Actual values of 10% tolerance resistors

Resistors and other electronic parts are rated to be within a certain percent of the specified value. A 1000 ohm resistor with a ±10% tolerance could have an actual value between 900 and 1100 ohms. So what’s the actual range of values in a bunch of resistors? Due to manufacturing techniques, it might not be what you expect:

A 10% carbon-composition resistor is made in a somewhat slipshod manner. The manufacturer tries to get it right, but some of the variables are just too difficult to control. They make up a batch, test them all, and then throw away the bad ones. What’s left is a distribution of values truncated on either side at the ±10% limits. The other main feature of the distribution is the big gap-toothed section in the middle. That’s where they pulled out all the good parts and sold them at a higher price with a ±5% tolerance. How else do you think they make 5% resistors?

The actual value of a 1000 ohm 10% resistor is usually between 900-950 ohms or 1050-1100 ohms. There will probably be none in the 950-1050 range, they were removed and sold as 5% resistors.

We like to see this kind of phenomenon up close, so we’ll send a Flash Destroyer to the first person who sends us a link to their DIY resistor tolerance testing robot.

This has been all over: via Make, mightyohm, tip line, and more.

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  1. And the 5% resistors probably have a hole where the 1% resistors were taken out, too.

    I’ve been toying with the idea of an automated resistor sorting robot on and off for a couple of years… and I’ve just had another idea for a measurement jig, so I guess it’s back on again…

  2. I don’t know how much costs a 1% resistor, but it seems that sorting those into the “+” and “-” group, and then connecting two 2R resistos in parallel, one from each group, will yield relatively precise values, (worst case 3%)

    Lets take 1K for example:
    R1 R2 R1 || R2
    2K+10% 2K-10% 990 (-1%)
    2K+10% 2K-5% 1019.5 (+2%)
    2K+5% 2K-10% 969.2 (-3%)
    2K+5% 2K-5% 997.5 (+0.25%)

    You can also notice that you have a bias towards lower resistance. This is obvious because R1||R2 == (R1*R2)/(R1+R2), and when (R1+R2)==const, the max value is for R1==R2.

    However, a small test case I did with Digikey shows me that 5% resistors costs only 5%-6% more than 10% resistors, so the idea above is hardly useful.

  3. Actually there are no bad resistors. The E-Series is designed in such a way that normally the tolerance bands of two adjacent resitors values touch or slightly overlap. So if a resistor is to large for a 1.0 k Ohm
    resistor it becomes a 1.2 k Ohm resistor. Due to historical reasons this is not true for all values. In the E12
    series there is a slight gap of ~ 0.5% between the multiples of 2.2 and 2.7.

    Klaus Leiss

  4. “A 10% carbon-composition resistor is made in a somewhat ………………………………….±5% tolerance. How else do you think they make 5% resistors?”

    I’ve read that they follow a similar procedure when selling you a 2.4GHz or a 2.2GHz processor for your computer. The silicon chips are all cut from the same wafer. After testing the ones which run stable at 2.4GHz are labeled so; the others are sold as 2.2GHz. :-)

    Kinda makes you feel a little cheated in a way :-|

  5. Carbon resistors are not generally available at 1% or better, only 5% and 10%, so I doubt you will see a 1% gap in the 5% models. 1% resistors are made with processes that are more repeatable to begin with and also have lower temperature coefficient and less aging problems. Selecting 1% carbon resistors by testing would have low yield and could easily change more than 1% under normal operating conditions.

    A lot of modern film resistors, especially the more accurate types are made by laser trimming rather than binning.

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