Solar Relay

Posted on Feb 5, 2014

With extended periods of bright sunshine and warm weather, even relatively large storage batteries in solar-power systems can become rather warm. Consequently, a circuit is usually connected in parallel with the storage battery to either connect a high-power shunt (in order to dissipate the excess solar power in the form of heat) or switch on a ve

Solar Relay
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ntilation fan via a power FET, whenever the voltage rises above approximately 14. 4 V. However, the latter option tends to oscillate, since switching on a powerful 12-V fan motor causes the voltage to drop below 14. 4 V, causing the fan to be switched off. In the absence of an external load, the battery voltage recovers quickly, the terminal voltage rises above 14. 4 V again and the switching process starts once again, despite the built-in hysteresis. A solution to this problem is provided by the circuit shown here, which switches on the fan in response to the sweltering heat produced by the solar irradiation instead of an excessively high voltage at the battery terminals. Based on experience, the risk of battery overheating is only present in the summer between 2 and 6 pm. The intensity of the sunlight falling within the viewing angle of a suitably congured sun probe` is especially high precisely during this interval. This is the operating principle of the solar relay. The trick to this apparently rather simple circuit consists of using a suitable combination of components. Instead of a power FET, it employs a special 12-V relay that can handle a large load in spite of its small size. This relay must have a coil resistance of at least 600 , rather than the usual value of 100-200 . This requirement can be met by several Schrack Components relays (available from, among others, Conrad Electronics). Here we have used the...

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