Resistor Tolerance - Page 1
Forum section: electronics / beginners

author: DW1961
author_rank: Frequent Contributor
timestamp: June 05, 2021, 05:44:40 am
content: I just got some resistor, an assorted pack, and I tested 25 of the 100ohm resistors. Most were within tolerance, which is +/- 1%. Three out of the 25 were out maybe .2-.5 ohms. The worst one was .8 ohms off.Is that about normal? For hobby stuff, would .8 ohms off matter in a application that called for 100 ohms?

author: Vgkid
author_rank: Super Contributor
timestamp: June 05, 2021, 05:54:51 am
content: That .8 ohms is still in tolerance for a 1% 100ohm resistor.Did you use a 4-wire measurement, or atleast null out your lead resistances for a 2-wire setup.Depending on the useage, that .8 ohms (0.8% error) would not matter for a circuir requiring a 1% resistor. If it is high, you can always trim it down.If low, add series resistance, though this is harder to do with smaller resistances.

author: Siwastaja
author_rank: Super Contributor
timestamp: June 05, 2021, 06:44:19 am
content: Small resistances are difficult to measure accurately, a cheap multimeter won't do.You would need to use a 4-wire meter with accuracy specified way better than 1% (say +/- 0.1%) to evaluate if 1% resistors are within tolerance.Also remember not to touch the terminals while measuring.What comes to the accuracy being ok or not, that completely depends on the application. Many, such as typical digital IO pullup resistors, are fine with +/- 50% accuracy; but sometimes you need +/- 0.1% resistors! You need to understand the circuit in question.

author: DW1961
author_rank: Frequent Contributor
timestamp: June 05, 2021, 07:39:30 pm
content: Quote from: Vgkid on June 05, 2021, 05:54:51 amI meant that is was out of tolerance .08. Tolerance is 1%, which would be as low as 99 or as high as 101, unless my math disability is messing with me again.As far as trimming the resistor, what do you man? They are two lead resistors, not SMDs.I used 3 different meters. They were mostly in concurrence with each other with a .01-.02 variance. I did set REL on them but it made no measuring diff.However I got the feeling as I measured them that it was not a perfectly accurate measurement down to the hundredth.

author: DW1961
author_rank: Frequent Contributor
timestamp: June 05, 2021, 07:41:51 pm
content: Quote from: Siwastaja on June 05, 2021, 06:44:19 amYeah I was getting to that point by measuring the resistors multiple times and having the hundreds walk around on me. Sheesh, I failed to even look up the tolerance of the meters. I'm going to look that up.Well, yeah, the meter (one of them) is 400 ohms +/- .8%+5 so no way I can get a 1% accurate reading on the resistors. All I can say is that the ones I tested were within the meter's ability to measure Ohms. Is that fair analysis? That being the case, most of them measured*well* within tolerance. That is, they measured .2-.4 of perfect.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: June 05, 2021, 09:04:07 pm
content: Simple question with a complex answer. Prior answers have covered much uncertainty in your measurements, though you haven't mentioned measurement temperature or specified temperature coefficient of the resistors so there are some other things to think about.So lets assume that the measurements are valid. Is it unusual to find some components slightly out of spec? Yes, if you are buying military or space grade components, or first rate components from a major distributor. Not at all uncommon when buying components from a surplus dealer or the lowest price components you can find on Ebay.Does it matter? As others have said, it depends on what you are doing. Many circuits will work just fine with 20% tolerances on resistors, which many decades ago was not an uncommon tolerance. As it became cheaper to produce tighter tolerance resistors the wider tolerance ratings just went away. Now it is uncommon to find resistors with tolerances much over 1% because the savings to the manufacturer from a wider tolerance band are negligible. The circuits haven't changed, just the components. If you are building a precision voltage divider or setting a critical threshold you care and these "out of spec" resistors might not be suitable.The earlier post about trimming values is talking about the technique of placing a high value resistor in parallel with another to slightly decrease the combined resistance. Can easily be done by soldering the extra resistor directly to the leads of the trimmed resistor. Increasing the resistance requires clipping one of the leads and inserting small resistor in series with the existing low value resistor. Not as neat or easy as paralleling.

author: TimFox
author_rank: Super Contributor
timestamp: June 05, 2021, 09:53:35 pm
content: If you only have a simple 2-terminal ohmmeter, you could still measure these parts with agoodfixture and a close-tolerance reference resistor. Suggestion: start with two quality binding posts on an insulated mount (standard 3/4 inch or custom spacing) with short wires to two quality banana plugs. You then calibrate the zero with a short piece of wire (here is where the clamping force of the binding posts is better than manual probing) and calibrate near your target value against a precision resistor. Mouser has 100 ohm 0.1% for < $2.00 (singles) and 0.05% for $3.12 (singles) in stock. The math for this calibration (near 100 ohms) is left as an exercise for the reader.A DMM in ohms mode will have worse accuracy than in DC voltage mode, since it includes a separate current source to generate a voltage proportional to the total resistance (including leads).

