fried resistors

I am doing a project using a 15000uF capacitor at 36 volts and i keep burning out resistorsi had three in series giving 66 ohms roughly 0.54A, they burned out as soon as i turned on the power i have no idea why any comments suggestions or ideas would be welcomed.

Posted by Anonymous on 02/03/2004, at 20:01 GMT

W = VI

Resistors are usually specified by their power rating. Most standard resistors are 1 Watt.
That means that they will dissipate 1 Watt as heat, before they expire.

You know that you're passing 0.5A at about 36V
W = VI
So W = 0.5 * 36

Is that > 1 Watt?

*fizzle*

Posted by Richard Senior (mec@richardsenior.freeserve.co.uk) on 04/03/2004, at 01:09 GMT

You need resistors with a 10W power rating

Power(in W) = voltage (in V) x current(in A)
or P = V x I in symbols.

Each of your three 22 ohm resistors has 12V across it and 0.54A passing through so the power developed in each resistor is 12 x 0.54 = 6.5W.

Your 22 ohm resistors must have a power rating of more than this, 10W would be suitable. Standard electronics resistors have power ratings of only 0.25 or 0.5W so these will be destroyed - as you discovered!

As a general rule you should check resistor power ratings if their resistance is low (<300 ohm) or if you are using a high voltage (>15V) because standard resistors may not be suitable in these cases.

For more information please see www.kpsec.freeuk.com/components/resist.htm#power

Posted by John Hewes on 05/03/2004, at 13:54 GMT