Principle of an Electronic Tube Light Ballast(Choke)

Gas discharge lamps, i.e lamps using the principle to make a gas electrically conductive and thereby light emitting, are a relatively old technique. Especially fluorescent lamps represent a very widespread lighting system. It is not possible to apply the line voltage directly to such lamp, be it AC or DC, a higher or a lower magnitude. Traditionally these lamps have always been operated on AC mains by means of a so-called magnetic ballast, which is nothing more than a reactor or choke, for limiting the lamp current. 







In recent years, as power electronics techniques came up, an alternative way of operation was introduced, the so-called electronic ballast, which converts the incoming mains frequency into a much higher frequency, usually in the range of 20 kHz to 80 kHz, to operate the lamp.

If we recall the wiring of a conventional tube light using the magnetic ballast, the complete circuit comprises of magnetic ballast, starter and the tube light. The role of starter and magnetic ballast together is to proved a high surge voltage using the principle self induction such that the gas inside the gas discharge lamp (in this case mercury vapor  gets ionized  Once the gas gets ionized  the electric current find an easy path trough the ionized gas tube thus lighting the tube light. The starter is out of circuit now as the same comes in parallel with the tube light and the tube light provides the low impedance path for the current. The ballast now provides only the 'regulating action' for tube light current. The magnetic ballast works at line frequency.

The function of electronic ballasts also is analogous to the magnetic ballast but accomplished with a higher frequency in the range of 40-80KHz than the line frequency. This technique is also applied in switch-mode power supplies (SMPS) to facilitate the use of a smaller transformer. The principle of transforming at higher frequencies is the same but with the advantage of compact size and less weight.


The working principle of the electronic fluorescent ballast is rather straightforward. The AC signal is first rectified and filtered using a bridge/capacitor configuration. The next stage is an oscillator stage. The rectified DC is applied to this stage which immediately starts oscillating at the required high frequency. The oscillations are typically square wave which is buffered via an inductor before it is finally used to ignite and illuminate the connected tube.