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# Electricity > Diodes

Diodes are components that allow electricity to flow in only one direction around a circuit. They act like valves in water circuits and some were originally known as valves. The direction the electricity can travel through the diode is denoted by an arrow. For this reason it is important that diodes are connected or soldered the correct way round in order for them to function correctly. On the outer casing of the diode there should be a small dark line at one end. It is that end that should be connected to the cathode or ‘-‘of the circuit. Circuit diagrams are usually marked with an ‘a’ and a ‘k’ for anode or cathode respectively. Below is the symbol for a diode with anode and cathode labelled.

Fig 1:
Diode symbol with anode and cathode

If you need to check if a diode is working, never use a lamp in a circuit as the high current may damage the diode. Instead use a simple circuit of an LED and resistor or alternatively use a multimeter.

Diodes can be used to convert A.C electricity into D.C. This is done with what is known as a bridge rectifier. Four diodes are used in a square formation which allows the alternating polarity of A.C to flow at a consistent polarity (D.C). Making a rectifier is sometimes done as a school practical. Using labpacks with both A.C and D.C power is useful as pupils can observe the differences between the two. Making a bridge rectifier is sometimes practiced alongside making a wind turbine. Turbines produce A.C electricity which needs to be converted to D.C in order to charge batteries or directly light lamps etc.

Fig 2:
Bridge rectifier

Zener diodes Are used to create a fixed voltage. They are used in reverse and ‘break down’ in a controlled way in order to maintain this selected voltage. The look similar to regular diodes apart from the fact they have their breakdown voltage and code printed on them.

The circuit diagram below shows how Zener diodes can be used in a circuit to fix an output voltage.

Fig 3:
Zener diode circuit