0]

Lowest part count: Si-diode (1N4148) + power transistor + resistor. Select the diode such that it's Vf is slightly below the transistors Vbe. Put it in parallel with the BE junction. Supply current to the base node through the resistor. If the transistor gets warm it's Vbe will drop, leading to it conducting.

Yes. This even works.

A more serious approach: Use an emitter follower fed by a voltage divider with a thermistor/NTC in the upper leg. The thermistor gets hot beyond it's thermistor temperature and it goes low-resistance rather quickly, leading to the fans spinning up.

(Actually, still just three parts. But I have a few hundred times more diodes than thermistors in my stock, so OTOH...)

[daybyter]

http://www.dx.com/de/p/my-428-ksd9700-thermal-sensor-temperature-control-switch-for-fan-black-golden-246147#.VUOkxcu-eYo