Intro

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:

Diode Circuit Symbol
Diode Circuit Symbol

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.

Uses

Diodes have a number of uses:

  • Generating light (LEDs)
  • Rectifying a signal
  • Logic gates
  • Lasers
  • Radio
  • 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.

Bridge Rectifier

A bridge rectifier be used to take an alternating current (AC) signal and convert this to a direct current (DC) signal.

Bridge Rectifier
Bridge Rectifier

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.

Logic Gates

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

OR Gate

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.

OR Gate
OR Gate
AND Gate

The AND gate circuit below uses the laziness property of electricity, it will always seek the path of lowest resistance to ground.

AND Gate
AND Gate

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).

Common General Diode Reference

Diode Part Type Peak Inverse Voltage (PIV) Max. Forward Current (IO(max) Max. Reverse Current IR(max) Peak Surge Current (Ifsm) Max. Voltage Drop (Vf(max))
1N34A Signal (Ge) 60V 8.5mA 15µA   1.0V
1N67A Signal (Ge) 100V 4.0mA 5µA   1.0V
1N191 Signal (Ge) 90V 5.0mA     1.0V
1N914 Fast Switch 90V 75mA     0.8V
1N4148 Signal 75V 10mA 25nA 450mA 1.0V
1N4445 Signal 100V 100mA 50nA   1.0V
1N4001 Rectifier 50V 1A 0.03mA 30A 1.1V
1N4002 Rectifier 100v 1A 0.03mA 30A 1.1V
1N4003 Rectifier 200V 1A 0.03mA 30A 1.1V
1N4004 Rectifier 400V 1A 0.03mA 30A 1.1V
1N4007 Rectifier 1000V 1A 0.03mA 30A 1.1V
1N5002 Rectifier 200V 3A 500µA 200A  
1N5006 Rectifier 600V 3A 500µA 200A  
1N5008 Rectifier 1000V 3A 500µA 200A  
1N5817 Schottky 20V 1A 1mA 25A 0.75V
1N5818 Schottky 30V 1A   25A  
1N5819 Schottky 40V 1A   25A .9V
1N5822 Schottky 40V 3A      
1N6263 Schottky 70V 15mA   50mA 0.41V
5052-2823 Schottky 8V 1mA 100nA 10mA 0.34V

Further Reading

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