author: tooki
author_rank: Super Contributor
timestamp: June 05, 2021, 10:39:14 pm
content: Quote from: DW1961 on June 05, 2021, 07:39:30 pmYour math is correct, but error is expressed as deviationfrom the nominal, not as deviation from the extrema of the tolerance band.So if you say “off by 0.08 ohms”, it means 0.08 ohms away from the nominal value. Bear in mind that many devices have different tolerances depending on external factors like temperature, so the same deviation from nominal could be within or outside of tolerance depending on the situation.Quote from: DW1961 on June 05, 2021, 07:41:51 pmWhen the meter’s accuracy is equivalent to or worse run the DUT, whether the DUT is in tolerance is unknowable. You can definitely tell if it’s far out of tolerance, but if it’s really close, it’s unknowable.The +5 counts here is the big gotcha, and how big of an absolute error this is depends on how many digits the meter has: on a 3.5 digit meter, 5 counts amounts to a much larger absolute error than it would on a 6.5 digit meter.

author: JustMeHere
author_rank: Frequent Contributor
timestamp: June 05, 2021, 11:36:42 pm
content: This might be some interest to you. It's about equipment you can use to test resistors.https://youtu.be/3mHemRWq_eg

author: DW1961
author_rank: Frequent Contributor
timestamp: June 06, 2021, 02:08:49 am
content: Thanks for that information. As always, very interesting information here on EEV. I'm going to plug in the DMM again and take a look. I think it read the 100hms resistor to .00 but it could have been .000 too. I'll have another look.

author: Zero999
author_rank: Super Contributor
timestamp: June 06, 2021, 08:09:02 am
content: One interesting thing to note is, if you take the average value of a large number of resistors, with the same resistance, it's normally closer to the specified value, than most individual components. The components with extreme values, either side of the tolerance band cancel one another out to some degree. This isn't perfect, because there will be a slight bias in the manufacturing process.This can be of practical use: if you connect four 100R 1% resistors in parallel strings of two in series, you'll get a resistor, with a closer tolerance, than 1%.

author: DW1961
author_rank: Frequent Contributor
timestamp: June 06, 2021, 05:31:22 pm
content: All of the meters have only measure .1 up to 1kOhm.I just went back and measured all of the 100hm and abut a dozen of the others, and they are all within tolerance given the addition of the meter tolerance.

author: mariush
author_rank: Super Contributor
timestamp: June 06, 2021, 07:40:00 pm
content: Don't forget about your multimeter leads resistance , the resistance of the probes (should clean them, could be oxidized or pitted, so depending on how much pressure or where the probes touch leads, there could be slightly different resistance).Also keep in mind multimeters have a error margin that could be different at different ranges .. for example the meter's precision could be +/- 0.5% + 2-3 digits... and also they'll drift a bit, depending on the temperature (the voltage reference inside meter can be affected by temperature)

author: Labrat101
author_rank: Unknown
timestamp: June 07, 2021, 06:25:57 pm
content: Quote from: Zero999 on June 06, 2021, 08:09:02 amThis is totally false the error will still be 1% ie 100@ 1% that's +- 1so adding 2 in serieswill multiply the error and then putting paralleled will make the tolerance 1% or worse never betterIf using china resistors that use steal wire , Non copper the wire alone has a resistance 1/4w example 0.01621..

author: TimFox
author_rank: Super Contributor
timestamp: June 07, 2021, 06:36:04 pm
content: This is a misunderstanding about statistics. The worst case error in any combination of resistors that all have a worst-case error of +/- 1% is, in fact, 1%.However, if you have a batch of resistors that have uncorrelated errors (a non-trivial assumption), then the combination of several units will have a lower statistical error (standard deviation) than the standard deviation of the batch.

author: Labrat101
author_rank: Unknown
timestamp: June 07, 2021, 06:54:27 pm
content: Quote from: TimFox on June 07, 2021, 06:36:04 pmHave you tried it in practice as 9 time out of 10 the resistor with a higher reading of 2% to 4% will have faulty end capsand will cause a problem . maybe not to home hobbies where 10% may not be noticed.I have tried this on my HP3457A 7 1/2 digit meter which I trust shows this not to be a reliable way to go.With Kelvin probes of course . Also when you apply a load the resistors will heat with a non uniformed drift overthe different values at best you will wind up with 3% .. in real life never better . sorry theory & Practice don't alwayssee eye to eye .

author: TimFox
author_rank: Super Contributor
timestamp: June 07, 2021, 07:02:02 pm
content: Resistors with faulty end caps do not satisfy the assumption of uncorrelated errors, and probably do not meet the specified worst-case error either. Statistical averaging of errors and noise are legitimate techniques, but must be used carefully. Sometimes, close-tolerance parts are the result of binning, so that the remaining parts have a bi-modal distribution. Nevertheless, if you combine the “double-humped” parts, the error will decrease (subject to statistical probability).Bad end caps can often result in excess noise as well.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: June 07, 2021, 07:53:23 pm
content: Some other hidden assumptions statistically here. And even data sheets don't help. If you truly need the answer you need to talk to the manufacturer. There are at least two definitions of what a tolerance means on a resistor. One says that the parts have been 100% tested, and under the test conditions they all were within the tolerance (with possible exceptions due to measurement error and noise. These two terms also need definition but often are held to less than a tenth of the tolerance). With another definition the production is not 100% tested, only sampled, and the statistics of the delivered parts indicate a high probability of compliance (usually somewhere in the 99% to 99.99% likelihood range).The two definitions give different results when combining groups of resistors. In the 100% test case the worst possible outcome is indeed the tolerance limit. For sampled populations the is no worst case limit. Either for single resistors or combinations.For both definitions combinations tighten the distribution, making the probability of a closer to ideal result higher. Strangely, the distribution of combined parts from a sampled distribution has a real possibility of being tighter than for a 100% test population, but in either case the probability of getting an edge of tolerance or worse result is quit low.

