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Which resistor do I need?

I am using DC current to power 4 computer fans. I have 3 fans hooked up now that draw out about 560mAh. I need to know what resistor to use to get 12V DC 1440A  down to 8V DC .25A. Please respond as quickly and informatively as possible. Thank you in advance.

Note: I got the 1440Amps from subtracting the usage from what the AC adapter puts out. The AC to DC adapter I am using puts out 12V 2Amps. If you can tell me the model number, and/or colour code.

andy707076 years ago
For a start, fans cant draw 560mAh, what you probably meant is mA. mAh is a measure of battery charge. also, if you say the fans are 250mA, 3x250mA=750mA, not 560, so either your calculation is wrong or if you used a multimeter, the fans aren't 0.25A. As someone already said, resistors are not a very effective way of dropping voltage, a voltage regulator would be better. For this, you could use a 7808 voltage regulator (can be picked up on ebay cheap) set up as follows:

1000µF electrolytic capacitor from pin 1-ground
220nF Mylar/Polymer capacitor from pin 1-ground
pin 2 to ground
100nF Mylar/Polymer capacitor from pin 3-ground
Input: pin 1
Output: pin 3
common ground.

This should work nicely and does up to 1A, which as you said, they are 250ma, should be fine, but if you need more power, use a 78S05 for up to 2A. Having said all that, it would probably just be cheaper and easier to buy standard 12v fans, 8v seems a bit strange.
guyfrom7up6 years ago
what do you need the 8 volts for? Cause in most cases, a resistor would be one of the worst ways. and I have no clue what you are talking about the 1440A, lol

but, if you are drawing a constant .25A, a resistor could work:

Vdrop= 4 volts
ohms law:
4=0.25R
R=16 ohms

Calculating wattage:
0.25 amps * 4 volts = 1 watt

So you should have a 16 ohm, 2 watt (or so) resistor (for safety).

But if you say that application, there'll probably be a better solution.
Probably the only thing worse then a resistor would be to use a potentiometer :-)
Guy's right, using a resistor on fans is a really bad idea. Use an adjustable voltage regulator, like the LM317