You work it out from the current through it and the voltage across it, where W=VI. W is in Watts, V is in Volts, and I is in Amps. It's as well to use a resistor that is well over the requirements for reliability reasons, the ratings given are absolute maximums.
Well, if you say that it's given: 1/2W...What do you need more? I think resistors come with only ONE value prescribed for each. So you would know the range of voltage and current you could operate the resistor without melting it.
You are right that very often power rating is not specified.
I don't think you know how to analyse circuit otherwise you
wouldn't ask for help. So before you go crazy and put
20Watt everywhere, here is one tip:
If the power rating is not specified either on the drawing or
in the text (articles in electronics magazies) you are pretty safe
to use 1/4watt everywhere.
thanks for the tip panic mode. i really appreciate it. but i want to make sure that it is really safe using 1/4 W resisitor according to the attached schematic. i got the schematic from zilog website on power phase control using Z8 Encore microcontroller.
i just realised that there is a topic on resisitor wattage on page 33 (if i not mistaken). anyway, i got a lot of information and thanks again for helping me. better be careful next time...