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By definition, an electrical ballast is a device that limits the amount of current in an electric circuit. In electrical gas discharge lights, such as fluorescent and neon lights, ballasts control the current flowing through the light.

Incandescent light bulbs produce light by running electricity through a metal filament inside the bulb, which heats it and causes it to glow and emit visible light. When fluorescent lights are turned on, electricity flows to two electrodes on opposite ends of the lamp, causing them to heat up. The electrodes, which are very similar to a filament in an incandescent bulb, then become hot and emit electrons which collide with and ionize noble gas atoms inside the bulb. This creates a voltage difference between the two electrodes, causing electricity to flow between the two electrodes, through the gas in the tube. These gas atoms become hot which vaporizes the liquid mercury inside the tube.

The mercury vapor then becomes excited and emits ultraviolet light, which hits a white phosphor coating that converts the ultraviolet light into visible light. Due to an effect known as avalanche ionization, without a device to regulate the electricity flowing through the bulb the gas would continue becoming more excited and higher intensity light would be emitted until the light failed. Thus it is necessary to have a ballast to regulate current through the gas. Modern ballasts supply the electricity needed to start the lamp and produce light, and then regulate the current so the lamp will produce the desired light intensity.

There are two main types of ballasts: magnetic and electronic. Magnetic ballasts use electromagnetic induction to create the voltages used to start and operate fluorescent lights. They contain copper coils which produce electromagnetic fields that control voltage. Magnetic ballasts, which have been used in fluorescent lights since their origin, are considered outdated and are being phased out by newer electronic ballasts. Electronic ballasts use solid state circuitry to control voltage to the lamp rather than magnetic coils, which makes them more energy efficient. One of the main problems with magnetic ballasts is that, while they can control the current to the light, they cannot alter the frequency of the input power. Because of this, the lamp illuminates on each half-cycle of the input power, causing the lamp to flicker and produce a low humming noise. This flicker can cause eye strain and headaches in some people, and the humming can be bothersome and distracting. Electronic ballasts can control the input frequency, eliminating these problems. Another advantage of electronic ballasts over magnetic ballasts is that one electronic ballast can control more than one lamp, allowing for multi-lamp fixtures to be controlled by a single ballast.

From these two main categories, fluorescent ballasts come in three different types: rapid start, programmed start, and instant start. Rapid start ballasts start lamps by simultaneously providing voltage to the electrodes and across the lamp itself. As the electrodes become hotter and emit more electrons, less voltage is required for the lamp, and eventually the cathodes will become hot enough to ignite the lamp on their own.

Programmed start ballasts are more advanced versions of rapid start ballasts. They have preprogrammed start-up sequences designed to give superior longevity to their lamps. Rather than supplying simultaneous voltage, programmed start ballasts first apply voltage to the electrodes to heat them up for a short interval, then apply voltage to the lamps. This helps to avoid a common problem in fluorescent lights called tube blackening, which occurs when the electrodes are damaged from voltages without sufficient heating. Programmed start ballasts have the longest lamp lives and are best used in locations with lights that are constantly being turned on and off, such as restrooms.

Instant start ballasts start lamps by providing a high voltage directly to the lamps without preheating the electrodes at all. Because there is no heating time, light is produced within 50 milliseconds, thus giving it the name instant start. Instant start ballasts have the highest energy efficiency of any ballast and the lowest cost, but they suffer from emissive material defects like the rapid start ballasts. Instant start ballasts are best used in lights that are not turned on and off very often.

For certain applications where a light is intended to be constantly on and off, such as a flashing light, there are ballasts which will keep the electrodes heated even when the light is off, which greatly increases the life span of the light.

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