# How the voltage of very high magnitude is obtained in a tubelight?

1. May 30, 2013

### Faraz Murtaza

i have a doubt regarding the starting of a tubelight that whenever we switch on the light then we require a very high voltage to create the arc for the continuous flow of current, then how do we get such a large amount of voltage to produce this arc

2. May 30, 2013

### davenn

I assume you are referring to a fluorescent tube light ?

if so, there is no need for extremely high voltage, there is no discgharge arc formed as in a long spark

very basically...
when, for example, mains voltage (120V/240VAC) is applied to the 2 filaments one at each end of the tube the filaments heat up and an electron cloud is formed. Since mercury vapor is a good conductor, these electrons make their way along the tube between the two filaments.
The electrons "colliding" with the mercury vapor atoms cause ultra violet light to be emitted. This UV light is absorbed by the phosphor coating on the inside of the tube and visible white light is emitted.

The Ballast in the circuit is used as a current limiter.

regards
Dave

3. Jun 3, 2013

### Faraz Murtaza

thank you so much

4. Jun 3, 2013

### sophiecentaur

Without some help from the 'starter circuit' you would need a high voltage for a metre of low pressure gas to 'strike'. It is virtually, an open circuit. Once the mercury has become ionised, that same column becomes a very low resistance path. As Davenn says, you need a way of limiting the current or a fuse will blow. An inductive choke is mostly used these days but, in the past, it was not uncommon to use a filament lamp in series, to limit the current.

5. Jun 3, 2013

### Faraz Murtaza

as you are saying that without the need of starter we will require a high voltage, so my question is how do we get this high voltage? as the supply is of 240V and as i found to create an arc for current flow we require about thousands of volt. So, how do we get such a large amount of voltage?

6. Jun 4, 2013

### the_emi_guy

Those big fluorescent bulbs we use for overhead lighting use thermionic emission from filament to get electrons going (hot cathode), so no need for really high V.

Back in the day, we used cold cathode tubes (CCFL) to backlight our LCD panels. More efficient since we are not heating a filament, but required high voltages.

Even though the bulbs were only couple inches long they required 1500V of strike voltage and then 400Vrms of sustaining voltage at 30KHz (we used Royer inverter). Got a nasty jolt more than once working in the vicinity of that. Don't miss them.

7. Jun 4, 2013

### sophiecentaur

You do need a starter and no one uses a high voltage to start the discharge these days.
If you want a design for a high voltage supply for another purpose then that is a different issue. You won't get one from a fluo tube supply.
Look up 'fluorescent tube starter circuit'.

8. Jun 4, 2013

### jim hardy

9. Jun 22, 2013