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reducing current on a power supply Answered

So I am trying to reduce the on the current power supply I have from an old router. The out put is 12v DC 800mA (.8A). I want to use it to run a man 12v .3A

I just wanted to make sure I did my math right and to make sure I didnt miss anything.

Ohms Law

I = V/R

R= V/I V = 12 I = .8

12/.8 = 15 ohms resistance in theroy
Current on the fan is .3 A, I am assuming that is the max. So desired current is .25
I = .25 V = 12 R is current resistance, X is additional resistance Q=(R+X)

I = V/Q

Q= V/I

R+X = V/I

X = (V/I) - R V = 12 I = .25 R = 15

(12/.25) - 15 = 33 ohm resistance needed

So I would need a minimum of 33 ohm resistor. Am I correct?
Sorry for the explicitness of the math, its been drilled into me to show every step.


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11 years ago

If it's a 12V fan it will be fine, you don't need to do any maths or add any resistors.
(If it's not a 12V fan, tell us what voltage it was designed for?)