04-08-2014, 09:03 PM
(04-08-2014, 06:16 PM)Skywave Wrote: I think we've all met the case of the high-ohmic value carbon composition resistor (1 MΩ, say) in an old item of equipment and upon measurement, we find that that 1 MΩ resistor reads something like 5 MΩ. Is such a significant change in the ohmic value caused - or accelerated - by excessive voltage?
Carbon composition resistors manage to go high all by themselves. I have a lot of NOS examples, and they all measure 10-30% high. Likewise, in transistor radios, where they will be exposed to no more than 18 volts, and typically much less, they all drift upwards.
There is a lot of speculation on the exact mechanisms, but I don't think I've ever seen an authoritative explanation.
It seems that they are hygroscopic to some extent, depending on the fillers used and how the resistor might be sealed after manufacturer (frequently with wax, which is often seen leaking from them!). From what I've read, pretty much all are affected to some extent, though apparently the Allen Bradley types (used by HP) are the best of the crowd. Interestingly, manufacturers today mention drift...
It's also possible that some physical changes take place with the carbon granules and fillers. They are manufactured under pressure, which might change during use.
Whether voltage accelerates the drift, I'm not sure. I don't do much with valves, but in my limited experience (of Hacker Mayflowers, mostly), I'd suggest that the voltage across the resistors doesn't seem to be a factor - but that's hardly a decent sample-size for any firm conclusions.
But but it's only my "take" based on what I've read - I don't pretend to know any more than the next person
