What is a diode
To use an analogy with water flowing through pipes. A diode is the electrical equivalent of a non return valve in a water system. In this analogy the water pressure (or head) is equivalent to voltage and the water flow is equivalent to electrical current. In other words current will only pass through the diode in one direction. Diodes are two terminal devices named the Anode and Cathode. Current will only flow when the anode is at a more positive voltage than the cathode.
Another minor characteristic of diodes is that they will not actually start to conduct until this forward voltage is greater than a particular amount. Typically this is about 0.7 volts, but can vary significantly depending on the particular type of diode. i.e. for voltages less than this, the diode acts like an open circuit (in both directions). One of the most common uses for diodes is in rectifying AC voltages. Since AC voltages alternately vary from positive to
negative the diode blocks the negative excursions and allows the positive through.
One of the most common forms of diode is the LED , or Light Emitting Diode. These produce light when there is current flowing through them, but none the less, still behave as a true diode, conducting in only one direction.
Common colours of LED's are green and red , although today it is common to find most colours from yellow, to blue and even white. Some LED packages incorporate two diodes back to back allowing different diodes to conduct according to the polarity of the applied voltage and may
even be different colours.. Other packages combine
multiple LED's into an array of indicators that can be used to form bar graph displays and similar. The light emitting diode also forms the basis of many opto-isolation techniques where the activation of a light source and detection by a light sensitive receiver isolates the source and destination of a signal using a beam of light (see opto-isolation).
The circuit symbols for various diodes are shown below