If you turn on a fluorescent tube-lamp, it flickers before lighting up.
If you then turn it off and turn it back on after 2 seconds, this time it doesn't flicker but lights up straight away.
If you wait for some time (say 10 minutes) and switch on the light again, it flickers before lighting-up, again.

Why does it flicker before lighting up?
And why doesn't it after a 2 seconds gap in turning it on and off?


3 Answers 3


There are several types of Fluorescent Lamps which each might have a different ballast circuit. In general, the flicker is caused because the gas inside the lamp is still relatively cold and cannot establish a glow discharge between electrodes. Once the lamp has been heated enough, the gas ionizes and the lamp glows. So if you turn it off for a few seconds and turn it back on, the gas is still relatively hot and ionized, which makes easy to start. When you wait 10 minutes, the lamp is cold, and will be harder to start again, generating a flicker.

  • $\begingroup$ nice answer. however, it is not an "arc" (which is lightning) it is a "glow discharge". $\endgroup$
    – Andreas H.
    Commented Feb 2, 2014 at 4:12
  • $\begingroup$ edited to reflect Andreas H. comment $\endgroup$
    – CAGT
    Commented Feb 2, 2014 at 4:32
  • $\begingroup$ @AndreasH. In the "flickering type" of lamps it's an arc due to the hot cathode. $\endgroup$
    – Calmarius
    Commented Jun 3, 2014 at 9:02

It doesn "blink" when the fourescent tube is still warm, hence, your observation that it turns immediately on after it was only briefly off.

The blinking occurs because the the starting circuit tries several time to establish a the current flow in the tube. It may not light up instantly because the gas inside needs to reach a certain temperature first.


There are many kinds of fluorescent lamps. Those neon tubes that tend to flicker before starting up is the hot cathode type. It has a circuit like this:

Neon tube circuit

image source

In these types a starting circuit randomly opens causing a inductive voltage surge in the tube to strike it. Generally the grid voltage is not enough to maintain the glow discharge, so the circuit tries again. Between the attempts the electrodes of the tube is being heated. When they got hot enough they emit electrons via the thermionic emission. This hot spot turns the glow into much brighter arc discharge which can be maintained on a much lower voltage. When the lamp is running cathodes are kept hot by the ion/electron bombardment.

CFL bumbs use cold cathodes. But this doesn't mean that their cathodes remain cold: when they struck, the grid voltage can maintain the glow due to their small size, which will turn into an arc as the cathodes heat up by the ion bombardment and the bumb gradually brightens up.

So if you turn off the lamp for few seconds the cathodes don't have time to cool down and the starting circuit can start the tube on the first try.



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