[R-390] Re: Parts Selection

[Drew Papanek]

From: Mark Huss <mhuss1 at bellatlantic.net>
To: Tim Shoppa <tshoppa at wmata.com>
CC: Drew Papanek <drewmaster813 at hotmail.com>,  r-390 at mailman.qth.net
Subject: Re: [R-390] Re: Parts Selection
Date: Fri, 26 Jan 2007 12:51:17 -0500

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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