Resistor uhms subtract of set max

Legofan

Joined Aug 21, 2021
1
Say I’m using a variable resistor however the highest uhms is too high. If in the path the the object I’m powering I put a resistor does the resistor subtract the amount if uhms the resistor has or does it lower the amount if it goes over the uhms of the resister.

Ian0

Joined Aug 7, 2020
3,785
No, they add up. Resistances can never subtract.
You can put a resistor in parallel if you want to reduce the resistance, in which case the combined resistances will be
$$R_{total}=\frac{1}{\frac{1}{R_1}+\frac1{R_2}}$$
There is a handy calculator here
http://sim.okawa-denshi.jp/en/teikokeisan.htm

crutschow

Joined Mar 14, 2008
28,227
What exactly are you using the variable resistor for?

Tonyr1084

Joined Sep 24, 2015
6,467
Resistors resist current. Two resistors end to end (series) adds to their value of resistance. That is to say that two 50Ω resistors in series is equal to a single resistor of 100Ω. That's in "Series".

Two resistors side by side (parallel) reduce the overall resistance. That is to say that two 50Ω resistors in parallel is equal to a 25Ω resistor. The reason can be likened to road construction. If one lane is open traffic moves slowly through the opening. If a second lane is added, same size and all, twice the traffic (current) can pass through.

If you have a variable resistor it's recommended to have a fixed resistor in series with it so that you can never take the variable setting all the way to one extreme or the other. A lot depends on how you're using it. You don't always need a fixed resistor. For instance, you could be using the variable resistor as a voltage divider/reference. All the way to one end of the resistor will select the full scale while all the way to the other end will select "Zero" scale. Most variable resistors don't handle much current and therefore can not be used to dim an incandescent light or control the speed of a motor. They simply will burn up.

Like @crutschow asks - how are you using it?