A ballast resistor is a device used as a resistor to regulate modifications (both expected and incidental) of the physical form of another device. There are two different types of ballast resistors, the fixed resistor and the variable resistor.
A fixed ballast resistor is used for devices that require a low amount of power. Examples of this would be neon lamps and devices that use LED lights. The fixed resistor is best for these types of devices as there is a large resistance with this type of ballast resistor. This will also allow the fixed resistor to control the device even when negative resistance with the device is occurring.
Ballast Resistor with Automobiles
A fixed resistor can also be used in automobile engines. This is a great addition to most vehicles as it can be used to decrease the amount of power being supplied to the ignition system once you finish turning the engine. Starting your vehicle (turning the engine) requires a lot of battery power and this must be regulated. Since the battery charge can decrease substantially while you are starting your vehicle it will need to be designed to safely operate with a low battery power.
As soon as you start the engine up the battery will be recharged fully. The quick decrease and increase of voltage can cause harm to the ignition system but with a resistor it can eliminate the harsh effects it could have had otherwise.
If the ballast resistor in your vehicle's engine fails then it will be easily noticed. The end result of a resistor failing would be that the vehicle engine tries to run while it is being started up. This is all done while the resistor is not being used for it's job. The result of this process will be your vehicle stalling as soon as you stop cranking the engine. When this is done the circuit will connect with the resistor again and it should start up normally on the next attempt. If any permanent damage is done then you may have to repair your vehicle.
Note: Most vehicles are no longer designed with a ballast resistor. Instead, electronic ignition systems are designed conveniently and do not require a resistor to be able to operate safely with a low voltage.
A self-variable resistor is a device used similar to the fixed resistor but the resistance is configured differently. The resistance will increase as voltage flows through the resistor. If the voltage flow that travels through the resistor decreases then the resistance will decrease as well.
With many small devices there is no need for a self-variable resistor as it does have a complicated design. The fixed resistor is usually fine as there is only a small amount of power being used anyway. However, if there is a need for a lot of power then the power load difference can play a role in the resistance necessary.
With a self-variable resistor you can regulate the power load more accurately and it can come in handy with high power load devices. Instead of dropping the full power load you can drop a partial load, and vice versa. With a fixed resistor this is not the case as you pretty much have a full charge or no charge at all.
This article should have gone over all the basic information you need to know on ballast resistors. If you are looking for information on the types of devices and what type of resistor they use then just complete a quick search online. You may also want to look for a photo or video that further describes the process of which these types of resistors work as it may be confusing to understand it simply from reading this article.