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'[EE] Carbon film vs Metal film resistors'
2006\02\09@070132 by Russell McMahon

face
flavicon
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I have grown so accustomed to using metal film resistors for
'everyday' applications that carbon film doesn't usually feature in my
thinking.

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 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* 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?



       Russell McMahon

* I've no doubt that when it makes sense to do so China can produce
products of superb quality. But I also have little doubt that the
burgeoning market / what the market will bear / caveat emptor / take
no thought for the morrow ... / aspects of the present market
explosion mean that quality will often be, at best, only as good as it
has to be.

2006\02\09@071017 by Bob Axtell

face picon face
I'm skeptical that the carbon film scheme will reliably dissipate the
heat without changing
value considerably.

The Chinese have an enormous problem here in the colonies- many
companies have pulled
back their plastics molds (if the chinese will give them back) due to
unethical business
practices on several levels.

Before I would change, I would run a bunch of tests.

--Bob

Russell McMahon wrote:

{Quote hidden}

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2006\02\09@074100 by Spehro Pefhany

picon face
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"
.....speffKILLspamspam@spam@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


2006\02\09@094616 by Paul James E.

picon face

All,

Another point of interest is in noise contribution.   Carbon film
resistors inherently have a higher noise figure than metal film.
So if noise is an issue, don't use carbon film resistors.  I believe
generally speaking, carbon film resistors have a noise value that is
typically twice that of metal film.   And in most cases, it is three
to four times as high.   Just food for thought.


                                          Regards,

                                            Jim



{Quote hidden}

> --

2006\02\09@095031 by Alan B. Pearce

face picon face
>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

If you are going to introduce significant cost differences by using metal
film, then you may find counterfeit items appearing using CF ones instead in
a rip off product.

My response to your requirement for using MF is do you really need the
tolerance accuracy? MF is normally supplied to 2% or tighter, CF is
typically 5%, but not having dealt with Chinese production, I don't know if
these are the tolerances they would supply.

2006\02\09@103943 by alan smith

picon face
If I recall....carbon film is a better choice in high voltage applications, less chance of arcing?  That thought was from a design I worked on some 10 years ago, where the application was high voltage RF, and the guy doing the design specifically put in carbon

"Paul James E." <jamespspamKILLspamintertex.net> wrote:  
All,

Another point of interest is in noise contribution. Carbon film
resistors inherently have a higher noise figure than metal film.
So if noise is an issue, don't use carbon film resistors. I believe
generally speaking, carbon film resistors have a noise value that is
typically twice that of metal film. And in most cases, it is three
to four times as high. Just food for thought.


Regards,

Jim



{Quote hidden}

> --

2006\02\09@104657 by Michael Rigby-Jones

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>-----Original Message-----
>From: .....piclist-bouncesKILLspamspam.....mit.edu [EraseMEpiclist-bouncesspam_OUTspamTakeThisOuTmit.edu]
>Sent: 09 February 2006 15:40
>To: Microcontroller discussion list - Public.
>Subject: Re: [EE] Carbon film vs Metal film resistors
>
>
>If I recall....carbon film is a better choice in high voltage
>applications, less chance of arcing?  That thought was from a
>design I worked on some 10 years ago, where the application
>was high voltage RF, and the guy doing the design specifically
>put in carbon

Carbon film is a better choice for RF because they have lower inductance than metal film.

Regards

Mike

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2006\02\09@105724 by Alan B. Pearce

face picon face
>If I recall....carbon film is a better choice in high
>voltage applications, less chance of arcing?  That
>thought was from a design I worked on some 10 years ago,
>where the application was high voltage RF, and the
>guy doing the design specifically put in carbon

Had that in Cockroft-Walton (sp?) multipliers for instruments in space.
carbon resistors specified for arc-over reasons.

