10% tolerance resistor... ...what is it good for?

Discussion in 'General Electronics Chat' started by Padapolis, Mar 20, 2013.

  1. Padapolis

    Thread Starter New Member

    Feb 21, 2013
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    While digging and sorting through Grandpa’s very old electronic parts, I’ve started to test the components that I find. Most of what I’ve found is a ton of carbon resistors that are easily from the 1960’s, apparently he loved resistors. While testing these resistors I’ve found that most have moved WAY out of tolerance, and those that are still within tolerance have moved towards the higher tolerance range.

    This shift in the resistors value is what led me to my question: Why would anyone today actually use a 10% tolerance resistor? It just seems to me like modern electronics have been developed with tighter tolerances in mind. So for today’s applications wouldn’t a 5% (or 1%) be just as inexpensive, or are there modern applications for this type of resistor?
     
  2. MrChips

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    Oct 2, 2009
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    I recently answered a thread as to why a designer chose a 10kΩ resistor in a circuit. Proper analysis would show that any value resistor from 1kΩ to 100kΩ would have worked in that circuit. That's over 3 orders of magnitude.
    In that application a 10kΩ ± 50% would still work fine.
     
  3. Markd77

    Senior Member

    Sep 7, 2009
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    I'm paying £0.0008 per 1% surface mount resistor and that's in quantities of only 100. I'm not sure why 5% resistors exist any more, but they do. You would have to use millions of them to save a worthwhile amount of money.
     
  4. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    The 5% and 10% resistors are not made intentionally. They are "artifacts" of the manufacturing process. The machine that makes the resistors tests them all and then they are sorted into where the fit in the tolerance, and then "banded" for that tolerance.
     
  5. MrChips

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    If the manufacturer sorted out the 5% resistors and tagged the remainders as 10% you would end up with 10% resistors with a bimodal distribution.
     
  6. karimC

    New Member

    Feb 1, 2013
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    bimodal distribution?
     
  7. ErnieM

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    Apr 24, 2011
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    Yep. Bimodal distribution. And that's exactly what they had.

    If you bought 10% resistors you might expect the median to be 0% and you could select those (meaning measure every one) that have 5% or lower tolerance, but for those who actually did this found no resistors within the 5% mark.

    Those were sorted and sold as 5% by the manufacturer.
     
  8. WBahn

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    Mar 31, 2012
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    Yep. If you bought a 10% resistor then you were pretty much guaranteed that it would be at least 5% away from the nominal value. And if you bought a 5% resistor you knew that it would be at least 2% away.

    But that's fine, because if you chose to use a 10% resistor you were asserting that as long as it wasn't any more than 10% off that it was good enough.

    I still have a bunch of unbanded (i.e., 20%) resistors. The next time I think about it, I should test some of them and see how far they are out after aging for probably at least 40 years.

    Now, even though there may not be any savings in buying 10% (assuming you can find them) over 1%, many people still design with 10% tolerances because it let's them stock just the E12 values.
     
  9. crutschow

    Expert

    Mar 14, 2008
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    The reason there were 5,10, and 20% resistors was that, way back when, the looser tolerance carbon composition resistors were much cheaper to manufacture than 1% film resistors (I think perhaps a factor of 5 or 10 difference). Today they've refined the manufacturing process so that 1% resistors are nearly as cheap as 5%, thus there generally is not much reason to use resistors will a looser tolerance than 1% anymore.
     
  10. MrChips

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    I just happened today to be sorting some resistors that are at least 25 years old. Most were 10% tolerance and all measured more than 10% over the nominal value.
     
  11. WBahn

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    Where they all carbon composition resistors? Or were some/all of them of different types?

    Do carbon film resistors tend to increase in resistance with age?
     
  12. MrChips

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    Oct 2, 2009
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    I believe they were all carbon composition. I will double check in the morning.
    I know nothing about this but I recall someone else commenting on AAC not too long ago that carbon composition resistors increase in resistance with age.
     
  13. WBahn

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    Mar 31, 2012
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    I've heard before that carbon comp resistors increase with age and the mechanism was explained to me at some point. But I recall neither the mechanism nor whether it applied to carbon film resistors. I would think that it should.
     
  14. WBahn

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    I just did a bit of checking and while I couldn't determine if carbon film resistors age the same way carbon comp do, I did find some stuff that makes me less confident that they should. Carbon comp resistors use various binders and fillers to achieve the different ranges. I suspect that carbon film resistors either don't use these or use less of them. So if the aging mechanism is primarily related to the binders and fillers, then carbon film resistors may be much more stable.
     
  15. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Most of the error is oxidation on the leads (increased resistance). Try some steel wool or even a pencil eraser to polish up a section of the leads and measure again.
     
  16. WBahn

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    I don't think I buy this. While it's an issue were eliminating, if that were the case, then it would almost always be mentioned in references that talk about aging effects and it would affect almost all kinds of resistors equally. Plus, many of the sites I looked through were clearly talking about changes in resistors in situ and not resistors that had been sitting in a parts bin for a couple decades.
     
  17. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    That is the point, the parts already soldered onto a board will have their leads protected by the solder connection. When they are sitting in a parts bin for 20 years, they will get a tin oxide coating on them that increases the resistance. If you have ever tried to solder an old (i mean old!) component to a board, the solder will bead up around the oxidized tin and not wet the surface well.

    This is the increased resistance I was asking the OP to check. I am guessing it will be a fairly constant value so the resistance will appear as a larger percentage on lower value resistors. I will try it myself with some antiques in my box and let you know what I come up with.
     
  18. Padapolis

    Thread Starter New Member

    Feb 21, 2013
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    Yes GopherT I did notice the corrosion on the leads of the resistors, and when I started testing them I used sandpaper to make the leads all pretty and bright. However, that didn’t change things much and many were still out of tolerance. Now sanding thousands of resistor leads wasn’t exactly in my top 10 to-do list, so I switched my meter leads to alligator clips. I figured the bite of the alligator clips would accomplish the same thing as sanding the entire lead, but with a much reduced time requirement. Still I’ve only found a small fraction of resistors that are still within tolerance.

    I have also noticed that all resistance changes have all been higher than the nominal value. And even in those resistors that are still within tolerance, none of them are below the nominal value. Which suggests to me that even those resistors have changed with age, despite still being within tolerance.
     
  19. richard.cs

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    Mar 3, 2012
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    Also known as a "Rabbit ears distribution" - a Gaussian with the centre chopped out (sharp edges at the cut, not two side-by side curves). This still happens with some parts but so far as I know isn't common for resistors any more.
     
  20. WBahn

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    Though you don't have the curve on the outside, either. You have sharp edges there for the same reason -- resistors more than the spec'ed tolerance are removed from the distribution. They become marked as being the next value away, except in the tiny regions in which the tolerance ranges don't overlap.
     
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