Diodes are one way conductors and under normal circumstances will allow current to pass from the anode to the cathode. The symbol for a diode is:
The arrow head (triangle) points towards the direction of current flow.
Placing a positive voltage on the cathode of a conductor will cause the diode to become an infinite resistance. A diode will like this until the reverse breakdown voltage is reached. At this point the diode will start to conduct. It is extremely rare to use or place a diode in this situation.
Diodes have a number of uses:
- Generating light (LEDs)
- Rectifying a signal
- Logic gates
- Prevent battery discharge in battery chargers
Light Emitting Diode (LED)
A Light Emitting Diode (LED) is a small compact device capable of generating light when a current flows though it. LEDs are available in a number of different colours. A fuller description of LEDs and their properties and used can be found here.
The alternating voltage (on the left) is placed across the bridge. The diodes will only allow the current to pass one way and this configuration produces a positive wave form that varies between 0V and the voltage applied. The capacitor helps to smooth the voltage out into an approximation of a flat DC signal.
Diodes can be used to produce basic logic gates such as AND and OR gates. A simple description of their operation is provided below. A fuller description can be found in this Wikipedia article on Diode Logic
In the circuit below, the 10K resistor acts as a pull-down resistor. If no voltage is present then the output (Out) will be at 0V.
If a voltage of 5V is applied to either of the inputs (A and B) then the diode will allow current to flow and the output will 5V.
The AND gate circuit below uses the laziness property of electricity, it will always seek the path of lowest resistance to ground.
If the inputs (A and B) at at 0V then the current from the 5V source will flow through the resistor to either A or B. The result is that the output (Out) will be 0V.
Placing a 5V signal on A and 0V on B will mean make the current flow through B. The 5V at A resists the current flow through the diode but the 0V on B provides a path through the diode to ground. Similarly, setting A to 0V and B to 5V will produce the same effect.
If both A and B are set to 5V then the current cannot flow through either of the diodes. This means that the current will flow though the output (Out).