How many Ohms do I need to run fans at 10V?

[Roger24]

I tested my 24V fans at various voltages today, and the lowest I could get them to run at was 10V (+5 and -5), and they sounded quite pleasant at that level. Now I've heard that modding voltages like that is not safe for the PSU (I used my old Aashima PSU for these experiments). I know that rheostats are variable resistors, but I figure that when I know what level I want to run my fans at, I can get by with a fixed resistance, right?
If you need to know, here's what the fans draw:
172mm: 24V, 1A
120mm: 24V, 11W

 

[brothernod]

Resistance = voltage/current

if you know what amperage and voltage you want it to run at just use that formula.

 

[Roger24]

You mean how many volts I want to cut away? So in the case of the biggest fan it's 2V/1A=2Ohm, since I want 2 volts lower?

 

[Szech]

You got it.

 

[schoolie]

Shouldn't that be 4.8 Ohms?

Consider the Fan as a 24 Ohm resistor in the circuit. When applying 12V to the fan and treating this as two resistors in series.

12 = 24*I + I*R2
I*R2 = 2V

R2 = 4.8Ohm , I = 5/12 Amps

Am I wrong?

 

[ButcherUK]

4.8 is right, you want a 5:1 ratio of resistance.

 

[Roger24]

So I'll need 10,48Ohm for my other fan then?

 

[schoolie]

Yes

 

[brothernod]

that's right, I forgot to factor in the resistance of the fan. Guess it isn't negligible.