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T12 Ballasts

T12 ballasts are ballast supplies for T12 fluorescent lamps. They are used to connect the bulb to the power line. Without a ballast between the lamp and the power line, the lamp would not work. T12 ballasts both allow the bulb to start and ensure it can run for an indefinite period of time. The reason for these ballasts has to do with how these lamps work.

T12 fluorescent lamps are, like all fluorescents, gas discharge tubes. When the lamp is turned off and not lit, the resistance between the two terminals inside the lamp is very high. All that exists between the two metal terminals is gas. As we all know gas does not like to conduct electricity. Therefore, if you were to plug a fluorescent lamp into the wall directly, it might not start. The resistance between the terminals would be too high. Instead, a much higher voltage must be generated.

When the T12 ballast generates the high voltage to start the lamp, it causes gas ionization between the lamp terminals. When the gas ionizes, it becomes conductive. Ionized gas is very conductive, almost as conductive as if there was a wire between the two terminals. However, the gas has one important property that a wire would not have — the ionized gas between the terminals produces light.

As electricity passes through the ionized gas between the terminals of a T12 fluorescent lamp, some of that electricity is converted into light. This is because gas is not usually an electrical conductor. Normally, if you try to send electricity through a gas it doesn’t work. Only when the gas is ionized is this possible. The gas is still not a perfect conductor, though. Engineers take advantage of this “imperfection” in the nature of ionized gas to make lights. The “imperfection” is that electricity passing through ionized gas (instead of passing through a wire) produces particles of light.

In order to make this phenomena last, some additional circuitry is needed. The ballast provides this as well. After the initial high voltage pulse ionizes the gas, the gas becomes much more conductive due to the ionization. The gas allows as much current to flow as the source of power allows. All the gas cares is that the initial voltage was high enough to cause ionization. If the supply allowed lots of current to flow, the lamp would overheat and self-destruct. Therefore, the T12 ballast also contains a built-in current limiter.

The current limiter in a T12 ballast comes into play once the high voltage pulse has ionized the gas. The current limiter allows only a certain amount of current to flow through the tube. The lamp chooses whatever voltage is right. The current limiter decreases the voltage until the lamp starts using less current, and then holds the voltage there. If the temperature changes and the lamp needs more or less voltage to keep the same current, the current limiter allows this change to happen. Therefore, the T12 ballast both starts the fluorescent lamp and allows it to run indefinitely.

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