For regulating a solar panel’s output, there are several possible ways. A linear series regulator can be used, but has the disadvantage of causing some voltage drop and having some internal power consumption at times when the sun is weak and the load is heavy. It’s much better to use a shunt regulator, which is inactive at such times, and springs to life only when there is excess energy. For this reason, most solar panel regulators use the shunt scheme, the one presented here being no exception.
D1 can be any diode that can safely survive the panel’s current. If the panel has a very low voltage output (less than 33 cells in series), it is an advantage to employ a Schottky diode in this place.
Q1 and Q2 are common power Darlington transistors. They need to be heatsinked for safe long-term operation at the 12 Watt dissipation level. That’s easy enough to do, but many newcomers misjudge how much thermal resistance is introduced by a mica insulator! Plan on 1K/W thermal resistance inside each transistor, two times as much in the insulator (if you use any), and 370K safe junction temperature. For typical environmental conditions, this makes you need a heatsink having a thermal resistance of about 1.3K/W. If it is larger, you get more safety margin.
R1 and R2 will have to be made by combining a number of power resistors in parallel. Yes, you need to make two resistor arrays of 4 Ohm, 80W each! This 80W figure includes a reasonable safety margin. These resistors will produce a lot of heat, and you may cook your coffee on them! Be sure to mount them in such a way that they have lots of ventilation, and that the heat from them will not reach the other components.
R3 and R4 may to have be built from parallel combinations too, because of the low value of only 0.15 Ohm.
U2 is a voltage reference IC. You cannot replace it by a standard Zener diode! Zeners are much too unstable! If you can’t find this chip locally, you may use the ubiquitous 7805 regulator instead, but the power drain from the battery will be higher. In this case, of course you don’t need R8, but you would need a 1uF capacitor at the 7805 output.
Q3 is a power MOSFET that has a very low Rds(on). You may use a different one, provided that it has a resistance that’s low enough for your application. You may use several in parallel. The one I used has low loss even at loads of 20A, and can handle much more!
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