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Probably a real newbie question but I can't seem to find an answer easily...

If I have 2 resistors both 1K but one is a 1% tolerance and the other is 10% and I measure them and they are the same. Let's assume exactly 1k, then are they effectively the same?

Could I use the 10% in a circuit that requires a 1% tolerance (as the actual value is within 1%?)

Or is there some other property Im not aware of?

Thanks

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If you're measuring the resistance in similar conditions to what it will experience in the circuit, then yes, you can use either resistor. If the circuit is going to heat up the resistor by 70C, then maybe not. Resistors, capacitors, and inductors can all be made out of different materials, each of which has its own properties. Here are some things that might matter aside from the component value and tolerance:

  • Power handling
  • Parasitic resistance, inductance, and/or capacitance (important at high frequencies)
  • Temperature coefficient
  • Max operating temperature
  • Max soldering temperature
  • Operating lifetime (very important for electrolytic capacitors)
  • Breakdown voltage and polarization (for capacitors)
  • Matching of multiple components (sometimes more important than tolerance)
  • Electromagnetic interference and compatibility (radiation/sensitivity)
  • Noise generation
  • Package size and weight (often related to power handling and parasitics)
  • Reliability of the supply chain

Usually only a couple of these will be major concerns in any particular application. For example, in audio systems, you're very concerned about noise. In automotive and aerospace, a high max operating temperature is required. In high-frequency design, parasitic inductance and capacitance are a big deal. For low frequency, low power, low precision circuits, just about any component will do.

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In my experience with electronics manufacturing a batch of 10% resistors will be within the 10% tolerance range, but may NOT be in a group centered around the stated value. There may be none that measure inside the 1% range because the manufacturer sorted out the closer tolerance parts and sold them as 1% resistors at a higher price, then sold the resistors that fell outside the 1% range, but inside the 5% range as 5% tolerance parts, cheaper than 1% but higher priced than 10%, and those outside the 5% range but inside 10% as 10% tolerance parts, etc. This allows the manufacturer to maximize their return on investment and still have reasonable rejection rates for the manufacturing process. Statistically the "tolerance" will fall in a bell curve around the center value, but the manufacturer only guarantees it will be inside the tolerance range. This applies to all mass manufactured products as part of quality and cost controls.

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Precision resistors may be better in a number of areas - the stability of the resistance over time and at different temperatures and the amount of noise the resistor adds to the circuit. There is a decent overview here.

If a circuit specifies 1% tolerance, it is generally because it requires it.

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  • \$\begingroup\$ I would suggest that the vast majority of circuits which specify 1% tolerance, at least with low-power resistors, do so simply because it is in many cases more practical to use 1% resistors for everything than to use 1% resistors in the places that "need" the precision and 5% resistors in the places that don't. Even if 5% resistors are "less than half the price" of 1% resistors, replacing fifteen 1% resistors with 5% resistors would (judging from Digi-Key's prices) save about one penny. Such savings are in many cases not worth the effort require to stock both 5% and 1% parts. \$\endgroup\$ – supercat Dec 5 '14 at 16:11
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Specifying a resistor as "1%" is not really sufficient to describe it, but it is often used as kind of a short form. One would expect a "1%" resistor to have tolerance within +/-1% (of course)- probably quite a bit better- see below, but also to be reasonably stable with temperature (perhaps +/-100ppm/°C or better- again probably quite a bit better typically) and to not change much with humidity and when it is soldered, and with time. By default it would likely be a film type (probably metal film), so would have limited pulse-handling capability, so you can't always substitute a 1% part for a 10% part.

If you use a 10% resistor selected for 1% in a measuring circuit you may find that it is unstable after soldering, with temperature, with mechanical stress, with time or with humidity and your circuit is not as stable as intended.

The materials used for a precision resistor can be quite different from those used for a non-precision part- as well as better trimming machines and so on. By the way, at least for the past 30 years or so, resistors are made by trimming on automated machines (even 5% resistors) you'll generally find some statistical correlation between values of resistors in the same batch. Usually they'll cluster around a value that's a bit different from the nominal value, and they'll typically be within about 1/3 to 1/5 of the nominal tolerance (so a 5% resistor is usually within +/-1% to 1.5% and a 1% resistor is usually within +/-0.2% to +/-0.3% of nominal. That's a consequence of wanting to get almost all the resistors within tolerance so none have to be discarded.

For example, a quick measurement of four 8.25K 1% 0603 resistors (Rohm) gives a mean value of 8.26128 and a standard deviation of 0.01433. If those statistics were representative (too small a sample), and the distribution was Gaussian, maybe one in a million would fall outside the 1% band.

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The individual resistors may have the same resistance but a large sample size of 10% tolerance resistors (a box of them, for example) will not all have the same resistance and many of them will not be within 1% of the desired value. You can use the individual 10% resistor that you measured to be within 1% tolerance in a circuit that requires 1% tolerance, but if you try to use 10% resistors in large quantities to build many such circuits you will have to measure all of them and reject most of them in order to obtain 1% tolerance. In that case it is better to use 1% resistors.

Think of the tolerance as a probability distribution centered around the desired value. The 1% tolerance resistors have a much tighter distribution and will always be within 1% of the desired value. The 10% tolerance resistors have a much wider distribution -- some of them will be within 1% by chance (like the one you measured) but most will not.

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  • \$\begingroup\$ The variance of a batch of resistors is not actually a normal distribution with the mean at the desired value. The variance of batches have their collective mean at the desired value, but each batch will have been cured at a different temp, had slightly different material, etc. So the mean value for each individual batch will not be at the desired value, but rather a normal distribution with the mean higher or lower than the desired value. If you need the mean value to be the desired value, mix resistors from different batches. \$\endgroup\$ – dotancohen Dec 5 '14 at 9:25
  • \$\begingroup\$ @dotancohen Yes, I know different batches will have different means, and not necessarily centered on the desired value. I'm just describing how the distributions look in general to compare the 1% tolerance distribution to the 10% tolerance distribution -- i.e. the former is much tighter than the latter. \$\endgroup\$ – Null Dec 5 '14 at 14:48
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If you measure two resistors accurately and they both measure exactly 1k then yes you can use that resistor, in your hand, that you just measured in a circuit that requires a 1% 1k resistor.

But what about the next one you buy? That 10% is the tolerance range of that part number so in theory the next one out of the package could be 900 ohms.

You pay extra for those 1% because the manufacturer guarantees that any of those parts you buy will be within 1%. Presumably they do extra testing or at least binning and they're more desirable so they cost more.

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