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My question is simple, though I'm not sure how to succinctly word it in the title. Often times when working with circuits (as a student) we need a specific resistor value, but only have resistors of other values, so we build an equivalent resistor from parallel and series combinations. I was wondering if there is any good method for choosing the optimal configuration (minimizing the number of resistors required) for creating an equivalent resistor from a fixed set of resistors. I'm not so much looking for an exact algorithm but rather a general heuristic for choosing the configuration.

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  • \$\begingroup\$ most applications in which you dont have access to the resistor you need do not require an exact value so an approximate is more than enough. this tends to allow you to use the easiest combination possible, not necessarily the most complex and precise one.. I'd say thats the general approach, except when you need very precise values for feedback.. \$\endgroup\$
    – Wesley Lee
    Commented Nov 21, 2015 at 1:45

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First off, the precision of your resistance value is going to be limited to the resistor's rating. Consider the list of standard values for resistors with a 5% tolerance. Any number between 10 and 91 is going to be within 5% of one or two of the preferred, E24 numbers. Your perfect resistor might even be in your supplies, labeled as one of these.

E24 (  5%): 10  12  15  18  22  27  33  39  47  56  68  82
             11  13  16  20  24  30  36  43  51  62  75  91

Trying to combine resistors in series or parallel will also combine their possible errors, so you may not be getting much closer. Make sure to measure the resistance of any such construction at the end.

See also: https://en.wikipedia.org/wiki/Preferred_number#E_series

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