author: Fungus
author_rank: Super Contributor
timestamp: June 07, 2021, 09:02:15 pm
content: Quote from: DW1961 on June 05, 2021, 05:44:40 amNo.

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: June 07, 2021, 09:09:57 pm
content: Quote from: Labrat101 on June 07, 2021, 06:54:27 pmNot sure where you are coming from, labrat. Putting several resistors together to make a "better" one (in some dimension) is a tried and true technique both in theory and practice. E.g. this is relied on to create a good Hamon divider.e.g.http://conradhoffman.com/HamonResistor.html(from our own Conrad Hoffman)Also tried and true way to reduce noise using multiple voltage references, opamps, etc.You are right--it will not make an unreliable resistor into a reliable one! So, yeah, if that is your problem statistics is not your solution.

author: TimFox
author_rank: Super Contributor
timestamp: June 07, 2021, 09:17:27 pm
content: In grad school, we knew that an experiment was a failure when a statistics textbook was required to save it.The statistical argument does not require a "normal distribution" within the batch, but does require that the values be statistically independent (uncorrelated) and works better when the mean value of the error distribution is zero. Obviously, if all the values cluster around 0.8% high, then the combination will give a narrow error band around that value.

author: Siwastaja
author_rank: Super Contributor
timestamp: June 08, 2021, 05:12:31 pm
content: Statistical improvement methods also require very large sample sizes and/or accepting out-of-spec outlier units to delivered or discarded during testing, latter of which is usually way more expensive than using better parts to begin with; and former even worse, why bother increasing average performance if some units are not any better?Assuming uncorrelated random errors, 4 resistors combined is betteron averagethan a single resistor, but in any single unit, that does not necessarily happen. A significant part of the batch will be units that are only slightly better. Someone can run the actual math but if you combine four 1% resistors you can maybe say the combined tolerance is very likely better than 0.9%, but unlikely to be even near to 0.25% unless you have one very lucky unit on your table. In the meantime, you could have just bought a 0.5% or 0.1% resistor which wouldn't cost much more than those 4 resistors did. I consider such component averaging methods as kind of "last resort" when suitable part in suitable price group just isn't available; for example, I have averaged several $3 MEMS gyroscopes because the next step with significantly higher specs would be thousands of $.Sometimes component manufacturers specify what their tolerances really mean, for example 3 sigma which would mean 99.7% of units fall within the tolerances.

author: TimFox
author_rank: Super Contributor
timestamp: June 08, 2021, 07:27:34 pm
content: Yes: there is an important difference between worst-case and probable errors.The theoretical “best estimate” for the standard deviation of the combination of four units from a parent distribution with 1% standard deviation is 0.58%. There is confusion between when to divide the sum-of-squares bynor by(n-1)in that calculation. Again, from grad school, the answer is “If it matters, then you need more data”.

author: golden_labels
author_rank: Super Contributor
timestamp: June 08, 2021, 09:00:29 pm
content: Resistors, as well as many other components, are not normally distributed. Due to binning.The distribution is still symmetric around the mean, the errors will cancel out, the CLT still holds and you are likely to get averaged value to be closer to the marked value, but don’t expect them to follow theoretical calculations for normally distributed variables.5% THT carbon resistors are a special case, because they are both the lowest group produced in that technology and — for most manufacturers — the only group. So they are not a subject to binning. But in general this is not true and if you order a bunch of 2% resistors, don’t even dream about finding anything better than 1% there or that combining a few of them will magically bring you into the high precision region.

author: TimFox
author_rank: Super Contributor
timestamp: June 09, 2021, 03:59:04 am
content: The reduction in standard deviation with combinations of units from a parent distribution does not assume a normal distribution, just uncorrelated (statistically independent) deviations.

author: Labrat101
author_rank: Unknown
timestamp: June 09, 2021, 11:39:42 am
content: Personally when I am doing calibration if i have a say 1kwith a rating of 1% . and measures995.0250I will add a 5and lengthen or shorten the wires until it within 0.004% .Resistance wire is also handy to have for supper fine.I have just recalibrated a pro decade resistance box to within 0.4per 1 meg .Example ::On cheap resistors just adding the loss . example 100k @ 1% could be out by as much as 900so adding what is the nearest value in your box 820 would bring it within a less than 1% good enoughfor most hobbyists .In real life I am using MIL grade equipment only