HowTo-Color (8) / Hacking Electronics / Simon Monk / 236-3 / Chapter 5
Figure 5-12 Battery backup and charging
CHAPTER 5: Batteries and Power
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and arrange to charge them at C/20 (assuming C = 2000mAh) or 100mA from the power supply. That way, the batteries would always be charged, and provide light whenever the power failed. Figure 5-12 shows the schematic for this. You may not have been expecting the extra diode D3. This is really just to account for the fact that we do not know exactly how the power supply is designed, so we do not know what would happen if the battery was connected to its output (via R1) when it was turned off. This may discharge the battery or damage the power supply. The diode D3 just protects it and makes sure no current can flow back into it. We want a charging current of 100mA to flow through R1, and we know that when both the power supply and battery are connected, there will be a voltage across R1 of 12V – 0.5V – 9V or 2.5V. So, using Ohm’s law, the value of the resistor should be: R = V / I = 2.5 / 0.1A = 25Ω The nearest standard value to this is probably 27Ω. Its power requirement: P = V2 / R = 2.52 / 27 = 0.23W This means a standard half- or quarter-watt resistor will be fine.
How to Use Solar Cells On the face of it, solar cells seem like the perfect power source. They convert light into electricity, and so in theory you need never change a battery or be plugged into a wall outlet again! However, as always, the reality is not quite so simple. Solar cells, unless they are very large, produce fairly small amounts of electricity and so are most suited to low-power devices and projects that are outdoors away from household electricity. If you are thinking of trying a solar project that will be installed indoors, unless it will be sited against a south-facing window, I really wouldn’t try it. Solar cells do not require direct sunlight, but to produce any useful amounts of electricity, they really need a good unobstructed view of the sky. Two solar projects I have developed are a solar-powered radio (the solar panel is as big as the radio and, yes, it needs to be next to the window), and a solar-operated chicken house door. If you
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