How do you choose wattage for resistors?

[antknee]

How do you choose the wattage for resistors? Is there a general principle? I tend to use 0.5W because it seems to work with everything but 0.25W are smaller so I'd prefer to use them where possible.

I have made up a schematic in Eagle and attached it. It has the following resistors-

R1 = R1 on a 555 timer
R2 = R2 on a 555 timer
FB1 = Feedback resistor on opamp
FB2 = Feedback resistor on opamp
Various potentiometers.

It has a +/-9V supply with ~ 1.5A.

Thanks.

 

[ericgibbs]

hi,
All the resistors could be 0.125W.

 

[audioguru]

Simply calculate how much power will be dissipated in a resistor then double it so the resistor does not get too hot.
The amount of power is:
I squared x R
V x I
V squared/R.

 

[antknee]

Oh good, thanks Eric, audioguru.

I will keep that in mind.

 

[Boncuk]

Simply calculate how much power will be dissipated in a resistor then double it so the resistor does not get too hot.
The amount of power is:
I squared x R
V x I
V squared/R.

Just to add: carbon film resistors have a tremendeously high positive temperature coefficient. If used at all stay well below 1/4 of the rated wattage, which is important for timing resistors.

Do not use carbon film resistors for high precision voltage dividers at all and use metal film resistors instead. Carbon film resistors change resistance just by the heat produced by the circuit - even in the low mW range.

Boncuk