# converting watts to milliamps

Discussion in 'Electronic Basics' started by Carl Farrington, Dec 13, 2003.

1. ### Carl FarringtonGuest

Hi there,

Please could someone explain to me how to convert watts into milliamps? I
want to compare the power usage characteristics of two devices (one is a
2.5" hard disk, the other is an IDE Flash Disk) but one manufacturer lists
usage in watts, the other in milliamps!

thanks,

Carl Farrington

2. ### Rheilly PhoullGuest

Amps = watts/volts
divide the result by 1000 for mA

thank you!

4. ### Dr. A.T. SqueegeeGuest

Assuming we're talking DC here (the AC formulae are slightly
different): Ohm's Law gives you everything you need.

P = IE, where P is power in Watts, I is the current drain in Amps,
and E is the voltage involved.

Taking this a step further, we get I = P / E

So: As an example, assume a 12V fan draws 4 watts of power. That's
4 over 12, which gives a current drain of about 0.333A, or 333mA.

We can double-check this by finding the resistance, and using
Ohm's Law again: R = E / I. Since we now know I, we can find that
R = 12 / 0.333 = about 36 ohms.

Now that we know resistance, we can use the formula P=I-squared
times R to find the power figure: P = .333 squared times 36 = 3.99
watts. Pretty close, heh?

You can also simply do P = IE, or .333 times 12 = 3.996.

Hope that helps. ;-)

--
Dr. Anton Squeegee, Director, Dutch Surrealist Plumbing Institute
(Known to some as Bruce Lane, KC7GR)
kyrrin a/t bluefeathertech d-o=t c&o&m
Motorola Radio Programming & Service Available -
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5. ### Ian StirlingGuest

You may want to look at the detailed datasheets, and work out just