What they look like:
How they are shown on circuit diagrams
Signal and Power Diodes
Diodes allow electricity to flow in one direction only. The arrow on the circuit diagram indicates the direction. Due to this, diodes are polarised and care must be taken when using diodes to ensure they are inserted with the correct orientation. Diodes are important for a number of purposes including:
Signal diodes are used in small current situations and are generally no different to power diodes in operation. They will be damaged easily if the maximum current is exceeded. They are typically used in radio and digital electronics.
Special Function Diodes
Zener diodes function as normal diodes and allow electricity to flow in the direction of the arrow with one special function. Zener diodes are categorised by a voltage eg 5.1 Volts (or 5V1). This voltage indicates the point that the diode will allow voltage to flow in the reverse direction.
For example, a 5V1 zener diode will not allow electricity to flow in the direction reverse to the arrow until the voltage across the zener diode reaches or exceeds 5V1. They are useful for:
Light Emitting Diodes - LED
LED's are used to visibly illuminate when electricity is applied to them in the direction of the arrow. They are usefil to provide visible indication to us when current is present. LED's will only handle small currents, typically 20 mA so care should be taken not to exceed this rating as permanant damage will occur if this current is exceeded.
How to identify them
Diodes (except LED's) are identified by labelling printed on their bodies. Careful examination and help from a magnifying glass will assist in identification.