Hello again.

I made one tool for electronics engineers. But first…

As an electronic hobbyist or professional sometimes we have to use resistor circuit in some form (like resistor divider, feedback loop for opamp, feedback for voltage regulator etc.). Calculating proper values for those resistors is quite simple. E.g. when we want to measure voltage from 0 to 24 V by ADC of microcontroller that accepts voltages form 0 to 3 V we can use resistor divider like below.

We can choose one resistor value, and then calculate the other. Let’s assume that R2 = 10k. Now we can calculate the R1 resistance. It’s quite simple task. The output value of resistor divider is:

\(\large Vout = Vin \cdot \frac{R2}{R1+R2}\)

When Vin = 24 V our circuit should have Vout = 3 V, so we can derive R1 from above equation:

\(\large R1=\left ( \frac{24\:V}{3\:V}-1\right )\cdot R2 = 70\: \mathrm{k}\Omega\)

And that’s it! But wait… We cannot find resistor with value 70k. The closest popular resistors from E6 series is 68k. We can accept that difference and calculate the real division factor. We can also start with another value of R2 and try to find better divider. Or finally….

**…we can use Resistor Selector Tool!**

Using this tool we can find two resistors form any series of resistors (E3, E6, E12, E24) that give us the division factor closest to expected. You can also put your own series (like resistor series from your stock, when you don’t want to buy new values of resistors).

Using this tool you can solve similar problems with other circuits (like calculating resistor values in opamp feedback loop etc.). Just try it and see.

By using this tool I calculated that the best values for resistors in given example (form E6 resistor series):

R1 = 33k, and R2 = 4k7 (and also 33M and 4M7…) which give us output value error of 0,265%.

This tool also allow you to get resistor values when one of resistors is actually parallel connection of two resistors like this:

In that case the best solution for this resistor is:

R1 = 68k, R2 = 330k and R3 = 10k, which give us output value error of 0,076%. Finding that values manually would take hours. By using this tool you can do it just by one click.

You can use any time by clicking the Resistor Selector Tool on left menu (in Tools section).

Feel free to test it and comment.