hi,
there are many power values of the resistors such as 1/4 W, 2/4 W, etc. so, how to determine the suitable power value for a resistor?

 

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.

 

thanks for the replies. i raised up this issue because some of the schematic doesn't state the power value of the resistor.

 

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.

Attached Images

schem1.jpg (52.1 KB, 179 views)

 

If the resister value is 1k or over you are safe to use low wattage, a 1/8 W will be happy to sit all day shorting out 240V mains voltage.

 

It's not quite so. Power disipation is calculated by

P=U*U/R

So if you use 220V and 1kOhm (1000 Ohm) heat disipation is
P=220*220/1000=48.4Watt.

1/8Watt (or 0.125Watt) is much less than that.

kekek, you can go ahead and use 1/4Watt or even 1/8Watt
for all resistors in your circuit - except R7 which is actually
AC load you are controlling.

 

thanks a lot guys... i was thinking of using power disipation equation before, but i'm not so sure if i am on the right track... anyway, thanks a lot.

 

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... ops: