ppm/c

[maxkracht]

Lots of resistor questions on here lately, wondering if anyone could answer the one I have had for a while. How does wattage rating relate to the ppm/c rating? Is a 2w resistor that has a 250ppm/c rating twice as quiet as a 1w resistor? And how does resistor tolerance fit into this? Is a 1% resistor actually quieter than a 5% if everything else is the same? Are calibration resistors special in some way?

[martin manning]

PPM/C is the temperature coefficient, which tells you how much the resistance changes with temperature. For a 50C temperature rise, the resistance of a 100 PPM/C resistor will change by 50 x 100/1000000 x 100 = 0.5%. There may be some correlation between temp coefficient and noise index due to the material required for say a low temp coefficient, but they are different metrics. Re tolerance, a resistor made of any material and wattage rating could be selected or trimmed to a very close close tolerance, but that will only hold for some reference temperature. The temp coefficient independently describes how much the value will change when the resistor gets hot (or cold).

[maxkracht]

So, all things equal, are the bigger resistors more betterer because they have more thermal mass and more resistance to temperature fluctuation resulting in a lower thermal noise? If so, is this a linear change or am I missing other factors?

[martin manning]

A larger wattage resistor will run cooler, and therefor produce less noise, which goes with the sq root of temperature.
Lots of good info on resistors here: https://eepower.com/resistor-guide/

[maxkracht]

Thanks Martin. I have read that before but wasn't smart enough to extract an answer. Namely, If attempting to select the lowest noise resistor of the same composition, do you buy the 1w resistor that is rated for 100ppm/c, the 2w resistor rated for 200ppm/c, or are they close enough that I should just buy whichever costs less or looks cooler?

[martin manning]

As I said above, I don't know if there is any correlation between temp coefficient and noise index. If you want low noise, go for the higher wattage. If you want the resistance to be temperature-stable, go for the low temp coefficient. 100 vs 200 PPM/C would be an 0.5% change for 50C vs. a 1% change. Probably not too important.

[maxkracht]

Thanks again. Not sure why I assumed higher temperature coefficient = more noise. I know these are nitpicky details for audio applications regardless.