resistor matter

[kekek]

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?

 

[Nigel Goodwin]

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.

 

[Agent 009]

how to determine the suitable power value for a resistor?

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.

 

[kekek]

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

 

[panic mode]

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.

 

[kekek]

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.

 

[dingo]

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.

 

[panic mode]

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.

 

[kekek]

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.

 

[kekek]

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...