First of all, this is for a PC style axial fan operating on DC power! I am assuming that you already know how to wire up a PC fan to a power supply. If you don't, just search around the city here as there are plenty of great step by step guides already posted on the procedure and its very simple, so it shouldn't be necessary for me to explain it again. To keep things simple, in this example I will be working with the specs of my own fans, which are listed below: - Fan Specs - Rated Voltage : 48 VDC Operating Voltage Range : 24~55.2 VDC Starting Voltage : 24 VDC (25℃ POWER ON/OFF) Rated Speed : 4000 RPM Â± 10% Air Delivery : 200 CFM Static Pressure : 0.70 Inch-H2O Rated Current : 0.39 AMP Rated Power : 18.7 WATTS Noise Level : 62 dB(A) Please be sure you have the proper specs of your own fans!! Don't assume anything, KNOW FOR FACT what you are working with! This can be quite dangerous to leave running unattended if you don't do it properly! So if you want to DIY a speed controller for your PC or axial style DC current fans, here are the calculations you need to make to do it safely and ensure it will last without burning out your components... -Use Ohm's law to figure your resistance in ohms- Resistance = Voltage/Amps So in my case, using the specs I posted before, you take the high end of your operating voltage range , which is 55.2v and divide that by your fan's rated current which is 0.39a and that will look like this.. 55.2 / 0.39 = 141.53846 ohms <-You're minimum amount of resistance required in Ohms. Next you will need to look at the combined wattage of your fans that will be running on the circuit, which should already corrispond with the wattage of your power supply. If you don't know the watts used you can figure it like this: Watts = Amps x Volts So for me thats: 0.39a x 48v = 18.72 watts So in my plans I am going to run one speed controller for 5 of my fans. Thats 5 fans at 18.72 watts each, so.. 5 x 18.72 = 93.6 watts So, this means that I am going to need a variable resistor such as a rheostat or potentiometer rated to support AT LEAST 141.53 ohms and 93.5 watts. When shopping for these resistors you won't find these exact figures so we will go ahead and round them up, NEVER ROUND THEM DOWN! A higher capacity resistor is fine, and will likely result in a longer life being as it won't ever be operating at its maximum load. Most of your larger potentiometers and rheostats increase capacity in increments of 25 or 50. So to keep things simple I'm going to look for a rheostat rated for 150 ohms and 100 watts minimum. Anything higher than this will also work just fine, these are only the minimums. I am actually using a rheostat rated at 100 watts and 300 ohms and it works perfectly with absolutely no problems! Simply splice your knob into your hot wire, and boom! Enjoy having absolute control over the speed of your fans! Hope this helps!! -d0p3- P.S. If you already have some potentiometers or rheostats on hand with a proper ohm rating, but don't know their wattage rating, a good test is to temporarily splice one in place, turn the knob to its max resistance, and let it run until it reaches a stable temperature, if its hot enough to cause discomfort when you touch it, then it can't handle the wattage and will burn out.