Garage sale purchase, box of 100 1M, 1 Watt, 5% resistors. Probably 50 years old. Thought I'd measure all 100 and single out the one or two that are possibly within 1% of nominal value. Well, only a single resistor read as low as 1.06M, the other 99 were reading 1.17M, on average.
I know my VOM is fairly accurate, since I can sample new 1/4 watt resistors and get evenly distributed readings on both sides of 1M. Measuring 1% resistors gives me readings of 1.005M or 9.995M, typically.
Well, the resistance of carbon comp units like those decreases with applied voltage. I bet that if you were dissipating even a sixteenth of a watt in those resistors (250V) they'd be a lot closer to 1M. ;)
Carbon resistors can be corroded. Oxygen and carbon turns to CO (yeah, carbon monoxide), and the carbon content will gradually diminish (unless you have ozone around, which makes the process much quicker). Some very good barrier coatings are available for carbon film resistors, but there's oxygen everywhere, and it has been decades since those resistors were labelled.
Carbon-black inks also fade with time, for the same reason; Gutenberg bibles have held up well, because he printed with metal-oxide inks.
Don't know, but I had no luck trying to hand select 1% metal film resistors. I never got a distribution over the whole range, but all clustered near one value, (sometimes two values.) I also found that for 10 meg 1% (1/4 W, TH) that when heated (solder them into place) the resistance would change slightly... on the 0.1%- 0.2% level.
Quite rapidly if they're dissipating several times their rated power.
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Paul and I have been arguing about carbon-comp, or CC, resistor drift for years. He claims to often observe up to 25% change; but measurements of my extensive 25-year-old collection doesn't revealed serious drift, with a few exceptions.
It's not surprising Ivan's whole batch has drifted together, it's be more surprising if they didn't.
Many if not most portions of a design can handle a 20% variation in value, we're used to that with caps for example. But if not, one should be using 1% parts. Given the low cost of 1% parts, many blokes have switched to using 1% as the default.
My CC collection is very well-made Allen-Bradley types, with crisp molding and bright color bands.
Here's my story: As a pulsed-power-nut, many of my designs rely on the CC resistor's superior ability to handle high peak transient power, far more than the normal 5x spec for most other resistors; and these parts perform reliably in my designs.
I've previously related here horror stories of failed commercial products that used ill-advised CC replacements unable to handle transient power.
Anyway, for the last 10 years I've been moving to newer type axial-lead bulk-material *pulse-rated* resistors, and as a result have been *struggling* with poor distributor inventory of these types, especially for 1W and higher ratings. Minimum orders and 16- to 20-week delivery times. :-(
The what-to-use issue is dramatically worse with surface-mount types. It's especially painful to try replacing 1W and 2W CC or 2 to 5W WW parts. Sheesh, forgettabout it. Series/parallel stacks.
There's a common failure syndrome in the Tektronix 2235 and similar oscilloscopes, in which a set of carbon-composition resistors in the high-voltage focus chain suffer from value-drift. The scope loses the ability to focus its spot properly.
I don't know whether the high voltage, or the relatively high power dissipation in the resistors is tending to cause them to drift more quickly than would otherwise be the case. Wouldn't surprise me, though.
My local surplus store did have 510k carbon-comp resistors in stock, I bought 8, and was able to find five that were close enough to 510k to work OK. Special high-voltage-rated film resistors would have been another choice, but would have been much harder to find.
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