I work at an electronics store and the other day a customer came in who was rebuilding a circuit board. I sold him some resistors, but later he came back in wanting to return some of them because they were only 1% tolerance, and he needed 5%. I'm not an engineer but I have a hobbyist's understanding of electronics, and I was under the impression that there are situations that would call for a more exact value, but never a less exact one. Am I mistaken?
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37\$\begingroup\$ It's highly possible they simply don't know themselves. They just know the repair schematic/BOM says 5%, and they got 1%, and don't know what "percent tolerance" means. I don't think there's a technical reason here, just a simple silly error. \$\endgroup\$– ShredderCommented Feb 20, 2023 at 9:13
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22\$\begingroup\$ Well 5% is greater than 1% so it means 4% more tolerance, more is better, right? \$\endgroup\$– floppydiskCommented Feb 20, 2023 at 11:38
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11\$\begingroup\$ I know of one major computer manufacturer that designed their own bus, in the spec was to use a %5 tolerance resister, this was to help randomize the retry delay in the event of a bus collision, using tighter tolerance resistors would result in more bus collisions. \$\endgroup\$– Glen YatesCommented Feb 20, 2023 at 17:23
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9\$\begingroup\$ @Glen Yates - But that by itself doesn't guarantee randomness. Resistors, especially if they're from the same batch (most likely), all are probably very close in value to one another. \$\endgroup\$– SteveShCommented Feb 20, 2023 at 19:38
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6\$\begingroup\$ @SteveSh Nobody said it was a good design... \$\endgroup\$– MichaelCommented Feb 21, 2023 at 0:01
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11 Answers
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If everything else is the same then there is no technical reason to prefer a looser tolerance, so I think you are correct.
Of course there are situations where a particular manufacturer and part number may be required for regulatory or other similar reasons and no substitutes are permissible. And there are many resistor characteristics that are important in some situations but are not specified by the nominal resistance value + tolerance + power rating (or package). For example, a composition resistor may be specified because of superior power pulse handling, but they cannot be made in tight tolerances due to the way they are manufactured. A 1% film resistor of the same value, package size and power dissipation rating might fail in short order.
answered Feb 20, 2023 at 1:44
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4\$\begingroup\$ If, as it seems from the question, it was a walk-in at a brick and mortar store, your second paragraph seems very unlikely. \$\endgroup\$– jaskijCommented Feb 20, 2023 at 20:00
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12\$\begingroup\$ @jaskij Yes, of course, it is included as a more general answer for completeness since answers here are expected to have longevity. \$\endgroup\$ Commented Feb 20, 2023 at 21:33
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There might be a hidden specification in wanting 5% - the circuit might be relying on carbon film (typically 5%, "beige bones") vs metal film (typically 1%, "blue bones") resistors. Carbon film has different behaviour under temporary overloads (in a bad design, a metal film resistor might fail) and looks (which might be relevant if someone is restoring vintage equipment and wants their resistors as beige as they were).
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8\$\begingroup\$ Looks! This is important for retro electronics. \$\endgroup\$– fraxinusCommented Feb 20, 2023 at 17:07
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If he truly did want 5% instead of 1% then he may have been trying to bin the resistors. That is, individually testing each one and selecting for a particular value.
Are you sure he did not say he needed 0.5%? Because no one says it is "only 1%" then proceeds to get 5%.
Otherwise you might prefer 5% if the resistor composition you needed simply did not come in 1%, but that's preferring one composition over another, not 5% over 1% and I would not expect anyone to phrase it as such.
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8\$\begingroup\$ I once measured a number of Philips metal and carbon film 5% resistors, and found them all to be within 1 % of the marked value, so hoping to "bin" resistors to get values slightly off a standard value is a lost cause with modern components. \$\endgroup\$ Commented Feb 20, 2023 at 6:49
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2\$\begingroup\$ I remember seeing in more than one ancient circuit diagram instructions that imply what now is known as "binning" - e.g. "choose R4 and R5 to be as much equal as possible, ideally within 0.5%". The fact that one cannot get real 5% or 10% resistors does not stop some retro fans from looking for them. \$\endgroup\$– fraxinusCommented Feb 20, 2023 at 10:42
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7\$\begingroup\$ Didn't lines of resistors exist where you were guaranteed NOT to be within 1% if you bought 5%, since those within 1% were binned at the factory and sold as 1%? \$\endgroup\$ Commented Feb 20, 2023 at 15:14
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2\$\begingroup\$ @Hearth: There may have been a time where there would be no convenient way to order an 8.45K resistor, and so the only way to get a 8.45K resistor would be to order 8.2K and maybe 9.1K resistors and hope for a value close to 8.45K. Nowadays, however, if one wants an 8.45K resistor one can simply order one. \$\endgroup\$– supercatCommented Feb 20, 2023 at 16:29
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5\$\begingroup\$ @rackandboneman not anymore, because the tolerance is expected to hold over the temperature range and the projected lifetime of the resistor. Resistors that are 5% are made with different technology and if you measure them new at room temp they may even be better than 1%. \$\endgroup\$– fraxinusCommented Feb 20, 2023 at 17:05
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There's no technical benefit. All resistors are ideally 0% tolerance. As they can't be, circuits should be designed for the part's worst value at tolerance. It's a design complication that leads to performance variance.
