Re: {Collins} Carbon Film vs Metal Film Resistors



Paul and Jim,

I agree with Paul's thoughts about film resistors being the better choice
in most applications.  However, besides surge issues, I also keep an eye on
the application of the resistor, especially if it will be carrying RF
currents.  For any circuit in which RF will pass thru the resistor, I
strongly suggest carbon film or carbon comp, as opposed to metal film.  The
metal film types are very stable and should last a long time, but their use
should be primarily in DC and audio applications in which any significant
self-inductance of the resistor would not alter circuit performance.

High value carbon comp resistors (1 megohm and greater) tend to be quite
unstable and tend to change value over rather short periods of time (a few
years, for example).  I have a pair of Heathkit resistor sub boxes that I
built in the late 1960s and early 1970s.  Heath used carbon comps for all
values.  During the past few years, I have had to replace about 1/2 of the
resistors (high values and even some low values) because they had drifted
out of tolerance (10% or 5%, depending).  I did use carbon comps for all
replacements, but if I have to change out resistors again, I'll probably go
with carbon film types.  

Since 47, 51, 75, and 100 ohm values are often used in RF circuits, I do
keep a stock of carbon comps on hand in those values, and in 1/2 watt, 1
watt, and 2 watt sizes.  It is tough to beat bulk body resistance for low
self-inductance.

73, Dale
WA9ENA
CCA Member 


> [Original Message]
> From: Paul Christensen <w9ac@xxxxxxxx>
> To: Jim Tills <jtills@xxxxxxxxxxxxxxx>; <collins@xxxxxxxxxxxxx>
> Date: 1/10/2012 9:00:59
> Subject: Re: {Collins} Carbon Film vs Metal Film Resistors
>
> Jim,
>
> Unless surge currents are involved, it's almost always desirable to use a 
> film resistor.  Metal film has the advantage of tighter tolerance and
nearly 
> zero drift with age.  However, unless there's a need for precise balance 
> using fixed resistors, I try to use carbon film rather than metal film. 
> Either type will generally result in lower noise over carbon composition 
> since noise is largely a function of current.
>
> I recently made mods to a 75S-3C and ignored my own advice.  To keep 
> consistency, I used the original Allen-Bradley 5-band, mil-spec 10% 
> resistors, even though I know better.  If you want to stick with carbon 
> composition, Ohmite still makes them and they're available through Mouser
--  
> albeit more costly than carbon film.
>
> Paul, W9AC
>
> ----- Original Message ----- 
> From: "Jim Tills" <jtills@xxxxxxxxxxxxxxx>
> To: <collins@xxxxxxxxxxxxx>
> Sent: Tuesday, January 10, 2012 9:03 PM
> Subject: {Collins} Carbon Film vs Metal Film Resistors
>
>
> > Hi everyone. Is there any preference in using Carbon Film or Metal Film 
> > Resistors when replacing Resistors in the old Collins equipment? OR - 
> > Should you try to find the old Carbon Composition Resisters if
available? 
> > Thanks CUL 73s Jim  KF0JB
> > _______________________________________________
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