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Electrical Aspects Of Operation Of A Fluorescent Tube

Failure Of Integral Ballast Electronics A fluorescent lamp is a negative resistance device, this means that as more current flows through it, more gas is ionized and the electrical resistance of the fluorescent tube drops, allowing even more current to flow through it.

This would mean that if you connected it directly to a mains current, it would very quickly ’self-destruct’ because of the uncontrolled flow of current.

Obviously this doesn’t happen in your normal fluorescent tube! This is due to the use of an electrical ballast that regulates the flow of current through the tube.

The ballast could take the form of a simple resistor, but this would waste a substantial amount of power so, instead, a reactane ballast is usually used (either an inductor or a capacitor).

When being operated from an AC mains power source it is common to use a simple inductor as the ballast (also known as a ‘magnetic-ballast’). In countries where they use 120V AC mains, the voltage is not sufficient to light one of the larger fluorescent tubes, so a step-up autotransformer is often used as the ballast.

With either type of inductive ballast a capacitor may aslo be used for purposes of power factor correction.

In the past a DC power supply may have been used to run a fluorescent lamp in order to provide enough voltage to strike an arc. The ballast used in this situation must have been resistive rather than reactive, which would have lead to power losses in the ballast resistor.

Operating from a DC power supply would have also meant that the polarity of the supply would need to be reversed every time the lamp was started, otherwise the mercury would accumulate at one end of the tube.

It is very rare for modern fluorescent tubes to be operated directly from a DC power supply. Instead an inverter is used to convert the DC supply into an AC one and also provides a current-limiting function.

There are more sophisticated ballasts called ‘electronic ballasts’ that use transistors or other semiconductor components that convert the mains voltage into high-frequency AC while, at the same time, regulating the flow of the current into the lamp.