User:Dimnsionofsound/Electrical Ballast Sandbox

Current limiting
Ballasts limit the current through an electrical load. These are most often used when a load (such as an arc discharge) has its terminal voltage decline when current through the load increases. If such a device were connected to a constant-[voltage] power supply, it would draw an increasing amount of current until it will be destroyed or caused the power supply to fail. To prevent this, a ballast provides a positive resistance or reactance that limits the current. The ballast provides for the proper operation of the negative-resistance device by limiting current.

A gas-discharge lamp is an example of a device which, under certain conditions, has negative differential resistance. In such a situation (after lamp ignition), every little increase in the lamp current tends to reduce the voltage "dropped" across it (supposing the lamp to be connected in series with other circuit elements). Let $$dI$$ represent the change in current I, and $$dV$$ represent the change in voltage V. Each variation can be positive (or negative) if its variable increases (or decreases). The differential resistance is the ratio between $$\textstyle dV$$ and $$\textstyle dI$$, $$\textstyle\frac{dV}{dI}$$ and it can be either positive or negative (and sometimes even null). This is quite a different concept from the resistance, which is always considered positive. In the case of a gas-discharge lamp, the differential resistance (i.e., dV/dI) really becomes negative because the positive variation for the current (dI) causes a negative variation for the voltage (dV) across the lamp.

For a mechanical analogy to negative resistance behavior, and how a ballast can limit current, imagine trying to push a heavy weight across a smooth surface. Applying a force (voltage) to the weight initially does not cause it to move (have positive current) because the static friction of the weight against the surface is more than the applied force. Once enough force (voltage) is applied, the static friction is overcome and the weight starts to move (now dynamic friction opposes the movement of the weight instead of static), but if the amount of force that was used to overcome the static friction is not changed, the weight will be pushed too fast across the surface. If we flood the surface with some depth of viscous oil (our ballast), we can continue to push as hard as it took to get the weight moving (leave our voltage constant) while simultaneously not allowing the weight to move too fast (limit current). The transition between static friction (high resistance) and dynamic friction as the weight moves (low resistance) is the negative resistance region - less force (negative $$dI$$) creates more movement (positive $$dV$$).

Ballasts can also be used simply to limit the current in an ordinary, positive-resistance circuit. Prior to the advent of solid-state ignition, automobile ignition systems commonly included a ballast resistor to regulate the voltage applied to the ignition system.

Series resistors are used as ballasts to control the current through LEDs.

Why I Chose to Edit This
Negative (differential) resistance is a relatively strange concept to try and understand, and it is integral to the concept of an electrical ballast. Analogies usually help people grasp concepts at an intuitive level before they understand them in a more formal way, so I aimed to give the reader an intuitive understanding of why an electrical ballast is needed that will most likely help them more appreciate the rest of the article.