There are plenty of resistor network finders out there, but I never came across one that worked off "just what I personally have to hand", they assume you have at least a full E24 or better set - if you are a cheap bugger like me, you don't keep a full set, just more common values in a few decades.
So anyway, long story short, I wrote my own equivalence finder that considers only those resistors you actually have, and figured somebody else might find it useful, so I put it on the interwebs -
Equivalent Parallel Resistor Network Finder Using Your Personal Stock List Of Resistors (http://http://sparks.gogo.co.nz/resistor_paralleler.html) (it's a mouthful, but trying to be friendly to the future googlers here).
Caution, you should use Chrome for best results. Firefox works as long as firebug is disabled. No idea what IE will think of it all.
Scroll down the page to enter in the base values you have decades/partial decades of. Select the number of decades you usually keep. Enter in any holes in your decades. And any bumps outside of the decades. You can hit the "Save (sort of)" button and bookmark the result for next time.
Once you've described your resistor stock, you can search for a desired value (you can enter it in the usual various styles people use - 1.0, 3k7, 1.5M, 0R5, 0.5R, 762913 ... you get the idea), specify how much error you will tolerate, choose when the search should give you a less-precise but less-resistors solution, the maximum number of resistors per stack, and if there are no solutions within your error tolerance you can select a Last Resort option to look for a solution of two parallel stacks in series.
NB: The first time you hit Search it builds an array the possible stacks, so that can take a few seconds depending on the Maximum Stack setting.
Anyway, there you go, hope somebody else finds it useful :)
Hey James:
That is a very handy calculator. I could have used it many years ago when I was designing active filters for a seismic data acquisition system. Not only did we need specific resistor values, but component sensitivity analysis showed how some values had particularly critical tolerances. Way back then, I wrote a fortran program that calculated all of the 2-resistor parallel combinations in the 1% resistor set (over x number of decades), sorted it all and printed the final table out on many, many feet of wide greenbar paper on the old lineprinter. Crude, but got the job done, and saved me hours of punching numbers into my trusty HP-15 rpn calculator. It was an FDNR anti-aliasing active filter design, which is a very cool technique using simulated-inductors, laplace transforms, 1/s stuff... Ahh, you can just look it up if you are interested. Very few folks seem to really know what analog circuitry to put before their ADC these days.
Now you can add a resistor calculator that does two-resistor voltage division, and then another that adds equivalent source impedance as well. I recently spent a bit of calculator time figuring the best resistor values for converting a 3.3V logic signal to a 1V signal, with 75-ohms-equivalent-driving-impedance, for a digital-audio output. But I suppose that is somewhat uncommon.
Anyway, thanks for the calculator -- handy indeed!
gil