Why we need Solar Panel Switch:
When a battery is charged by a solar panel during the day, it may become (partly) discharged via that panel after sunset. This may, of course, be prevented by a diode, but that has the drawback of a forward voltage drop of 0.7 V or, in the case of a Schottky diode, 0.4 V. In many applications, this is not acceptable.
How Solar Panel Switch works?
The present circuit, the solar panel switch, replaces the diode and connects the solar panel to the battery via a relay contact. It is powered by the solar panel. When the supply voltage is too low the relay is not energized so that the battery is not connected to the solar panel. When the panel output is sufficient to energize the relay and light-dependent resistor (LDR) R2 receives enough light to cause T1 to conduct, the relay is energized and the battery will be charged.
Solar Panel Switch Circuit Diagram:
Owing to its inherent hysteresis, the relay will remain energized even if the potential across the panel drops to a certain degree. A connected and charged battery cannot actuate the relay when the light begins to fail because R2 will then switch off T1. At what brightness this occurs is preset with P1.
Since the current drain is determined primarily by the relay, it is important that is a miniature type with a high coil resistance, which can nevertheless switch up to 10 A (e.g., Siemens Type V23037-A0002-A101*).
*Available from ElectroValue, 3 Central Tradina Estate, Staines TW18 LIUX. Telephone (0784) 442 253.