Often an experimenter needs a variable resistance and adds a trimpot to the circuit. For instance I want a resistor that can be varied between 220 and 330 ohms. The easiest solution would be a 220 ohm resistor and a 100 ohm trimpot in series. But I don’t have a 100 ohm trimpot, which is a very uncommon value. I have a 1000 ohm trimpot, so can I use that instead?
Let’s see how we can do that. The circuit doesn’t say that the resistor or trimpot has to be a single part. You can put two resistors in parallel and get 220 or 330 ohms. Let’s try this: leave the 220 ohm resistor to be the minimum, and replace the 100 ohm trimpot (that would have been in series) with two resistors that can be varied between 0 and 100 ohms.
I take a guess at the resistor that I want to go in parallel with the 1000 ohm pot. I guess 150 ohms. So I take my calculator and enter 150 and press the 1/X key. Then I press + and enter 1000 and pres the 1/X key. Then I press the = key and 1/X key and I get 130.43 ohms.
That’s a bit too high, so I take another guess, and enter 120 into the calculator and press the 1/X key and + key. Then I enter 1000 and press 1/X key, press the = key and then 1/X key. I get 107.14 ohms. That’s just about right for the maximum.
As I vary the 1000 ohm trimpot, which is in parallel with the 120 ohm resistor, it is going to give a resistance that is between 0 and 107 ohms. The linearity of the pot will be affected. The resistance will change less at the high resistance end of the trimpot, and change more at the low end of the trimpot. But we can now use a 1000 ohm trimpot to do the job of a 100 ohm trimpot.
Any two resistors in parallel can be found by adding their conductances. The conductance is equal to 1 divided by the resistance. So before adding the resistance you entered, you press the 1/X key to change it to conductance. The unit of conductance is the Siemens, which at one time was called the Mho. This procedure also works for capacitors in series. But if you have two resistors or capacitors of the same value, just divide that value by two. Two 1000 ohm resistors in parallel equal 500 ohms, etc.
Back to experimenting…