[Milsurplus] CarbonFilm / Metal Film Resistors

[Bob Camp]

Hi

This all gets a bit weird. If you want to get into the original papers 
on this stuff they all are from better than 50 years ago.

At any frequency you are likely to use them at the spiral cut in the 
metal film or carbon film resistor does not matter for values over a 
few hundred ohms at any wattage. For quarter watt and smaller size 
parts you have to get below ten ohms for it to be measurable, let alone 
significant. The main RF "parasitic" element in a carbon film or metal 
film resistor is the capacitance between the end caps. Anything that 
cuts down on the size of the end caps reduces this effect. That's why 
things like chip resistors are a bit better at RF than leaded film 
parts.

Carbon comp resistors are *strange* devices to model at RF. They stay 
purely resistive as frequency goes up. This is different than the end 
cap effect in a metal film resistor. The strange thing about the carbon 
comp resistor is that even though it's purely resistive it's value 
decreases as frequency goes up. Strange but true ...

There is about only one place where this subtle difference actually 
matters. The classic example is the resistors you see across RF tuned 
circuits in some tube radio front ends. A 470K ohm quarter watt carbon 
comp will be about 20% low at 20 MHz from what I recall. If you are 
going to substitute a carbon film part, just drop down a value or two.

If you get into one or two watt parts the values and frequencies drop a 
bit. It's been an awful long time since I have seen a two watt 100K 
carbon comp in the active part of an RF circuit though ....

Bottom line - unless you have a very unusual application you can pretty 
much interchange carbon film, metal film, and carbon comp resistors in 
RF applications. As you go to VHF or UHF you really should be using 
something else anyway.

	Enjoy!

		Bob Camp
		KB8TQ


On Mar 13, 2005, at 11:05 PM, <jimandterri at bellsouth.net> wrote:

> When replacing carbon composition resistors, at  what frequencies do I 
> need to be concerned about the inductive effects of the newer carbon 
> film and metal film resistors, I assume the metal film would be far 
> more of an issue  I have seen some vague suggestions but not much 
> specific info. Has anyone seen any real world problems in this area?
> JMS.
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