resistor question

[soundcollage]

The koa speer 1/4Watt Metal Film Resistors
that mouser sells come in either 50ppm or 100ppm. What is this and what is the difference?

[Mike Burgundy]

I just had a look at Mousers resistor page ( I don't order from them cause they don't ship overseas) but I can't really find any ppm rating in there?ppm I know as "parts per million" but that'll be a little off the ball with resistors.
That said, a resistor is, theoretically, a resistor.
In theory a carbon comp is exactly the same as a metal film of the same value is exactly the simulated resistor in your CAD program.
Reality works a little differently - side-effects have a more or less (depending on the circuit - a lot is HYPE) significant effect on the components response.
Metal film has better tolerances, smaller sizes for the same power,less drift,  and lower thermal noise. Plus, theyre mostly easier to get. which is a good thing: cheap, silent, reliable and available is an unbeatable combo especially when most of the carbon-comp "quirks" can be easily faked with metals and caps.

I don't know what code you're looking at - got a link? - but you want to look at size (regular resistor type, forgot the type designation for now), value (duh) and power handling (for pedals 1/4W is almost always fine. Should you have a higher-rated resistor, it WILL work fine. This only goes for power handling though.)
hih

[R.G.]

It's almost certainly their temperature coefficient of resistance.

That is, the resistance changes by 50 parts per million or 100 parts per million per degree C. 50 ppm is 0.000050, 100 ppm is 0.000100, so they vary 0.005% per degree C or 0.01% per degree C.

Whether this matters to you or not depends on how precise your circuit design is.

[Mike Burgundy]

that makes a lot of sense.
Since they're metal films, and were talking about a proven circuit I highly doubt there's any problem.
Thanks RG.

[soundcollage]

Thanks for your quick response. I was just curious- there is a price difference of  9 cents(although at $.03 and $.12 neither will break the bank). I imagine there would be little difference in any of the circuits I will construct.

[Paul Perry (Frostwave)]

It's almost certainly their temperature coefficient of resistance.
That is, the resistance changes by 50 parts per million or 100 parts per million per degree C. 50 ppm is 0.000050, 100 ppm is 0.000100, so they vary 0.005% per degree C or 0.01% per degree C.

...or it might be that temperature induced resistance variations are guaranteed to be less than 50 or 100 ppm. I certainly wouldn't want to rely on them being exactly 50 or 100ppm! plus, I bet the tempco isn't anything like linear either.. sometimes people assume that the temperature sensitive offset voltages in semiconductors are linear & try to null them out with ingenious circuits, but alas!! :roll: