Low-loss circuit powers solar lantern

The solar-lantern circuit in Figure 1 is a low-loss configuration that uses a 7W, four-pin CFL (compact fluorescent lamp) and a 12V, 7-Ahr, sealed, maintenance-free battery. The inverter features greater-than-85% efficiency, less-than-2mA quiescent current, and a shunt-charge controller with deep-discharge and overcharge protection for the battery. The low quiescent current and the deep-discharge and overcharge protection ensure long life for the battery. The preheating feature in the inverter avoids the blackening of the end of the CFL, thereby ensuring long life. The circuit finds application in rural areas as a reliable, compact, portable light source and in urban areas as an emergency-lighting system. The shunt charge-controller circuit comprises IC1, a low-current, voltage-reference 2.5V LM385, and IC2, an LM324 comparator. IC2A, with resistors R1 through R8 and transistor Q1, provides protection against deep discharge of the battery.




Figure 1: This solar powered lantern driver can serve as an emergency lighting system

The circuit switches off the load, including the inverter and the lamp, when the battery voltage falls below 10.8V and thus protects the battery from deep discharge. Under a no-load condition, the discharged battery voltage is approximately 12.2V. Hence, the circuit provides a deep-discharge reset level of 12.3V to avoid oscillations. Red LED1 indicates a low-battery condition. IC2B with resistors R9 through R14 and transistor Q2 provides protection against overcharging the battery. Q2 switches on and shunts the solar array when the battery voltage exceeds 14.8V and thus protects the battery from overcharging. Q2 turns off when the battery voltage drops below 12.5V and thus enables battery charging. D2 is a reverse-blocking diode. It prevents the discharge of the battery through the solar cells when the cells are not generating electricity. Amber LED2 indicates that the battery is in full-charge condition. Green LED3, along with IC2C and resistors R15 through R20, provides an indication of charging.





Tables 1, 2 and 3 give core and winding details for the magnetic components in the circuit. The inverter uses a Class D, push-pull, force-driven topology with MOSFETs as switching devices. IC3, an SG3524, drives the inverter. The force-driven topology ensures trouble-free start-up in all environmental conditions. The switching frequency is approximately 26kHz. Q6, along with resistors R29, R30, and R31 and capacitor C10, forms the preheating circuit. In addition to the 12V, 7-Ahr sealed, maintenance-free battery, the circuit uses a 10W, 12V single-crystalline-silicon solar-cell panel. The recorded backup time is approximately eight to 10 hours for a fully charged battery with a light output of 370 lumens using a 7W, four-pin CFL.