Designing Embedded Hardware, 2nd Edition

Capacitors

While a resistor is a component that resists the flow of charge through it, a capacitor stores charge. Capacitance is measured in Farads (or more formally, "Faradays") with an equation symbol "C" and a unit symbol "F." Typical capacitors you will use range in value from uF (micro-Farads) down to pF (pico-Farads).

The relationship between current, capacitance, and voltage is given by:

   I = C * dV/dt

where dV/dt is the rate of voltage change over time.

The schematic symbols for capacitors are shown in Figure 4-26. The component on the far left is bipolar, while the other two are unipolar. A unipolar capacitor has a positive lead and a negative lead, and it must be inserted into a circuit with the correct orientation. Failing to do so will cause it to explode. (Unipolar capacitors have markings to indicate their orientation.) A bipolar capacitor has no polarity.

Figure 4-26. Capacitor symbols

Applying a voltage across a capacitor causes the capacitor to become charged. If the voltage source is removed, and a path for current flow exists elsewhere in the circuit, the capacitor will discharge and thereby provide a (temporary) voltage and current source (Figure 4-27).

Figure 4-27. Capacitor charging and discharging

This is an extremely useful characteristic. A given voltage source may ...