Using 1/4w and 1/8w resistors in same circuit

[GuitarGas]

Is it ok to use different watt rated resistors in the same circuit?
The reason I'm asking is because I can't seem to find any 3.3k resistors in 1/4w on ebay, but there are a tonne in 1/8w.
I'm preferring ebay for these small one off purchases as the costs are a lot lower than placing a big order through tayda electronics
Cheers

[induction]

GuitarGas wrote

Is it ok to use different watt rated resistors in the same circuit?

Yes, it's fine. The power rating just tells you the maximum voltage you can put across a given resistor.

Power = Voltage^2/Resistance. So for a 10k resistor (for example), a 1/8th watt resistor can take a maximum voltage of V = sqrt(P*R) = sqrt(0.125*10k) ~= 35V.

Even if you don't know the amount of voltage across a particular resistor, you can use the difference between the positive and negative rail as an absolute maximum. In the above example, if your circuit is powered by 9V with no charge pump, then you know you will never get close enough to 35V to worry about burning out the resistor. In practice, I think you can use 1/8 watt resistors in most pedals with no problems.

I believe 1/4 watt resistors are more popular with hobbyists only because they are (or at least used to be) more widely available. That's becoming less true, these days. 1/8 watt resistors have the added benefit of being smaller and easier to fit into a build without standing them up.

[GuitarGas]

Ok thanks heaps, although...

induction wrote

Power = Voltage^2/Resistance. So for a 10k resistor (for example), a 1/8th watt resistor can take a maximum voltage of V = sqrt(P*R) = sqrt(0.125*10k) ~= 35V.

I have absolutely no idea what this means???

[Silver Blues]

Let's try and translate...

Power = voltage squared divided by resistance. So for a 10K resistor (for example), an 1/8W device can take a maximum voltage of the square root of (power times resistance) so since the device is rated for a maximum of 1/8W and is 10KΩ the maximum voltage that can be placed across it is the square root of (0.125x10000) which is the square root of 1250 which equals 35.3V (i.e. about 35V).

Does that help?

[GuitarGas]

Silver Blues wrote

Does that help?

Yes, a lot. Thanks!