Anode (+)                    Cathode (-)

Component Theory - Diodes

A Rectifier Diode is a two terminal passive component that is designed to only allow current to flow from the Anode to the Cathode. They therefore control the flow of current in a circuit.

A diode has 3 main uses within a circuit that you need to know -

  • To control the flow of current in a circuit
  • To protect against incorrect polarity
  • To protect against 'back EMF'

When the Diode allows current to flow is it considered to be 'Forward Bias'and when it does not it is 'Reverse Bias'. If the diode receives excessive negative current it can 'breakdown' and fail to control it entirely.


When using electro-mechanical components such as motors, 'back EMF' can occur and cause damage to your connected IC. As a standard DC motor has no brake the motor may spin after being engergised. This continued rotation cause the motor to generate voltage  as it is electromagnetic. This voltage can damage the output pin of an IC or PIC and is sent back into the motor using a diode to dissipate.​


The Zener Diode is specifically designed to 'breakdown' at a set voltage known as the 'zener voltage'. ​This can allow for voltage regulation as the specific voltage diode would be used to ensure the correct voltage is being supplied to the circuit. If not, the Zener would not conduct.

A Bridge Rectifier is a combination of diodes that converts Alternating Current (AC) to Direct Current (DC) in a circuit. It does this by controlling the rapidly changing + & - of AC current into a constant + & - DC. 

These are commonly found in most power supplies for digital electronics which require a constant DC input but receive AC from mains electricity. The AC wave form looks like an Analogue wave and is rectified into a peaked positive voltage line as shown blue. This can then be smoothed using capacitors to a constant DC voltage which is shown in green.

Anode (+)                  Cathode (-)

1N4001 Rectifier


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