Why you use resistor with variable resistor?

Answer 1

Often we want to easily change a resistor value, so we use a variable resistor.

For example, we may want to change the resistor that controls the power sent to a LED, so we can easily make it brighter or dimmer.

Often if we use a variable resistor, there is only a very narrow range that is useful.

Continuing our example, sometimes we use several LEDs, and we use the variable resistor to set them all to the same brightness.

In this case, the resistance range that sets the LED to be twice as bright as the the other LEDs, and the resistance range that sends so much power to the LED that it is permanently destroyed is even less useful.

So we add a fixed resistor in series with the variable resistor -- the fixed resistor sets the minimum net resistance, no matter how we turn the knob on the variable resistor.

In our example, the addition of the fixed resistor allows us to turn the variable resistor throughout its whole range, and the LED gets brighter and dimmer; without that resistor, a certain range of the knob on the variable resistor would allow so much power to go to the LED that it would be destroyed.