A RFI suppression capacitor is shown in the image. Opening the capacitor unveils the subsequent view.
RFI suppression capacitor has the following functions in the tube light circuit:
a. Absorbs the electric noise created by discharge around the electrodes in order to suppress the radio frequency interference with other electric devices.
b. Attenuates the initial striking voltage from the ballast and make it wide too in order to ensure more reliable starting operation.
c. By avoiding arching between glow tube contacts, it ensures a long contact life.
When power is applied to a tube light circuit, this voltage is not sufficient to ionize the gas inside the main tube. However, this power generates an electric potential across the contacts of small tube of starter. This electric field is large enough to ionize the gas inside the small tube and hence a current flow in the two contacts through the ionized gas. The heat generated due to the flow of current expands the bi-metallic plate towards the other plate and within a few tenths of seconds, it touches the other plate. This serves two functions – first, it will de-ionize the gas and second, it will increase the current through the filaments of the main tube.
Now the gas of the main tube gets ionized and current starts flowing through it. Thus the bi-metallic plate of starter cools down re-opening the gap between the two contacts. This gap will remain open until the tube light will start next time.