Simple fan Speed Control

Not so. A rheostat burns out if you exceed the current rating. Calculate the current rating from rheostat wattage and rheostat resistance, using the power law W = I²R, so a 3W 50R rheostat will safely carry 245mA, 3W of fans.

But a 3W 10R rheostat will carry 550mA, 6.6W of fans. And a 3W 100R only 175mA, a 2W fan. Though you can overclock wire-wound ones a bit.

So if you want to run several fans on one rheostat, you need to pick a resistance to suit as well as a wattage.

Check out Vishay's datasheets for more examples.

 

cpemma : thanks, it kinda helps me, but i do not really understand your calculations... 3W 10R rheostat will carry 550mA, 6.6W of fans. And a 3W 100R only 175mA, a 2W fan. if u ask me, i would have thought that 3w 10r will go for 635 miliwatts of fan and 3W 100R for a 6.25 watt fan... im still kind of lost, if u can tell me how much watt of fan i can put on a 20ohm rheostat that im puting on a 12 volt rail, that might help . thanks for teh post again! (if my calculations are corect, it should be able to take 1.25 watt of fans)

 

From W = I²R, for W=3, R = 10; I² = 3/10, I = 0.5477A

Fan Watts = V x I = 12 x 0.5477 = 6.573W

Overclocking by 25% is usually safe, so 8W fans shouldn't cause smoke on that rheostat. And as you turn the speed down, the current drops so even less risk of damage.

 

thanks! now i understand the calculations and my rheostat takes 20% tolerance! thank you for replying so fast, i now get o mod a bit more, hehe, got myself another rheostat too, once again, thanks alot!