Fans these days are quiet - that problem has been solved. Trouble is that when there are a load of grilles, cables, drives and other stuff in the path of the air from the fan the turbulence caused often creates more noise than the fan.

A quick look around and you can buy all sorts of fan controllers with pretty coloured lights, flashing lights even, digital readouts etc., but even the simplest controller is often overkill and relatively expensive.

DIY Option

A quick scrounge through components I had kicking about and here's a simple design that anyone with reasonable soldering skills and who understands basic electronics can cook up.

It's basically a pot to set the input voltage with an emitter-follower buffer to drive the fan. Trivial stuff and only takes a few £ of bits to build.

What you need

• A fan adaptor cable to chop up
• A ~10K trimpot
• A ~10K Resistor (low power)
• A general purpose low-power NPN transistor with gain >100
• A general purpose medium-power PNP transistor with gain >20
• A general purpose diode like 1N400x
• A ~47µF Cap
• A small piece of aluminium to act as a heatsink/substrate
• Some heatshrink tubing

Schematic

The basic idea is this:

I used BC547 and MJE350 transistors but there are loads that will do the job.

The cap provides a bit of output decoupling in case some fans out there draw significant gulps of current in a short time, and the diode provides a discharge path and helps avoid reverse biasing of the emitter-base junction of the NPN transistor during power-down which will cause it to degrade over time. In many cases leaving out the cap and diode will probably make no noticeable difference and are just playing safe.

Heat

Many fans draw about 100mA or so and if you are dropping any reasonable voltage across the transistor then it is going to warm up. With 6V/100mA output there will be 600mW dissipated. Not a lot, but it will run hot without a heatsink.

If you have a more power hungry fan or multiple fans then you may find it gets dangerously hot.

Playing safe I cut a small strip of scrap aluminium I had to help dissipate the heat over a larger area so it barely gets warm.

Assembly & Testing

I started off with the medium-power transistor bolted to one end of the aluminium strip and assembled everything "rats-nest" style.

To test I used a bench supply (with current limit set) and a meter and once I was happy that it was all working as expected with no shorts or any other problems I connected it up on the fan cable and gave it a quick test in a PC.

Once I was satisfied it was all good I blobbed some glue on the circuit to stop any knocks from shorting bare wires together and then used heatshrink sleeving over the lot to protect it.

One thing go be careful of is that you make sure that there is enough voltage to start the fan at power up. Often fans will run down to much lower voltages once spinning but can struggle to start up on low voltage. Make sure you have plenty of margin on the fan starting up as with wear startup may become more problematic.

Results

My one works great and has allowed me run the fan slowly enough that there is no obvious airflow noise with only a 3°C temperature increase (without the fan it would be more like 20°C hotter).