HowTo-Color (8) / Hacking Electronics / Simon Monk / 236-3 / Chapter 5
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Hacking Electronics
Just the Battery
Figure 5-11 Diodes for battery backup
If only the battery has a voltage greater than zero (in other words, the power supply is not plugged in), then the situation is as shown in Figure 5-11a. The 9V from the battery will be at the anode of D2, and the cathode of D2 will be pulled toward ground by the load of the light bulb. This will cause D2 to be forward-biased and conduct the current through the light bulb. A forward-biased diode will have an almost constant voltage of 0.5V across it, which is why we can say that the voltage after the diode is 8.5V. On the other hand, D1 will have a higher voltage (8.5V) on its cathode (right-hand side in the diagram) than its anode (0V), so no current will flow through D1.
Just the Power Supply If just the power supply is connected (Figure 5-11b), then the role of the diodes is reversed and now the current flows through D1 to the light bulb.
Both the Power Supply and the Battery Figure 5-11c shows the situation where both the power supply and the battery are connected. The 12V of the power supply will ensure that the cathode of D2 is at 11.5V. Since the anode of D2 is at 9V from the battery, the diode will remain reverse-biased and no current will flow through it.
Trickle Charging As we already have a battery and a power supply, we have most of the ingredients we need to charge the battery. We could for example use six AA rechargeable batteries in a battery box
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