Resistor and Capacitor Values

Question

I am curious if anyone knows the thinking behind the sequences of values that are used for Resistors and Capacitors. While going through some old parts the other day I noticed that at one time whole number values such as 10, 20, 30, 40, --- were more common. Now of course it is 22, 27. 33, 47, 56, 68, ---. I suspect there may be a mathematical or functional reason behind the sequences but I do not know what that might be.

Thanks John

johnbeetem · Accepted Answer

There are good articles about this at Wikipedia: https://en.wikipedia.org/wiki/Resistor#Preferred_values which links to https://en.wikipedia.org/wiki/Preferred_number#E_series

Each resistor has a tolerance like 10%, 5%, or 1%. You want resistors that can cover all possible values, but since they vary there's not much point in having 10% resistors with values 1% apart. So instead you cover a decade of resistors (e.g., 10 to 100 Ohms) with a minimal number of values that overlap each other when you consider the tolerance. The values have a geometric progression, where each resistor is (for example) 10% larger than its predecessor.

For example, here are the familiar values for 10% resistors: 10 12 15 18 22 27 33 39 47 56 68 82 100

michaelkellett · Answer

E14 just choked on my link to you !!!! There is no reason not to use SM parts at home and it isn't hard to solder Rs and Cs down to 0603 using only a headband magnifier and a fine tipped iron. To do 0.5mm pitch QFPs you'll probably need a microscope. You can use copper clad board to prototype on - scratch out gaps with a scalpel or Dremmel (or copy of one). Google 'copper clad board rf' and look at the pictures - I did post some links but E14 dies when it tried to understand them. if you hang out on sci.electronics.design and look for picture links posted by John Larkin you'll get some ideas of how to do this kind of prototyping. (Be warned - there is a lot of Off topic noise on sed !!) MK

DAB · Answer

Hi John, A lot of the values were selected based upon a lot of the early tube circuits. AM radios and amplifiers were mostly standardized around a few designs for cost and performance. Since there was a ready DIY market around the world, having a steady supply of kit parts helped manufacturers to decide what to produce. In many ways, your basic low pass, band pass and high pass RC circuits remain unchanged from tube, transistor, to IC implementations. So many of the old favorites are still useful today. I am not sure how long the discrete components will still be available with all of the SMT and internal circuitry available on SOC designs. I know I have a small stock of parts that I dip into all the time for projects. DAB

Resistor and Capacitor Values

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