In the past, 5% resistors have been significantly cheaper than 1% resistors. That becomes important when buying in volume rather than handfuls. Nowadays, 1% resistors are the standard part and there's no cost benefit in 5% resistors.
My guesses here:
Your customer was buying replacement parts for an existing circuit and so wanted like-for-like, not understanding there were no downsides to a 1% part
Your customer was buying against a spec' or parts list and simply wanted the specified part
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\$\begingroup\$ In some cases, devices have regulatory approvals which are contingent upon using particular specified parts. I recall a semiconductor vendor discontinuing a lower-grade part except on a special order basis, and suspect that orders for the lower-grade part would have been filled with parts that failed testing for the higher grade but passed the lower grade, if any were available, and otherwise filled with parts that passed the higher grade. If all parts that pass any testing pass the higher grade, there would be no need to sell any lower-grade parts except... \$\endgroup\$– supercatCommented Feb 20, 2023 at 16:14
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\$\begingroup\$ ...to customers who need a part which is labeled XYZ123-3 (the 3 representing some particular grade which is inferior to any parts that are still being sold). If the company's policy has always been to fulfill orders for XYZ123-3 chips using parts that pass more stringent tests if no inferior parts are available, the fact that a company starts fufilling all orders for such parts in such fashion would not represent a change requiring new design approval. \$\endgroup\$– supercatCommented Feb 20, 2023 at 16:21
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1\$\begingroup\$ I'm not the downvoter. I thought your second scenario painted a picture of someone who naively wants a particular part, rather than ignoring the fact that in some industries even the most benign substitutions require lots of paperwork. If a board is assembled with 1% parts instead of 5%, all of the documented inspection criteria will need be updated to accommodate that, procedures for guarding against counterfeiting may need to be reevaluated, etc. Issues that can be dealt with, but may be expensive enough that special-ordering parts labeled with poorer tolerance may be cheaper. \$\endgroup\$– supercatCommented Feb 20, 2023 at 18:52
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2\$\begingroup\$ @supercat, no problem but scenario 2 doesn't anywhere suggest the customer's naive, just someone quite reasonably following the specified process in the released documentation ('simply wanted the specified part'). May be technically knowledgeable, may not but doesn't matter in that context. Had you misread the meaning there? \$\endgroup\$– TonyMCommented Feb 20, 2023 at 19:01
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2\$\begingroup\$ Sorry @supercat, I'm afraid you're really reading something into it that just isn't there and it's generating many long comments. 'Simply' means there's no other factors, no personal psycho-whatsit stuff :-D 'I simply want a sandwich' etc. \$\endgroup\$– TonyMCommented Feb 20, 2023 at 20:26
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For a random signal noise source, you may want noisier resistors that tend to come with larger tolerances because of different technology. Admittedly that's extra-handwavy because the additional noisiness of, say, carbon resistors comes from shot noise while a good random signal noise source would rather tend to rely on Johnson noise (which is more dependable).
Another handwavy reason might be that more precise resistors tend to be trimmed, and bein suitable for trimming implies an underlying structure that is more susceptible to stray inductivity.
Note that either of those reasons is really grasping at straws as an intellectual game.
The straightforward answer is "no", really.
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Customers don't know everything, I once returned a suitable product as unsuitable just because I don't know any better.
However customers are always right (tm)
With beer 5% is better than 1%, perhaps your customer was thinking along those lines.
answered Feb 20, 2023 at 11:26
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5\$\begingroup\$ I recently learned that expression in your second paragraph was actually corrupted from the original: "The customer is always right in matters of taste" which to be honest makes a lot more sense to me. \$\endgroup\$– MichaelCommented Feb 21, 2023 at 0:05
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\$\begingroup\$ I'm accustomed to low precision resistors having higher tolerances to overvoltage. \$\endgroup\$– JoshuaCommented Feb 21, 2023 at 3:10
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Of course agreeing with all other answers that generally say "no", I see those four cases:
In analog musical circuits not-tight values may produce "the tone", when all loose tolerances in filters and other elements sum up, making one's tone less reproducable. Thus, even when you build a clone of your sound processor and give it to a friend, you two sound slightly differently. Though this does not apply to Hi-Fi equipment - rather artistic ones (e.g. guitar effects, synthesizers etc.).
It was the point to measure them and prepare some report/statistics - for example for students classes.
