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ResCalc - A resistor combination calculator
Here's a resistor combination calculator utility that Mark Lovell and Morgan Jones wrote, called "ResCalc"::
It's quite cool... just type in any arbitrary resistance value that you want, and the program will calculate a combination using standard values that will get you close to what you want. It even shows you the color bands that will appear on the required resistors!
Note that to achieve a tight tolerance, you need to start with at least one resistor that also has a tight tolerance. If you try and use two 5% resistors, your result will have a tolerance no better than 5%. So, in the example above, if you parallel a 1.3k 5% resistor and a 24k 5% resistor, you will wind up with a 1.233k resistor, but still a 5% tolerance.
In some cases only one resistor needs to have a tight tolerance. For example, if you parallel a 10.0k 1% resistor with a 1M resistor, you'll get within 1% of 9990 ohms even with a 1M 10% resistor. You get the idea...
"E24" values are the standard values that 5% tolerance resistors are normally available in: 1k, 1.2k, 1.3k, 1.5k, etc. and the "E96" series are the standard 1% values (1.00k, 1.02k, 1.05k, etc). Some vendors do sell E24 values in 1% tolerance, but they are less common, so if you a want 1% tolerance result, you're better off using E96 values.
This example shows the same desired value (1.234k) using E96-series resistors. E96 values are what we normally have available (at least in the US) as 1% standard value resistors.
If you click the "0.1%" box under "Required Tolerance", keep in mind that to achieve 0.1% result, you will need to use 0.1% tolerance resistors to start with.
Download RESCALC.ZIP (32kB) and extract the .EXE file.