Storing Charge in a Capacitor

CHAPTER 4: LEDs 79

Storing Charge in a Capacitor

To make the brake lights stay on for a few moments after the car has stopped, you will need a capacitor to store charge.

If you think back to the idea of electricity as water flowing in a river, then a capacitor is a bit like a storage tank. Figure 4-32 shows how a capacitor can be used to store charge.

Figure 4-32a shows a tank (c1) being filled with water through A. Throughout this, water will also flow along the top and drive a water wheel, turning electrical energy into motion, a bit like how a light bulb or LED turns electrical energy into light. The water falls out of the bottom, returning to ground. Imagine a pump (like a battery) pulling the water back up for another circuit. If the water stops flowing into C1 through A, then C1 will still be full of water that will keep the water wheel turning until the water level in C1 drops below that of the outlet of the water well.

Figure 4-32b shows the electronic equivalent of this circuit. While the voltage at A is higher than GND, C1 will fill with charge and the light will be lit.

When the voltage at A is disconnected, the capacitor will discharge through the light bulb, lighting it. As the level of voltage drops in the capacitor, the bulb will gradually dim until it goes out as it reaches GND.

Figure 4-32 A capacitor as a tank