[R-390] Re: Parts Selection

[Mark Huss]

Don't remember what the value was. we just grabbed a handful of what 
there was a pile of in the parts bin. The key is not to stress it too 
much. Put ten times the wattage through it and you get a nearly instant 
open. But keep it around two to five times the wattage rating, and you 
will blow it slow enough you can see the effect. A lot of times, I will 
see carbons in high heat areas that have reduced in value. I figure that 
is why. Then again, come to think of it, high heat usually means low 
resistor values to start with.

Tim Shoppa wrote:
> Mark wrote:
>
>   
>> I had a similar discussion with an Army Instructor.
>> He claimed that a Carbon resistor acts as a fuse,
>> and always fails open. That it will open under
>> overcurrent(I.E. Heat) faster. Finally he told me
>> to put my money where my mouth is. We took
>> a handful of carbon comp. resistors, a power 
>> supply, and an ammeter. Then put 1 Watt through
>> the 1/2 watt resistors. Every time, current went
>> up for about thirty seconds, climbing higher 
>> and higher, until the carbon resistor was nearly a short, then it 
>> opened.
>>     
>
> Your experience mostly matches mine, Mark. I think there's
> a good element of variability because carbon compositions
> span 6 orders of magnitude in terms of resistance rating
> (and therefore the "composition" part must be highly
> variable depending on value and maybe maker)
> but what you describe is the failure mode I often observed
> (meaning, when I was a kid I would burn up resistors
> this way on purpose!) for carbon comp resistors in
> the 10 to a-few-hundred ohm range.
>
> In the 2.2K range things seem to work different simply
> because I find so many charred 2.2K's that measure
> to be 5K or 7K or 10K at on the ohmmeter.
>
> On the other hand, for metal films, the failure mode is
> ALWAYS that exceeding the wattage rating by a factor
> of a few will make the resistor so hot that the solder
> on the leads melts and drips off. Measuring the resistance
> then shows that the resistor is almost exactly the same
> value as when you're started.
>
> Now, exceeding the power rating by a factor of twenty
> will make the metal film resistor glow red, and generally
> it won't have the same value anymore if it remains
> intact :-).
>
> Others here obviously have a different attitude than me,
> but I regard it as a FEATURE of a circuit if a failure results
> in obviously burnt-until-inoperative-or-arcing-or-glowing
> components. Choke fires are a common element that all
> my transmitters have survived. Sometimes I finish the
> QSO before I turn off the transmitter :-). Please
> remember that I'm also the guy who advocates not
> baby-stepping your electrolytics up but just turn
> on all the volts and let 'em blow if they're weak!
>
> Tim.
>
>   


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Some people are like a Slinky .. not really good for anything,
but you still can’t help but smile when you shove them down the stairs.
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