Resistor rateing confusion

[adamasd]

I have been order parts and came across the Vishay Dale resistors that I can not figure out. They are CCF55 and CCF60 series and they are listed like this;

CCF55
1/4WATT Metal Film Resistors
Vishay/Dale 1/4WATT Metal Film Resistors 1/4watt 100Kohms 1% Rated to 1/2WATT

CCF60
3/4WATT Metal Film Resistors
Vishay/Dale 3/4WATT Metal Film Resistors 1/2watt 100Kohms 1% Rated to 1WATT

The data sheets did not clear anything up, are they saying that the CCF55s will stay within their 1% tolerence up to 1/2 watt of dissapation but are optimal when used under 1/4. That would make them 1/2 watt resistors though. Then there is the CCF60s which get three wattage rateings. What is going on here?

thanks,
adam

 

[Ptownkid]

wierd...

I think you've got the answer, but 'm not positive.

 

[adamasd]

The answer for the CCF55s maybe, but the CCF60s, I can not even begin to sort that one out.

adam

 

[bcarso]

I think it's not based on temperature-coefficient-based shift in value so much as long-term reliability. Typically the -55, -60, -65, and -70 suffixes relate to old military wattage ratings, which involve high ambients (70 C at least usually, sometimes even 125 C) and long-term stability issues. Most everyday commercial apps have far lower ambients with consequently lower resistor tamperatures for a given dissipation and ambient. Also, manufacturers have figured out better formulations and processes, so feel confident in a non-mil rating now well in excess of the old days. And it's easier to quote such as an alternative than to change a mil-spec, if you want to sell to the gov't---hence two ratings.

For a Mil-spec-oriented datasheet see for example:

http://www.vishay.com/docs/31027/cmfmil.pdf

EDIT: I see now that there are a number of "dual rated" resistors, independent of mil-spec per se. I still think that the rating pertains to the expectation of long-term stability, but it does appear that no one wants to explain this in the standard datasheet. Probably some lengthy reliability document from the manufacturer will disclose the parameters.

 

[adamasd]

Ok, that clears it up, the CCF55 is pretty much 1/2watt and the CCF60 is 1 watt as far as my needs are concerned then.

Thanks,
adam

Edit: Your edit has returned some of my confusion over these resistors. Complete and thurough data sheets, it seems so sensable.

 

[PRR]

The CF55 is the physical size that we have called "quarter Watt" for almost 50 years.

It seems you can now put 1/2 watt in that size, but I think that's not a good idea. You see a LOT more problems with over-heated resistors than "under"-heated resistors. Buy half-watt resistors, they don't cost much more. If you need a Watt, buy 2-Watt jobs.