Method Of Starting A Fluorescent Lamp

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In order to start a fluorescent tube the mercury atoms need to be ionized before the arc can ’strike’.

Starting a small fluorescent lamp proves to be no problem as they require only a small amount of voltage for this to happen, however, larger tubes require a lot more voltage, somewhere in the range of 1,000 volts.

This is exactly how it is done in some instances, instant start fluorescent tubes simply use a high enough voltage to start the arc conduction.

These types of tube can be identified due to the fact that they have a single pin at each end of the tube and the fluorescent light fittings they go into have a disconnect socket situated at the low-voltage end which ensures that the mains current is removed automatically to prevent the person replacing the tube getting a nasty high-voltage electric shock.

When this method is not being employed, a separate starting aid must be used. In some fluorescent tube designs, such as preheat lamps, a combination filament/cathode is positioned at each end of the lamp and a mechanical or automatic switch initially connects the filaments in series with the ballast, thereby preheating the filaments before striking the arc.

In countries that use 240V mains supplies, these systems are standard and generally use a glowstarter. Sometimes electronic starters are also used with these electromagnetic ballast fittings.

During the preheating, the filaments emit electrons into the gas column by a process of thermionic emission, which creates a glow discharge around the filaments.

Then, when the starting switch opens, the inductive ballast and a small value capacitor across the starting switch create a high voltage which strikes the arc.

Although the tube strike in these systems is reliable the glowstaters will often cycles a number of times before letting the tube stay lit, this causes the flashing during starting that sometimes occurs.

Once the arc has been struck, the filament/cathode is then kept hot which allows the continued emission.

If, for some reason, the tube fails to start, or starts then goes out again, the starting process is repeated.

With automated starters like glowstarters, a failing fluorescent tube will cause the starter repeat this cycle endlessly, flashing again and again as it tries to start the worn out tube which then goes out as the emission is not enough to keep the cathodes hot enough and the lamp current is too low to keep the glowstater open.

Some more advanced starters time out in this situation, and do not attempt repeated starts until power is reset. Some older systems used a thermal overcurrent trip to detect repeated starting attempts. These require manual reset.

Newer rapid start ballast designs provide filament power windings within the ballast; these rapidly and continuously warm the filaments/cathodes using low-voltage AC.

No inductive voltage spike is produced for starting, so the lamps must usually be mounted near a grounded (earthed) reflector to allow the glow discharge to propagate through the tube and initiate the arc discharge.

Electronic ballasts often revert to a style in-between the preheat and rapid-start styles: a capacitor (or sometimes an autodisconnecting circuit) may complete the circuit between the two filaments, providing filament preheating.

When the tube lights, the voltage and frequency across the tube and capacitor typically both drop, thus capacitor current falls to a low but non-zero value. Generally this capacitor and the inductor, which provides current limiting in normal operation, form a resonant circuit, increasing the voltage across the lamp so it can easily start.

Some electronic ballasts use programmed start. The output AC frequency is started above the resonance frequency of the output circuit of the ballast; and after the filaments are heated, the frequency is rapidly decreased.

If the frequency approaches the resonant frequency of the ballast, the output voltage will increase so much that the lamp will ignite. If the lamp does not ignite, an electronic circuit stops the operation of the ballast.