Resistor Calculator

Dave, I wrote one a while back in BASIC and have the source code and exe file at

It does not automatically select the tolerance, but instead has a quick reference of 5% and 1% values. In other words, here might be a slightly less clunky program until you find something better.

Schematica has something like that tool as part of their 555 Designer Pro and Etcetera offerings. The 555 Designer is pretty inexpensive and a real time saver when a quick sim/stim oscillator is needed. The resistor picker handles E12, E24, or E96 families in serial or parallel pairs.

Another handy, and cheap, tool is the voltage divider helper from

MiscEl does this. It's a great little program all around. Use the prefered components tab to set E24, etc. and the series/parallel circuit design tab to get what you want.

Also,

Cheers, John

Newer version here

Nice, clean prog!.

Oops, meant this one, ("nice clean prog!").

Maybe its too simple, but I just take the next lower available value, and add one resistor in series to make up the difference.

Actually I prefer to do this in parallel, use the next higher available value, and calc a parallel value to reduce it to the required value.

Either way the calculation is trivial.

Trivial it aint :) Yesterday built a RF step attenuator using 5 off SPDT toggle switches and some surface mount resistors. The 10dB and 20dB sections needed

I ended up writing a very crude "brute force" method. I wanted to be able to only use values we carried in stock. So I simply stepped through every series/parallel combination of 2 and 3 resistors, from our stocked values, recording the best "match".

This was to calculate voltage dividers or gain setting ratios, but the principle should work for other optimisation "targets".

I agree, it ain't always that trivial. Not when you have to try and match E values you have in stock or can get in a hurry, match a required tolerance and/or power spec, and then do it over and over again...

Dave.

That's EXACTLY what I want, thanks! It just gained an icon place on my Desktop.

Dave.

Hey! Great find! I've upgraded. Thanks.

John

Adding another set of constraints of course makes the problem harder. Thats a major one.

I haven't got exactly what the OP asked for, but I wrote a simple Excel spreadsheet that does resistive divider computations and shows all the possible combinations with their theoretical error. Makes it easier to choose from the values you have on hand. There is a 1% and a 5% worksheet, covering a range to 100:1.

The tolerance parameter is used to hide combinations that are too far off. Find the more optimal choices by setting this value smaller.

I'll post on a.b.s.e under this topic

Steve