Metal film temperature coefficient

A general question. While working late last night, I noticed that as a piece of equipment warmed up, a 470 ohm metal film resistor I had just replaced, began to reduce slightly in value. I was surprised because I thought metal film resistors had a positive temperature coefficient. (And carbon film resistors had a negative temperature coefficient.) So I checked all of the rest of the same type 470 ohm resistors and they too dropped in value as they were heated. However other 470 ohm resistors from a different manufacturer showed a positive temperature coefficient. I then checked several other metal film resistors and they all increased as they were heated. Only two values showed negative and they were both from the same manufacturer in China. They certainly look like typical 1% metal film resistors with 5 color bands. I did notice these 470 ohm resistors didn't drop in value as rapidly as a carbon film resistors would, but it still dropped.

Am I correct in assuming that all metal film resistors increase in value as they heat up?

hi
Look thru these links:

https://www.pc-control.co.uk/resistor-types.htm

https://www.electro-tech-online.com/custompdfs/2008/05/Chapter2018old.pdf

https://www.electro-tech-online.com/custompdfs/2008/05/MFRPDF.pdf

gray5596 said:

Am I correct in assuming that all metal film resistors increase in value as they heat up?

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You can't.

Contrast to carbon film resistors which almost always have a negative TC, the temperature coefficient of metal file resistors are often specified as +/- certain ppm so it is safe to assume that some would have positive TC while others has negative TC, depending on resistance value, film material and manufacturing process.

The manufacturer mixes various metals together to try to achieve a zero temperature coeficient. The normal slight variation in the mix can push the coeficient to either the positive or negative side of zero.

Thanks for the inputs and URLs.

I had been laboring under the false idea that there was a general rule for temperature coefficient for carbon and metal film resistors. So the resistor rules are: Metal film can be either positive or negative, carbon film is usually negative, carbon composites are usually negative and always noisy, and wives are always right. Did I get that correct?

gray5596 said:

... and wives are always right. Did I get that correct?

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Very true! Wives are always right concerning small things like the brand and colour of a new car. The big things, e.g. who becomes next president, they leave to the husbands.