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# Which resistor should I buy

[From: ] [author: ] [Date: 11-05-18] [Hit: ]
You need to use a voltage regulator like a LM317. They are cheap and very easy to hook up and give you the option of varying the fan speed as well.http://www.national.com/mpf/LM/LM317.An even better option would be to use a 12V power source.......
I have a 12V 0.14A pc fan which will be connected to a 35V power supply. Which resistor do I need to run the fan at 12V and and which resistor to run it at 7V (so it's more quiet).

Thanks for any help.

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That is not a good way to reduce the voltage to the fan. You are wasting energy in those resistors and the resistors will get hot.

You need to use a voltage regulator like a LM317. They are cheap and very easy to hook up and give you the option of varying the fan speed as well.
http://www.national.com/mpf/LM/LM317.htm…

An even better option would be to use a 12V power source.

If you still want to use resistors you need to use ohms law.

to run the fan at 12V you need a voltage drop over your resistor of 35 - 12 = 23V
with I = 0.14A then R = U/I = 23 / 0.14 = 164 ohm

to run the fan at 7V you need a voltage drop over your resistor of 35 - 7 = 28V
with I = 0.14A then R = U/I = 28 / 0.14 = 200 ohm

The values above are for the resistors in series with the fan.

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go get a regulator IC or use voltage drop circuit
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