On Sep 8, 4:34*pm, pete wrote:
Does anyone know of an online utility that can calculate
what combination of series and parallel resistors are needed
to get a particular value?
Specifically, I'm trying to calculate the best way to get
close to 5250 Ohm, using E12 preferred values. Power consumption
is not an issue and I'd like the value to be +/- 2% as that's
the resistor tolerance.

I'm not looking for the answer, I'm looking for the way to
find the answer. There are lots of websites where you can
tap in resistor values and have it calculate the result, but
that gets long winded. I've got a combination that gives
5253R with 4 resistors, but I'd like to do better


People have suggested a main resistor plus a parallel tweaker
resistor, which is a fine way to go. A close main R plus a relatively
high value parallel tweaker can often get you there with just 2 Rs.

But one error to avoid is to calculate your tweaker r value using the
nominal value of the main R. Instead use the real value of the main R
to work out what tweaker R you need, and the result will be far more
precise.

For some apps you can use basic 5% Rs to get 1% accuracy, but for some
you cant. Wider tolerance Rs are so for 2 reasons. First is simply
selection, its cheaper to stick them in the E12 range than E24. The
other one is tempco, Rs do change value with temp, so if you keep your
R temp steady you can use cheap Rs for quite good accuracy.

PS one of the key ideas of the E ranges is that every real value of R
produced can be placed in a nominal value band and be sold.


NT