2006\02\09@113413 by Mike Harrison

flavicon
face
On Thu, 9 Feb 2006 15:57:20 -0000, you wrote:

>>If I recall....carbon film is a better choice in high
>>voltage applications, less chance of arcing?  That
>>thought was from a design I worked on some 10 years ago,
>>where the application was high voltage RF, and the
>>guy doing the design specifically put in carbon
>
>Had that in Cockroft-Walton (sp?) multipliers for instruments in space.
>carbon resistors specified for arc-over reasons.

Probably solid-carbon though - film resistors are not good at HV, due to poor surge capability, and
spiral-cut ones can arc between turns.
My marx generator (http://www.electricstuff.co.uk/marxthree.html) used to eat 10KV-rated film resistors due
to surge currents - after replacing with solid-carbon ones it's been fine.

2006\02\09@114946 by Alan B. Pearce

face picon face
>>Had that in Cockroft-Walton (sp?) multipliers for instruments in space.
>>carbon resistors specified for arc-over reasons.
>
>Probably solid-carbon though - film resistors are not good
>at HV, due to poor surge capability, and spiral-cut ones
>can arc between turns.  My marx generator
>(http://www.electricstuff.co.uk/marxthree.html) used to eat
>10KV-rated film resistors due to surge currents - after
>replacing with solid-carbon ones it's been fine.

Yeah, I think they were. They were the ones that relate to the story a while
back about resistor values of carbon resistors, and I told a tale about
checking their resistance as they were warmed up to expel moisture.

2006\02\09@115543 by Bob Blick

face picon face
> I have grown so accustomed to using metal film resistors for
> 'everyday' applications that carbon film doesn't usually feature in my
> thinking.
>

The real world uses carbon film except where conditions dictate otherwise.

If you don't trust the supplier, what does it matter if they provide
crappy metal film or crappy carbon film? They are a bad supplier, find
someone else. Besides, you can always specify accepted manufacturers and
part numbers.

Cheerful regards,

Bob


2006\02\09@132051 by Rich Graziano

picon face
I have carbon film resistors in stock if you need some.
Rich
----- Original Message -----
From: "Russell McMahon" <apptechspamspam_OUTparadise.net.nz>
To: "PIC List" <@spam@PICLISTKILLspamspamMIT.EDU>
Sent: Thursday, February 09, 2006 7:01 AM
Subject: [EE] Carbon film vs Metal film resistors


{Quote hidden}

> --

2006\02\09@132912 by Rich Graziano

picon face
This is very true.  The carbons are less inductive.
Rich
----- Original Message -----
From: "Michael Rigby-Jones" <KILLspamMichael.Rigby-JonesKILLspamspambookham.com>
To: "Microcontroller discussion list - Public." <RemoveMEpiclistTakeThisOuTspammit.edu>
Sent: Thursday, February 09, 2006 10:46 AM
Subject: RE: [EE] Carbon film vs Metal film resistors


>
>
>>{Original Message removed}

2006\02\09@180459 by Rich Graziano

picon face
Yes. That is true.  They are noisier. But for high voltage applications they
are better because of the lower inductance.
{Original Message removed}

2006\02\10@215536 by Rich Graziano

picon face
Is that carbon film or carbon composition?

----- Original Message -----
From: "Alan B. Pearce" <spamBeGoneA.B.PearcespamBeGonespamrl.ac.uk>
To: "Microcontroller discussion list - Public." <TakeThisOuTpiclistEraseMEspamspam_OUTmit.edu>
Sent: Thursday, February 09, 2006 10:57 AM
Subject: Re: [EE] Carbon film vs Metal film resistors


> >If I recall....carbon film is a better choice in high
>>voltage applications, less chance of arcing?  That
>>thought was from a design I worked on some 10 years ago,
>>where the application was high voltage RF, and the
>>guy doing the design specifically put in carbon
>
> Had that in Cockroft-Walton (sp?) multipliers for instruments in space.
> carbon resistors specified for arc-over reasons.
>
> --

2006\02\12@204343 by Russell McMahon

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flavicon
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> Is that carbon film or carbon composition?