Preparation for bigger order: as 5%s are cheaper, he searched for best under that price label.
Thinking about next serviceman: seeing 1% would think "why it is so tight and do I really need the same for replacement?". About half a year ago I had to replace 1200 uF capacitor in somehow regular switching power supply. As a hobbyist I was wondering for a while if I really need this value (I had only 1000 uFs in my box and live in a place without electronic store by the corner). I had used 1000 uF I had and since then owner of the equipment hasn't complained - and we are still friends! ;D.
The customer may have not mentioned that being sure that you will provide him with 5%s as defaults.
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depending on the circuit you are making, you don't need the precision. For example, if it is a current limiting resistor it will do the work either at 1% or 5%.
Honestly the main reason to prefer 5% over higher precision like 1% is cost, the other is power, it is very hard to guarantee very high precision for higher power components.
Edit: now for your specific case, I can't see a case where you would want less in general, if everything else is equal then yeah 1% is within the 5% and should work in all conditons.
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People forget economics
Why would you want a 5% tolerance resistor? Because it's cheaper than a 1% tolerance resistor and your circuit can stand the variation.
A good example of such a circuit is a voltage divider where both resistors are 5% tolerance. Generally (not perfectly, but generally), resistors of the same tolerance will vary in value in the same way under identical environmental conditions. Thus, it's not worth paying for 1% tolerance when all you care about is the unitless ratio of the two resistances.
In this case, if the rebuilt board already had a 5% tolerance resistor, another 5% tolerance resistor would be required or the ratio would vary with environmental conditions, which can be bad.
Note: This isn't perfect, obviously, but I've not only seen this done, I've done it myself, especially in microelectronic design where you know for a fact that the manufacturing process of the two resistances is identical. Heck, we'd use short strips of polysilicon rather than actual resistors that would take up more real estate so long as they had the same orientation to guarantee the same processing encroachment.
I can't help it... I'm reminded of an old joke from my college days. A mathematician, a physicist, and an engineer are asked to answer the question, "what is 2+2?" The mathematician proudly answers "4!" The physicist smirks and says "4.00000!" The engineer looks a bit bewildered by their answers and responds, "any damn thing that works."
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1\$\begingroup\$ 4! = 24. Also, a related joke that 2+2=5 for sufficiently large values of 2. \$\endgroup\$– MichaelCommented Feb 21, 2023 at 0:08
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\$\begingroup\$ @Michael Hah! That's a twist on the mathematician's answer. In the case of the joke, the exclamation point is literally that and not a factorial operator. \$\endgroup\$– JBHCommented Feb 21, 2023 at 0:14
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\$\begingroup\$ Apparently a good answer but how could it be worth the gas to return and exchange? \$\endgroup\$– JoshuaCommented Feb 21, 2023 at 3:11
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\$\begingroup\$ @Joshua Without an explanation of the user's circuit, that's next to impossible to answer. But if the resistors were used in a current or voltage regulator, then the fact that the regulator isn't stable would justify the gas and time. ... And that's assuming your customer actually knew what he was doing, which I hope was the case, but might not have been. \$\endgroup\$– JBHCommented Feb 21, 2023 at 3:13
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1\$\begingroup\$ But 1% resistors may be more economical. Our automated PCB assembly area had reels of standard value resistors (4.7 K, 5.1 K, 10 K, ...) and capacitors (0.1 uF, 0.01 uF, ...) on the pick and place machine. The resistors for the most part were all 1% film resistors. It would cost more to not use those 1% parts, and replace them with 5% tolerance components (that might require a hand operation) than to use the better tolerance, slightly more expensive parts. \$\endgroup\$– SteveShCommented Feb 21, 2023 at 13:17
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Yes. If you are building a run of (say) 100 RC oscillators and you want them to have a spread of frequencies from (say) 95-105kHz. So you buy a lot of 5% resistors and select to get that range.
Of course there are better ways to do this, but it isn't an impossible scenario.
In other words, the customer is always right.
(Just playing devil's avocado here.)
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\$\begingroup\$ You can't guarantee that a batch of 5% resistors is going to have any sort of distribution (Gaussian, uniform) across the tolerance range. \$\endgroup\$– SteveShCommented Feb 21, 2023 at 13:19
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\$\begingroup\$ no, that's why you'd have to select from a batch. In fact you might have to buy a lot, because 10 adjacent resistors on a reel might be 4% away from nominal, but very close to each other. As I say, I am just playing the game here. Yes, there just might be a reason - although not a great one. \$\endgroup\$– danmcbCommented Feb 21, 2023 at 13:56
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Although answered in several ways here correct, it boils down to the the type of circuit. If one is building an audio amplifier, some looseness is good, and the ceramics are more forgivable under load. Of course the cost factor of larger production scale adheres too, but when you need to bias another component for a precision instrument (sensors, for example,) you want a small tolerance resistance.
