Designing Embedded Hardware

Diodes

Diodes are extremely useful semiconductor devices. They have the interesting characteristic that they will pass a current in one direction, but block it from the other. They can be used to allow currents to flow from one part of a circuit to another but prevent other currents from “backwashing” where you don’t want them.

The schematic symbol for a diode is shown in Figure 2-29. The arrow indicates the direction of conduction. The arrow represents the anode , or positive side, of the diode, while the bar represents the cathode , or negative side, of the diode. A higher voltage on the left of the component will allow current to be passed through to the right. However, a higher voltage on the right will prevent current flow to the left.

Figure 2-29. Schematic symbol for a diode

Diodes have a forward voltage drop when conducting. This means that there will be a voltage difference between the anode and the cathode. For example, a diode may have a forward voltage drop of 0.7V. If this diode is part of a larger circuit and the voltage at the anode is 5V, then the voltage at the cathode will be 4.3V.

Diodes are useful for removing negative voltages from a signal, a process known as rectification. Four diodes may be combined together to form a bridge rectifier, as shown in Figure 2-30. The bridge “flips” the negative components of the wave so that only a positive voltage is present ...