Designing Embedded Hardware

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 will range in value from μF (microFarads) down to pF (picoFarads).

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 2-10. The component on the 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 2-10. 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 2-11).

Figure 2-11. Capacitor charging and discharging

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