Hi, There are 480,000 different combinations of standard 1% resistor values. If you need 2.75 volts from a 5.00 volt reference, which of those 480,000 pairs will give you the closest result?
For this example there are 9 resistor pairs that will get you to within
0.5%, but one resistor pair will get you within 0.15%. Only 1 out of 480,000 combinations is the best pair!!
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YOu can not be sure of getting .5 % or less error if you have 2 of the 1 % resistors to start with.
You did not list the resistance of the source and load. If the source will not supply the needed current for low values, they are out. If you have a low resistance load , the resistors will have to be on the low end of the standard values.
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Regards,
Bob Monsen
"I cannot persuade myself that a beneficent and omnipotent God would
have designedly created parasitic wasps with the express intention of
their feeding within the living bodies of Caterpillars"
-- Charles Darwin
Yus.... I wasn't really on about that but I think it is a monster brilliant way of doing things.
I was on about the idea of a 2K74 or 976R resistor versus 2K7 and 1K. Like the numbers imply 0%1 (snicker) resistors what no-one is going to specify in a design but they appear in application notes in places where no-one would specify them.
I think there are a couple of reasons for the three-digit value. One, in a 1% resistor, you have to have the next value within 1%, right? (or 2, or
0.5, or whatever - that's not the point). As for why such a tight tolerance, there are probably real engineers who do take resistor values into consideration when they're designing something, but, you know, now that I think about it, maybe it's an evolutionary thing. Maybe the resistor manufacturers' process started turning out batches that were so close to each other, so reliably, that they found they could characterize a whole batch within 1%, and then they started dicking around with their process, and found that they could reliably make a batch of _any value they want in the world_ to better than 1% tolerance, so all of the resistor manufacturers had a powwow, and said, "OK, so what values should we assign?" And they came up with that familiar 1% table.
Why design engineers use them, well, I could only speculate, other than that the value might make a difference, and when they're cheaper than carbon, they're the logical choice, so you just pick something. ;-)
Cool, but I often find I havnt got a full set of resistors and so if I want to make a divider thats more exact than i can make with any 2 of the ones I have I have to fiddle about trying to work out what set of 3 resistors that I do have in series/parallel whatever combination will give the best result.
(I realy must get some 9k resistors, I so often want to make a 10x non inverting op amp gain stage)
Along about 20 years ago several resister manufacturers were able to produce
1% resisters cheaply and the 1% standard values were already set. The military and instrument manufacturers were already pushing for 0.1% devices. Small wonder that 1% and 0.1% parts are available cheaply now, all the patents have run out.
2K74 is a standard value for 2% tolerance or better resistors (EIA preferred value for the E48 or higher range) 976R is a standard value for 1% or better resistors (EIA 96 or higher range). These values were derived by choosing a set of values whereby the bands of resistance given by the nominal value
+/- the tolerances just overlap, giving 6, 12, 24, 48, 96, or 192 values per decade (e.g. between 100 Ohms and 1K) for E6, E12, E24, E48, E96 and E192 ranges respectively. E96 is pretty common now as it makes sense for 1% resistors. E192 is used for tighter than 1% tolerances.
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