Where to find carbon SMD resistor

Thread Starter

Dritech

Joined Sep 21, 2011
847
Hi,

Below is a quote from the LT1167 datasheet:

"Adding an external 5k input resistor in series with each input allows DC input fault voltages up to ±100V and improves the ESD immunity to 8kV (contact) and 15kV (air discharge), which is the IEC 1000-4-2 level 4 specification. The input resistors should be carbon and not metal film or carbon film."

Where can I find SMD carbon resistors please. I only managed to find carbon film resistors.

Thanks in advance.
 
Last edited:

Papabravo

Joined Feb 24, 2006
12,890
They may not exist in the SMD form factor. They are very nearly obsolete in thru-hold form as well. Hamfests and flea markets might be your salvation.
 

Lestraveled

Joined May 19, 2014
1,946
I think what Linear Technologies is looking for is a non-inductive resistor. Inductance is the only parameter that carbon composition resistors have over film resistors. DigiKey stocks some carbon comp chip resistors but not many. They do make non-inductive film resistors.
 

ian field

Joined Oct 27, 2012
6,539
Hello,

I have never seen them in SMD, but in through hole.
Rapidonline seems to sell them, but a lot in packages of 1000:
http://www.rapidonline.com/electronic-components/arcol-0-5w-carbon-composition-resistors-79895

Bertus
These days; carbon composition resistors are very rarely used - except sometimes for RF work where parasitic inductance would be a problem.

AFAIK: most SMD resistors are basically just a slice of ceramic coated on one side with carbon or metal film, and end contacts plated on.

Anything under M-Ohm will be just that and no more inductive than a PCB track the same proportions. Very high resistance values may involve a zig-zag film track on the ceramic which may incur some parasitic inductance - but I'm fairly sure the manufacturer will minimise that as much as possible.
 

ian field

Joined Oct 27, 2012
6,539
Those are carbon composition resistors.
Composition are OK for dissipating transients, which is probably why the datasheets specifies them. They're not great for continuous dissipation because they drift.

Film resistors are pretty much the opposite, they handle continuous dissipation pretty well but can fail open if subjected to transients.

The application probably only needs "the letter of the law" as laid down in the datasheet, if the equipment is used in an electrically harsh environment.
 

Lestraveled

Joined May 19, 2014
1,946
@ian field
In this application, accuracy is not needed. 5K verses the 1000 gigi-ohm input resistance will have no measurable impact. An inductive resistor would create voltage errors from transients in a noisy environment.
 

SLK001

Joined Nov 29, 2011
1,543
I agree with Lestraveled that the problem with the carbon film and metal film resistors is that they are spiral cut to the proper resistances. This "inductor" would make any transient pulse worse. I would bet that a regular SMT resistor would work quite well in this application.

You could always contact the engineers at Linear and ask them for clarification. Specifying a virtually obsolete part to use with their part seems most "unwise".
 

ian field

Joined Oct 27, 2012
6,539
@ian field
In this application, accuracy is not needed. 5K verses the 1000 gigi-ohm input resistance will have no measurable impact. An inductive resistor would create voltage errors from transients in a noisy environment.
At that range of values an SMD film resistor wouldn't be any more inductive than composition - maybe even less if its shorter relative to its width.

It was merely a recap of the relative merits of the two types.

The datasheet has to anticipate worst case scenario - in an average environment, the resistor type may not be *ALL THAT* critical.

Composition resistors are very forgiving of transients that far exceed their voltage rating, whereas spikes can chimble away at film resistors until they become high or open.

Chances are; the chip would fail long before the limiting factor became significant with either type of resistor.
 

ian field

Joined Oct 27, 2012
6,539
Over what time period was that test conducted?

Transients of 8kV as previously mentioned exceed the resistors voltage rating by quite a few orders of magnitude. For a narrow transient; the dissipation is well within spec, but each transient chips away a little of the film. Carbon composition is much more resilient to this type of abuse.

As I stated previously: the datasheet has to take into account "worst case scenario" film resistors might be adequate in a mild environment.

At the end of the day: The manufacturer only guarantees the part reliability/performance if you operate it within the specifications laid out in the datasheet. But very often those specifications are a little over cautious and you can get away with certain things.
 
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