Preheat starter for electronic ballast

Arthur E Edang, Don Bosco Technical College, Mandaluyong City, Phillipines -- EDN, 1/4/2001

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, IC_1D's output is low as C₁ and C₂ charge toward V_CC. IC_1D's low output pulls IC_1C's inverting input low, which causes V_O to clamp high. A high level at V_O closes the switch and forces current through the filaments. After approximately 0.5 sec, IC_1D's output changes state and allows IC_1C to accept the high-frequency signal at its noninverting input. IC_1A is a square-wave oscillator, which causes V_O to be a high-frequency-pulse series that lasts for approximately 1 sec. At the end, C₂ reaches a high enough voltage to force IC_1B to pull down IC_1A's noninverting pin to ground. With a grounded IC_1A output, V_O 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.