Preheat starter for electronic ballast
Arthur E Edang, Don Bosco Technical College, Mandaluyong City, Phillipines -- EDN, January 4, 2001
Electronic ballasts for fluorescent lamps use various techniques to turn on the bulbs. The design usually involves a compromise between turn-on voltage and lamp life because the two are inversely related. One way to reach a reasonable compromise is to initially allow a momentary inrush current to warm the filaments, followed by a series of interrupted short circuits across the lamp that generate the required high voltage to trigger the fluorescent. With a preheated filament, the necessary strike potential reduces to half.
The trigger circuit in Figure 1 controls the electronic switch across the bulb. At start-up, IC1D's output is low as C1 and C2 charge toward VCC. IC1D's low output pulls IC1C's inverting input low, which causes VO to clamp high. A high level at VO closes the switch and forces current through the filaments. After approximately 0.5 sec, IC1D's output changes state and allows IC1C to accept the high-frequency signal at its noninverting input. IC1A is a square-wave oscillator, which causes VO to be a high-frequency-pulse series that lasts for approximately 1 sec. At the end, C2 reaches a high enough voltage to force IC1B to pull down IC1A's noninverting pin to ground. With a grounded IC1A output, VO clamps low.
The high-frequency switching strikes the preheated lamp. In case the bulb fails to start, the circuit turns off and then on again. Residual charges on the capacitors discharge through D1 and D2 to ensure precise timing.
