Like Tom, I install my carbon composition Beverage termination resistors fully
exposed to the weather. I measure them annually and over a 20 year period just
a few have changed resistance outside of their marked tolerance band.
I'm sure there are better ways to protect them, but open air has been very
reliable for me.
73
Frank
W3LPL
---- Original message ----
>Date: Tue, 12 Mar 2013 14:01:51 -0400
>From: "Tom W8JI" <w8ji@w8ji.com>
>Subject: Re: Topband: Antenna terminations
>To: <topband@contesting.com>
>
>> Depends on the method used to provide the conductivity. If it's carbon
>> black filled, the shrinking is goin g to push the molecules of the carbon
>> black closer together and that results in a net decrease in volume
>> resistivity. It may not matter however, as carbon filled stuff is going
>> to be in the K-ohms range in the lowest case. Other fillers may have
>> lower values.
>
>This is of interest to me, because I use carbon resistors and heat shrink so
>often.
>
>I just measured several carbon composition and film resistors, and none
>changed value with pressure (in a table vice) until they fractured.
>
>I can't find any conductive heatshrink, either. This potential issue really
>should concern all of us, because it is a serious hazard if we heatshrink
>power line leads or HV cables.
>
>I'm not doubting the results, but the only resistors that I've seen change
>value outside of age or abuse are large carbon bar resistors when submerged
>in a liquid. For dummy loads, like a Cantenna, the resistor has to be
>specially treated before spending any time submerged.
>
>I routinely mount Allen Bradley carbon composition and Ohmite metal
>composition termination resistors right out in the open air, directly
>exposed to weather, and they stay good for years! They can look really ugly
>and still test good. This isn't saying that is a good practice, just that I
>haven't ever seen an issue outside of direct lightning hits. I have some
>pretty ugly resistors retrieved when redoing antenna end connections (the
>fence wire rusts with age), and they still are good!
>
>If there is a certain brand of materials that is sensitive, it would be
>helpful to learn what it is.
>
>I do not use metal or carbon films outside, or in high overload
>applications. BTW, some carbon films look **exactly** like carbon
>composition resistors!!
>
>73 Tom
>
>_________________
>Topband Reflector
_________________
Topband Reflector
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