A ballast resistor is a resistor inserted into a circuit to compensate for different changes. Others might give a different answer to the question “what is a ballast resistor” like this:
A resistor that has the property of increasing in resistance as current flowing through it increases, and decreasing in resistance as current decreases. This article is the first from the basic electronics series.
Ballast resistors can be used to
They are sometimes used in car engines produced with breaker-points type ignition. Here is a video that shows how to test if your car has ballast resistor.
One simple use for it can be seen when we connect one LED in series with a resistor. Every LED has a different operating current and voltage drop. The resistor value can be calculated like this:
R (Ω) = (Supply Voltage – LED voltage drop) / I (A)
For example we have a 9V battery and a white LED with a typical voltage drop of 3.1V. The desired current is 13 mA so by applying the formula results:
R = (9V – 3.1V)/0.031A = 452Ω
We choose a 470Ω value for the ballast resistor. Read more here.