Fan bus Q

[jpnelson83]

Hokay, I'm making a fan bus and I have it all set up to do the 7v trick. However there are some rumors flying arround that this is not necessarily good for the rest of your components. So I've decided to do the 7v mod using resistors. now using the formula that resistance(ohms)=(supplied voltage - forward or required voltage)/Current(A). By this calculation, using a .1A fan, that I'm going to need a 50 ohm resistor to knock the voltage down to 7v. All I really want is some one to confirm that I'm calculating this correctly and that I'm not going to fry something. Secondly should I just use a 1/4 watt resistor or should I use something larger like a 1/2 watt or something? Thanks.

 

[jpnelson83]

Anybody

 

[mattg2k4]

I've never seen that formula you're using, but using the voltage divider formula Vout = Vin * R2 / (R2 + R1) and saying your fan R2 is 120 ohms (12v / .1A), a 50 ohm resistor would give you 8.47 v. An 85 ohm resistor would give you 7v.

With an 85 ohm resistor in series I would expect your current draw to be about 58ma (12v/205 ohm). Since you're dropping 5v across your resistor, 5v*.058A = .293 watts. A half watt resistor is the minimum. I would use a 2watt or larger if I were you, because that sucker is going to get really hot.

edit: I wouldn't worry about 7v'ing your fans. If it creates problems you can always undo it, and it almost always works just fine. Plus it doesn't waste power like the resistor method does.

 

[specialk]

If you are using a fan that doesn't draw too much, use some diodes to drop the voltage. DON'T use a voltage divider because they depend on a constant current draw (and constant parallel resistance with R2). You can use 7 silicon power (not signal) diodes in series to drop 7*0.7V (4.9V) bringing you to 7.1V.

Or even better, use a linear regulator (such as a LM317).

Or even better, use PWM to slow down the fan.

-special [k]

 

[jpnelson83]

ok, thanks for the input guys.