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Recant of using 3 Terminal regulators for fan control

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Inactive Moderator
Feb 13, 2001
Twin Cities
Several days ago, I embarked upon composing an article describing how to implement commonly available 3 terminal variable voltage regulator ICs to control the speed of your heatsink fan. I purchased the components, photographed each step of assembly, etc. What emerged was a beautiful piece of work, but in the end, not an acceptable substitute for the commonly available rheostat. In the design phase, i forgot a simple tenet of 3TRs. Because of the 1.2V reference source in them, the maximum voltage they can deliver at the output is the source voltage minus 1.2V. I was used to designing around a source voltage that was more than the desired output voltage. Here's the barb. That last 1.2V often accounts for a great deal of the fan RPM. That is to say, the process is not linear. Using my own Delta 68 cfm fan as a test bed, going from 10.8V to 12.0V made a great deal of difference.
So, if your HS fan delivers more cooling than your setup needs and you want to reduce the velocity/noise, I recommend using a rheostat. Check at the local electronics surplus warehouse and you may find one for a very reasonable price. I found a whole case of new, 35 ohm units that are great for this and they were only $2.50 each. Yes, you're wasting a few watts of 12V power using the rheostat, but for non-tinkerers, it is a much easier solution. Here's a picture of the 3TR fan control assembly. Worked great!
Ive tried 2 places today in the UK. Maplin and a small electronics shop near me that has an entire 20ft wide by 14ft high wall covered in a shelving unit made up of those tiny 4" x 2" plastic drawer...madness, sheeeer madeness.

I digress...nehooo none of them stocked Rheostats, i was very surprised as people on these posts seem to find bags off them lying around. Now the fella in the crazy shop 'o' shelf advised me that i would be better off making a variable voltage regulator...but this is what you did aint it? And you say you lost the last 1.2v which equated to quite a few rpm.

Soooo is there any other solution, and if not (i am a very basic electronics tinkerer) does anyone know where i can get rheostats in the UK?