A diode is a two terminal electronic component and most diodes used today is made of a semiconductors. The two terminals are called the anode (the arrow side) and the cathode (the bar side). A diode has the property to let current pass in one direction from the anode to the cathode but it blocks current flowing in the other direction from the cathode to the anode. The arrow in the symbol points in the opposite direction as electron flow because it was based on conventional current flow before the discovery of electrons. In other words current will flow through the diode if the anode (arrow side) is positive and the cathode (bar side) is negative.
The diode is mostly used today in the following applications:
- Rectifying circuits in power supplies.
- Protection of relay coils from back EMF generation.