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Re: Resistor in series

 

Maybe a new thread should be started, as this seems to have drifted (pun intended).

But just to add to the noise, my statistically insignificant data is that high-value carbon comps tend to drift much more than do low-resistance ones. I would hesitate to declare a boundary between low and high, but it seems to be in the tens of kilohms. Roy's data is at least consistent with that. I would not have expected gross drifts with a 300 ohm resistor. But 300k? I would bet that a fair fraction would read as near open circuits.

That also comports with the common problem with resistor strings in HV circuits (not just Tek's).

--Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070

On 12/1/2020 20:37, Roy Thistle wrote:
On Tue, Dec 1, 2020 at 11:10 AM, Richard Knoppow wrote:

I wonder about improving stability by baking.
This is not really replying to Richard... but, just mostly reiterating what I previously claimed about carbon composition resistors.
We have hundreds of IRC SR series 2W fixed carbon composition resistors. These are more than 5 decades old. They are painted as Orange Black Brown Gold
and so have, Nominal value: 300 ohms, Nominal Range: 285 to 315
I measured 10, taken at random, from the box.
333,322,337,321,339,315,334,314,314,329
Are these resistors way out of spec.? I wouldn't say so, given the uses to which they are properly put. They are more like silver banded (10%) than gold.
If you notice they all measured 315 ohms or greater. This I knew was almost certainly the result of "moisture" absorption. After time in the oven, they mostly measured to be less than 315 ohms... and so mostly back in spec.
The original MIL spec. for these resistors was MIL-R-11C, which is long obsolete.
Table XII "DC Resistance Test Voltages", pg.13, of MIL-R-11G give the voltages different values of carbon composition resistors are to be tested at... to measure their resistance.
MIL-HDBK-217F Section 8.1, page 9.2 gives the reliability of carbon composition resistors.
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