resistor PPM

[surfsup]

I noticed people say carbon comp are noisy
Metal film can be "grainy"
carbon film are preferred by many

Looking at carbon film, I see mouser has some where they are more expensive and the only difference is the % PPM of carbon. The higher the percentage, the higher the cost.

I know carbon costs money, but is there another reason for this and isn't "more" carbon contributing to noise, or am I totally whack on that last point?

[selloutrr]

can you post a link to what you a referencing?

It also has to due with heat dissapation, size, and purity of materials used.
Wattage plays a big part in the "noise" and "grain" of the resistor, how hard it's working / heating vs. it's ability to hold it's value under stress.

[Firestorm]

I think you're reading the temperature coefficients of the resistors, which is given as ppm/c or ppm/K. This is a rating of how much temperature affects the resistance.

All resistors have thermal noise, dependant only on the value of the resistance and the current. But different types of resistors have other noise and that is what you're trying to control. Wirewounds are quietest, but impractical. Then MF, CF and CC. I like metal film where I have to use large resistances (1M on the input jacks, etc.), but if you don't like the sound of the metals, use CF or CC, but use higher power ratings than you need (1W where you technically need only 1/2W). The larger physical size will reduce excess noise.

[surfsup]

The column is titled "Temperature coefficient" and it will read like so:

"+/- 350 PPM / C "
or
"+/- 400 PPM / C "

in comparing like products (10W wirewound ohmite resistors or 1w, can't remember now)...the only difference I could see was in this column

One was 0-400PPM and the other was 0-700PPM I believe, and the 0-700 was about 50% more in price (say 1.50 for one and 2.25 for the other)

I assumed PPM to be parts per million and C meaning "of carbon".

I couldn't find anything on TAG discussing this in the past.

[triodecr]

PPM/C means parts per million per degrees Celcius. It tells you how much the value changes per 1 deg. C of temperature change. The higher the number the more the resister value drifts with temp change. You usually pay a higher price for lower PPM/C so there may have been other differences in the resistors you were comparing.

[surfsup]

could be but I went over the values 10 times and didn't see anything. I've never seen anything with "more" ppm being a good thing. thanks for the explanation. I bought the lower value ppms.