Out of spec NOS A-B resistors query

[Zim]

So, I’ve been getting together a few NOS Mil-spec Allen Bradley resistors for an La2a build.

just got 20 x 2.7k carbon comp resistors and all are measuring somewhere around 3.3k. the colour codes are correct for 2.7k so the values have obviously drifted.

Ultimately my question is should I use them as 3.3k resistors now, still use them as 2.7k or should I just not use them at all?

cheers

Zim

 

[5v333]

If they measure 3k3, then u can use them as 3k3.

 

[JohnRoberts]

I would be very suspicious of resistors that far off value, resistors are generally inexpensive, I'd bin them.

JR

 

[scott2000]

I've always wondered if there were factory mistakes out there that could explain some large numbers of similarly speced goofy components. Usually only see them at surplus places. Would be interesting to know.

 

[Zim]

I would be very suspicious of resistors that far off value, resistors are generally inexpensive, I'd bin them.

JR

That was kind of my thinking. I was just surprised that out of 20 they were all so close in value to each other (at there new value) that I wondered if it was to be expected.

They were so close to each other that my initial thought was that I’d been sent the wrong ones until I checked the colour code

 

[Zim]

I've always wondered if there were factory mistakes out there that could explain some large numbers of similarly speced goofy components. Usually only see them at surplus places. Would be interesting to know.

Missed this while I was replying to John... but, if true, that would certainly go some way to explaining these values

 

[zamproject]

Hi
Carbon resistor drift over time, I hardly find out that it could be in the +50% +100% range for a 50' tube device, and changing 100 resistor in 12 tube PTP device is a pain
Moisture seem to be the major factor, carbon react like a sponge, at low level structure the carbon density lower, increasing the resistivity.
Best
Zam

 

[Rob Flinn]

I've had to replace CC resitors that are as much as 2 or 3 times their rated value. That is why I don't use them on my own builds.

 

[JohnRoberts]

I have some old 5% NOS carbon film resistors that have been sitting around for half a century. They are decent quality (ROHM). I just measured a 27k and it measures 27.33k so around 1% high... not bad for 50YO penny resistors.

I have had good luck with decent quality carbon film. I have seen those ROHM resistors turn red hot and return to with a few percent of rated resistance. That said I have learned from cheap Chinese builds that they can find parts too cheap to work.

Several years ago I repaired a battery charger for my old (now RIP) neighbor's mobility scooter. I didn't have a schematic but it wasn't rocket science to figure out the charger circuit. Oddly it turned out that a small 1/4W resistor was way way out of tolerance. It took me longer to find than it should have because bad resistors like that are not common in my experience so I wasn't looking for it. It was not overheated or stressed, just a cheap POS component way out of tolerance.

Resistors are cheap. Pay the extra penny or two and get something good. Reduce the number of variables you have to worry about.

JR

 

[emrr]

I have a pretty large quantity of NOS carbons, most are higher than rated. I think it may relate directly to humidity absorption. You can solder them in and measure them a few minutes later, they'll be different. It's what they do now, with age accumulation.

 

[Zim]

I have a pretty large quantity of NOS carbons, most are higher than rated. I think it may relate directly to humidity absorption. You can solder them in and measure them a few minutes later, they'll be different. It's what they do now, with age accumulation.

But you use them at their ‘new rating’ right? So if it’s a 2.7k that measuring 3.3k you wouldn’t use It where a 2.7k was needed would you.

probably a dumb question, but would they start to come back into spec once they were in circuit and started getting some DC passed through them?

(I’m still inclined to bin them, but if they’ve got some potential to still be useful I’d rather not - just don’t want to compromise another circuit by using them)

 

[Tubetec]

Ive noticed typically that older higher value carbon comps are the worst offenders as far as drift ,

I did get a bunch of resistor lucky bags from Maplin way back , contained some nice 3watt Allen bradly carbon comps , what was marked as 1200ohms was actually 12kohms , the measured value of several of the wrongly marked items was incredibly close so I discounted drift and put it down to a mistake in the colour banding at the factory .

 

[Tubetec]

I guess you could always apply the appropriate voltage/current to one of the suspect resistors , run them fairly hot ,then re-check values after 'burning in'

 

[emrr]

But you use them at their ‘new rating’ right? So if it’s a 2.7k that measuring 3.3k you wouldn’t use It where a 2.7k was needed would you.

probably a dumb question, but would they start to come back into spec once they were in circuit and started getting some DC passed through them?

You might experiment with baking them some long period of time, see if what they look like afterwards. If they've absorbed decades of humidity, they'll be noisier also, so any dehumidifying is good. You use them at whatever value they actually measure. I've put ones in based on the way they measured, then took them back out when soldering caused large unpredictable drift.

 

[5v333]

Wouldnt humidity lead to higher conductance?

 

[zamproject]

Wouldnt humidity lead to higher conductance?

Probably not when H2O substitute graphit powder, which have higher conductance anyway (or lower resistance, G=1/R)
Best
Zam

 

[Tubetec]

At first look you might expect moisture to cause a drift lower than spec'd value , but maybe theres something more complex going on in the chemistry of the resistor once moisture gets in . I got boxes of ancient stock Erie carbon resistors , stored in a cow shed for years and years , drift wasnt so bad on lower values, values of 100k ohms or more showed extreme drift in the high direction , didnt find any which drifted low as far as I remember.

 

[5v333]

Probably not when H2O substitute graphit powder, which have higher conductance anyway (or lower resistance, G=1/R)
Best
Zam

you are right of course. Carbon has much much lower resistivity than H2O.

 

[PermO]

Would something like a food dehumidifyer work for this ? (you leave these on for a couple of days)

I've got a small one to dry herbs and fruits and vegtables.

Or would a higher temp be required... like a kitchen oven at 50°C ?

 

[Tubetec]

I'd be wary about baking off resistors or any other electronic components in the same place you cook your food .
I seem to remember Morgan Jones speaking about heat treating wirewound resistors , it produced some small drift in initial value but seemed to be more stable after being put into service.

A small cheap counter top grill/oven combo might be a plan , but it might be best to sheild the components from the direct heat of the element also.