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One of my projects involves a number of voltage dividers... but if I'm worried about precision (which I'm really not, but I want to understand how these things actually work) can I realistically use "standard grade" 5% resistors for these dividers and calibrate around the error?

What I'm really saying is that if a given resistor is "off" by 3.5% I don't really care... as long as it's always off by the same 3.5%. But if from one measurement (voltage? current?) to another it might be +2% one time and -3% another time, then I need to get higher quality components...

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Aging and temperature should change the values but I'm not sure to what extend. As a side note, metal film 1% are so cheap that I don't see why bother with 5% series. Does your question relate to carbon type or metal film? I think the temperature coefficients are different. –  alexan_e Jan 31 at 20:25
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Just to give you a quick answer. From my experience it's pretty much constant value (at same temperature), but it differs from the labeled value by maximum ±5%. –  Phataas Jan 31 at 20:25
    
Data point: Modern eg 5% resistors from reputable manufacturers TEND to cluster around the nominal value relatively tightly BUT this can not be relied on - outliers do occur. –  Russell McMahon Feb 1 at 0:04
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3 Answers 3

The answer to your questions is mostly covered in the data sheets. A 5% tolerance resistor will also have a specification for temperature drift, "load life" (drift with time under certain environmental conditions) and so on. It's possible to make a 1% resistor that is just as crappy as a 5% resistor in stability, it's just trimmed closer to begin with (and at a certain temperature).

Calibration can reduce the initial inaccuracy, but it won't reduce the other kinds of drift. The drift will determine whether you can make a 0.1% circuit with 1% resistors or a 0.5% circuit with 5% resistors.

Here's a typical temperature characteristic for a cheap resistor. enter image description here

There are many other types of drift described on page 6 of the above linked PDF file.

For example,

enter image description here

It's up to the designer to come up with an error budget and interpret the data sheet numbers to see if the part will fit the job, given the environment it will live in and the desired product life and required reliability.

Generally if a part has a close tolerance and a low temperature coefficient of resistance (TCR) the other drift specs will be good as well, but you do have to check the data sheet if it matters for your application. For a pullup resistor, a change of 20% makes little difference, but if it's a voltage divider on a measuring circuit, a 0.1% change might be very important.

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Very helpful. In my particular application the temperature drift is not likely to be a meaningful source of error, but the explanation helps a lot. –  ljwobker Jan 31 at 23:16
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There are many things that can cause a variation in a resistors resistance. For the specifics of the OriginalPost IF a +-5% resistor actually was +3.5%, it will not, between operations change +2, -3% ... PURELY based upon tolerance.

Now there are other influences on such parts that do cause variation. One is temperature not only from the local ambient (and thus similar resistors would additionally vary by the same amount) but from their own self-heating (a resistor operating at 99% power rating will obviously have a slightly different resistor value from one that was operating at 1% power rating.

There is also ageing which is related to temperature (rule of thumb we use is a 80% continually stressed 1% will be 4% over 25years, a 0.1% will be 0.2% for <50% stress over 25years) . Any decent resistor manufacturer will tell you all the information you need to know

Here is a good place to start.

http://www.vishay.com/docs/49562/49562.pdf

My advice if you have a bunch of voltage dividers is to create a spreadsheet and have a column per resistor. Have different rows that then go:

MIN from tolerance ( ie R *0.95)

MAX from tolerance (ie R *1.05)

MIN_TEMP_MIN_RES ie TCR equ at min temp and lower tolerance

MIN_TEMP_MAX_RES

MAX_TEMP_MIN_RES

MAX_TEMP_MAX_RES

Then for the three temperatures (min, ambient, max) take the worst-case voltage divider combinations (upper_max & lower_min, upper_min & lower_max) ...

for 5% resistors you will not get an overall accuracy spread better than 10% (just bear that in mind)

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Purchasing a batch of ±5% resistors means that the resistors you receive will be anywhere between 95-105% of the intended value (@ std temperature). For example, a batch of ±5% 100 Ω resistors are guaranteed to be anywhere from ~= 95 Ω to 105 Ω.

This will harm your accuracy,and as someone said before, ±1% resistors are pretty cheap.

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This doesn't fully answer the question. –  JYelton Jan 31 at 20:43
    
This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post - you can always comment on your own posts, and once you have sufficient reputation you will be able to comment on any post. –  Nick Alexeev Feb 1 at 6:07
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