Film !

   RM

2006\02\12@205809 by Russell McMahon

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> The real world uses carbon film except where conditions dictate
> otherwise.
>
> If you don't trust the supplier, what does it matter if they provide
> crappy metal film or crappy carbon film? They are a bad supplier,
> find
> someone else. Besides, you can always specify accepted manufacturers
> and
> part numbers.

A significant issue is that I have essentially no control of what
happens after the project assumes a life of its own. I can set gross
parameters which may well be adhered to (such as CF or MF resistors)
but if I get down to part numbers and suppliers there is no certainty
that that will not be changed without me knowing. And I have no good
information on the quality of the available Asian manufacturers. MF,
being a dearer and more 'complex' product, arguably set a higher entry
level to manufacturers offering them. One could also argue that the
potentially higher profit levels may attract inferior producers :-).
All in all, it seems more likely that I will get a dependable result
with MF if the cost difference as a % of total component cost is not
severe.

Here I tend to just say "SFR16" and all is well :-).
(That sort of spec transcends manufacturing location. I note that the
first box of them that I grabbed says "Made in Brazil". I have no idea
how good the typical quality of Brazilian electronics components is
but I have some confidence in the people who decided to get them made
there).


       RM



2006\02\13@054132 by Gerhard Fiedler

picon face
Russell McMahon wrote:

> (That sort of spec transcends manufacturing location. I note that the
> first box of them that I grabbed says "Made in Brazil". I have no idea
> how good the typical quality of Brazilian electronics components is but
> I have some confidence in the people who decided to get them made
> there).

I don't think there is such a thing as "typical quality" of produced
anything for a country. There maybe could be an average and a standard
deviation (if there were a satisfactory scalar measure for quality), but
even if there were, in almost all cases the variety of different qualities
produced in any country makes a "typical" quality practically inexistent.

Gerhard

2006\02\13@060120 by Alan B. Pearce

face picon face
>> Is that carbon film or carbon composition?
>
>Film !

Russells project may well have the manufacturers looking at carbon film (as
being easier to manufacture), but in the Cockroft-Walton multiplier that I
dealt with they were composition.

2006\02\13@133338 by Peter

picon face

On Mon, 13 Feb 2006, Gerhard Fiedler wrote:

> Russell McMahon wrote:
>
>> (That sort of spec transcends manufacturing location. I note that the
>> first box of them that I grabbed says "Made in Brazil". I have no idea
>> how good the typical quality of Brazilian electronics components is but
>> I have some confidence in the people who decided to get them made
>> there).
>
> I don't think there is such a thing as "typical quality" of produced
> anything for a country. There maybe could be an average and a standard
> deviation (if there were a satisfactory scalar measure for quality), but
> even if there were, in almost all cases the variety of different qualities
> produced in any country makes a "typical" quality practically inexistent.

I think that there is. The average level of services, manpower training
and pay, causes average businesses to produce an average quality class
of products.

Peter

2006\02\13@170124 by Gerhard Fiedler

picon face
Peter wrote:

> I think that there is. The average level of services, manpower training
> and pay, causes average businesses to produce an average quality class
> of products.

That's probably true. Unluckily there are few average businesses. Most of
them are either above or below :)

Gerhard

2006\02\13@171545 by David VanHorn

picon face
>
>
> That's probably true. Unluckily there are few average businesses. Most of
> them are either above or below :)


About half, which is all part of the reptoid's agenda.

2006\02\14@152926 by Peter

picon face

On Mon, 13 Feb 2006, Gerhard Fiedler wrote:

> Peter wrote:
>
>> I think that there is. The average level of services, manpower training
>> and pay, causes average businesses to produce an average quality class
>> of products.
>
> That's probably true. Unluckily there are few average businesses. Most of
> them are either above or below :)

And all those which aren't are pushed towards the average by local
constraints (pushed up for the ones below, and pushed down for the ones
above).

Peter

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