Ballast Circuit Types - Advance Transformer Ballast Mfg. The Preheat circuit, which applies power to the lamp electrodes to preheat them before the lamp is started, was the original system developed for the operation of fluorescent lamps.
Hot electrodes are necessary to establish the electrical arc within the lamp.
This preheating is accomplished by using a separate manual or automatic starter switch along with the ballast. When power is applied, the starter switch is closed and the ballast delivers a controlled amount of current through the electrodes to preheat them. In a few seconds the electrodes attain the proper temperature and the starting switch automatically opens. The opening of the starting switch, which acted as a short circuit across the lamp, leaves the gas in the lamp as the only other path for the current to travel. Because the filaments are hot and the arc is established, the lamp emits light.