How a fluorescent tube lights up: the ballast and starter

A fluorescent tube needs two helpers to light: a starter that preheats the electrodes and a ballast that limits the current. What each one does and how they fail.

Anyone who has switched on the lights in an old office or school corridor has seen a fluorescent tube blink once or twice before it settles. That delay is not a fault; it is how the circuit works. A tube needs two helpers to light, the starter and the ballast, and each has a clearly defined job.

What happens inside the tube

A fluorescent tube is a glass tube holding low-pressure mercury vapour and a small amount of inert gas. At each end there is a coiled electrode. When a discharge forms between the electrodes, the mercury atoms give off mainly invisible ultraviolet radiation. The phosphor coating on the inside of the glass absorbs it and re-emits visible light. The shade of white a tube produces depends on the phosphor blend.

The catch is that a discharge does not start by itself in a cold tube. Two things stand in the way: the electrodes are cold and not ready to emit electrons, and mains voltage alone is not enough to strike the gas. The starter and ballast exist to overcome exactly these two obstacles.

The starter: a switch that preheats

A classic glow starter is a small cylinder containing inert gas and a bimetallic contact. It is wired in series with the tube's electrodes. The start-up sequence runs like this:

• when the wall switch closes, voltage appears across the starter and a faint glow discharge forms inside it;
• the glow heats the bimetallic strip, which bends until the contacts touch;
• current now flows through the ballast and both electrodes, and the electrodes heat up;
• with its contacts closed the starter's glow stops, the strip cools and the contacts spring apart;
• at the moment the circuit is suddenly broken, the ballast produces a short high-voltage pulse and the tube strikes.

Once the tube is running, the voltage across it is lower than what the starter needs to glow. So the starter stays dormant and takes no further part. If the electrodes were not hot enough on the first attempt, the cycle repeats, which is where the blinking comes from.

There is usually a small capacitor inside the starter. It reduces sparking when the contacts open and cuts down radio interference.

The ballast: pulse maker and current limiter

A magnetic ballast is a copper winding on an iron core, in other words an inductor. It has two jobs. The first, as described above, is to produce the striking pulse when the starter breaks the circuit. The second, less well known, is to limit current once the tube is lit.

A gas discharge has an awkward property: as current rises, the tube's resistance falls, which lets even more current through. Without a limiter the current would run away and damage the tube and wiring. The ballast opposes changes in alternating current and holds the discharge at a stable level. That is why the ballast has to match the wattage of the tube.

Magnetic ballasts have downsides. They get warm, may start to hum with age, and because they operate at mains frequency the light has a flicker that is hardly noticeable to the eye but can sometimes be seen on rotating parts. They also draw reactive power, so such fittings often include a power factor correction capacitor.

Symptoms and their causes

• The tube keeps blinking and never lights. Usually a worn starter or electrodes at the end of their life. Swapping the starter is the cheapest and quickest first check.
• The ends glow orange-red, the middle stays dark. The starter contacts have welded shut and current flows through the electrodes continuously. Replace the starter, or the electrodes will soon be ruined.
• Blackened ends. Electrode material gradually deposits on the glass. It means the tube is nearing the end of its life.
• Loud hum or a burning smell. The ballast is overheating or has a shorted winding. Switch the fitting off straight away and have an electrician look at it.
• Hard to start in the cold. A discharge is harder to strike at low temperatures, a typical situation in unheated storerooms and garages.

When buying a starter, check that the wattage range printed on it covers your tube. Different starter types are used for a single tube and for two short tubes wired in series in one fitting.

Electronic ballasts and LED tubes

Modern fittings replace the ballast and starter with an electronic ballast. It preheats the electrodes itself and runs the tube at high frequency, so start-up is quick and there is no flicker or hum. Such fittings have no starter socket at all.

If you plan to swap fluorescent tubes for LED tubes, read the wiring diagram on the pack. Some LED tubes work in a magnetic-ballast fitting once the starter is replaced with a special dummy starter; others require the ballast to be removed from the circuit. Rewiring is a job for an electrician. A broken fluorescent tube releases mercury vapour, so air the room and do not throw the fragments out with ordinary rubbish.

For premises that still run fluorescent systems, the Zaferoğlu Elektrik catalogue lists fluorescent tubes and starters alongside its LED lamps.