At 01:01 AM 2/10/2006 +1300, you wrote:
>I have grown so accustomed to using metal film resistors for
>'everyday' applications that carbon film doesn't usually feature in my
>thinking.

I suppose you're talking about leaded resistors.

>A Taiwanese manufacturer has asked whether they can substitute carbon
>film for metal film in a design. They claim that cf costs are far
>lower. Volumes are only moderate (1,000 - 10,000 products per year at
>most). Most resistors are operated at far below their power and
>voltage ratings.
>My initial reaction is that whereas most resistors could technically
>be changed to CF, the cost savings would be minimal compared to other
>costs

High purchasing costs of unpopular resistors in such tiny quantities
may be influencing their request. With the carbon film, they may have
them around or they can buy reel quantities for a few dollars and
not worry about it.

>and that sacrificing the reliability of metal film would not be
>worthwhile. I'm also dubious about the merits of allowing CF as they
>would almost certainly be Chinese*

Taiwan has a number of quality resistor manufacturers, many (all, probably)
of which now have their factories on the mainland. The brand is more important
than where the factory is plunked down- because the QC will depend
on management and how much they value their name. What you probably don't want
is a 'lowest bidder' type of generic part, which is more of a crap-shoot.

>sourced with unknown (indeed
>non-existent) provenance and aimed at bottom of the market,
>quality-irrelevant applications. (I've little doubt that there is a
>reason that they are cheap).
>
>Thoughts? - or recent experiences with modern CF resistors.
>Am I too prejudiced against a new 'mature' technology?

Depends on the circuit. And if you trust the manufacturer to use name-
brand parts. In China you also have the problem of counterfeit parts.
BTW, I've seen MANY MORE reliability problems with metal film,
especially Taiwanese metal film. CF outright failures and serious drift
issues are very rare, IME. They make so many billions of them...

There's another option- you can supply the resistors. I've done that for
really cheap parts where I'm worried about quality issues. They probably roll
their eyes at the dumb big-nose (ben dan da bi tzi) who pays 10 times as
much for about the same thing, but so what?  Of course leaded parts are
an order of magnitude more expensive to ship than SMT.

>Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
->> Inexpensive test equipment & parts http://search.ebay.com/_W0QQsassZspeff


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