# resistor rating help

Discussion in 'General Electronics Discussion' started by screwball, Jan 9, 2012.

1. ### screwball

89
1
Jan 9, 2012
Hello,

Im using a 100ohm resistor which is 1/4 watt max
the power i am using is 6v DC Mains and i am powering a 0.25amp PC fan,

is the 1/4 watt max enough or does it need to be higher?

Gary

2. ### jackorocko

1,284
1
Apr 4, 2010
What is the purpose of the resistor?

Last edited: Jan 9, 2012
3. ### screwball

89
1
Jan 9, 2012
purely to reduce the current so the fan doesnt rotate as fast

4. ### jackorocko

1,284
1
Apr 4, 2010
I believe it is the voltage that you will be reducing to control the fan speed. With that said, you will need to figure out the voltage drop across the resistor to figure out it's power requirements.

I am not sure I know how to figure that value when motors are involved.

5. ### screwball

89
1
Jan 9, 2012
it doesnt matter whether its voltage thats reduced or amps as both work in parallel which ohms law states,

i know 100ohms is what resistance resistor i need, i was wondering if a 1/4watt one is suffice for a fan using 0.25amps

6. ### Resqueline

2,848
2
Jul 31, 2009
Ohms law states:
Max U = square root of (P * R) = sqr (0.25W * 100Ω) = 5V
Max I = U / R = 5V / 100Ω = 0.05A

7. ### screwball

89
1
Jan 9, 2012
does this mean i can use this resistor or not?

i dont care what speed it will give me as ive already tried and it didnt blow but im just curious that its heating up ever so slowly and could eventually blow (obviously itll eventually blow from being used for days and days and days but you know what i mean) or something?

thanks for all the help up to now anyway

8. ### Resqueline

2,848
2
Jul 31, 2009
I don't know what fan you have there or how it behaves voltage/current/speed-wise.
All I can say is how much the resistor can take; and it can take 5 of the 6 Volts you have, leaving only 1 Volt (& 50mA) for the fan.
Fans usually won't start or run at 1V, so if the fan runs then chances are the resistor sees less than 5V and so it'll certainly survive.
Resistors that small reaches thermal equlibrium in half a minute or so, so if it doesn't turn brown in a minute there's no reason to worry.

9. ### screwball

89
1
Jan 9, 2012

I see what you mean now more clearly, i will leave it on for 10minutes to triple check it will be okay before installing it

thankyou

1,087
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Dec 13, 2010