The inverter circuit shown produces a low-voltage dc supply suitable for use with compact fluorescent lamps (CFLs). The main feature of this inverter is the inclusion of a pre-heating circuit, with high-voltage ignition and steady-state currentlimiting. The incorporation of a preheating feature helps to prevent blackening of the bulb, while improving both the efficiency and the voltage and currentwave shapes across the bulb. This, in turn, lengthens bulb life. The circuit is applicable to solar-powered lanterns, solar street-lighting systems, automobile interior lighting, and emergency lighting systems.
The inverter configuration is a selfoscillating push-pull arrangement. Transistor Q3, along with the relay (RLY), resistor R6, and capacitor C4, forms the preheating circuit with a preheat time of approximately 400 ms. The relay opens and C3 is connected across the bulb, which improves the shape of the voltage and current waveforms and the bulb’s power factor. Diode D1 and a fuse have been included to provide reverse-polarity protection.
This circuit has been built and tested using a 12-V dc input, resulting in a light output of 370 lumens when used with a 4-pin, 7-W compact fluorescent lamp. The circuit’s efficiency was measured at better than 75% with a switching frequency of approximately 25 kHz.