Solar Light Cheaps Out on Fill Factor
Discover Circuits founder Dave Johnson found this broken solar powered path light near a railroad track. Like many engineers who find things and think the parts might one day come in handy, he stuffed it in his pocket. (Why does broken stuff seem to have a natural affinity for railroad tracks??) Upon closer inspection, he realized that instead of filling the 3-square-inches of available space with solar cells, the unit skimped out, using four skinny cells that yielded an active area of only .83 square inches (28%).
He writes in the Discover Circuits forum: “I measured the open circuit voltage at 2.3V and measured the short circuit current at 37ma in sunlight.. I started tracing out the circuit used in this path light assembly. It appears that they used a 1N5819 Shottky diode between the solar panel and a single 1.2v NiMH rechargeable AA cell. The 1.2V from the single cell is too low to power any LED directly, so they used a transistorized (no ICs) DC to DC converter, to boost the voltage high enough to drive a single, most likely yellow, LED. During sunlight charging, the four solar cells act as a constant current source, feeding a maximum 37ma of current to the single battery cell. Other solar powered path lights use a higher solar panel voltage, which can charge two AA cells. The advantage of two cells is that the LED driver circuit is nothing but a current limiting resistor connected to a simple circuit, which turns off the LED when there is sunlight. This is typically done by monitoring the voltage from the solar cells. The thing I picked up seemed to be a cheap knock-off. But, would it work?” . Dave’s further calculations revealed that while the light would indeed light, it would only be good for about six hours — maybe good enough for London in June but not nearly long enough for the interminable December nights in Colorado. His conclusion: “This design stinks. The manufacturer was trying to save money by using only one rechargeable cell, and a very small solar cell area.”