What is a Diode

The Diode
One component which has been shown but not described is the diode or ‘rectifier’.   This is a device which has a very high resistance to current flowing in one direction and a very low resistance to current flowing in the opposite direction.   The base/emitter junction of a transistor is effectively a diode and, at a push, can be used as such.   A proper diode is cheap to buy and has far greater voltage and current handling capacities than the base/emitter junction of a transistor.

Diodes are mainly made from one of two materials: germanium and silicon.   Germanium diodes are used with very small alternating currents such as radio signals coming from an aerial.   This is because a germanium diode needs only 0.2 Volts or so to carry a current while silicon needs 0.6 to 0.7 Volts (same as a silicon transistor base/emitter junction).   Germanium diodes (and transistors) are very sensitive to temperature change and so are normally restricted to low power circuits.   One very neat application for a silicon diode is as an ‘un-interruptible power supply’ where mains failure is caught instantly:

In this circuit, the mains voltage drives the Power Supply Unit which generates 12 Volts at point ‘A’.   This provides current to the Load.   The diode has +12 Volts at ‘A’ and +12 Volts at point ‘B’ so there is no voltage drop across it and it will not carry current in either direction.   This means that the battery is effectively isolated when the mains is functioning.   If the Power Supply Unit output were to rise above its design level of +12 Volts, then the diode would block it from feeding current into the battery.

If the mains fails, the Power Supply Unit (‘PSU’) output will fall to zero.   If the battery and diode were not there, the voltage at point ‘A’ would fall to zero, which would power-down the Load and possibly cause serious problems.  For example, if the load were your computer, a mains failure could cause you to lose important data. With a battery back-up of this type, you would have time to save your data and shut your computer down before the battery ran out.

The circuit operates in a very simple fashion.   As soon as the voltage at point ‘A’ drops to 0.7 Volts below the +12 Volts at point ‘B”, the diode starts feeding current from the battery to the Load.   This happens in less than a millionth of a second, so the Load does not lose current.   It would be worth adding a warning light and/or a buzzer to show that the mains has failed.


LEDs:
There is a widely used variation of the diode which is extremely useful, and that is the Light Emitting Diode or ‘LED’.   This is a diode which emits light when carrying current.   They are available in red, green, blue, yellow or white light versions.   Some versions can display more than one colour of light if current is fed through their different electrical connections.

LEDs give a low light level at a current of about 8 or 10 mA and a bright light for currents of 20 to 30 mA. If they are being used with a 12 Volt system, then a series resistor of 1K to 330 ohms is necessary.   LEDs are robust devices, immune to shock and vibration.   They come in various diameters and the larger sizes are very much more visible than the tiny ones.

SCRs and Triacs:
Another version of the diode is the Silicon Controlled Rectifier or ‘Thyristor’.   This device carries no current until its gate receives an input current.   This is just like the operation of a transistor but the SCR once switched on, stays on even though the gate signal is removed.   It stays on until the current through the SCR is forced to zero, usually by the voltage across it being removed.   SCRs are often used with alternating voltages (described below) and this causes the SCR to switch off if the gate input is removed.   SCRs only operate on positive voltages so they miss half of the power available from alternating power supplies.   A more advanced version of the SCR is the ‘Triac’ which operates in the same way as an SCR but handles both positive and negative voltages.

Opto-Isolators:
Another very useful variation on the LED is the Opto-Isolator.   This device is a fully enclosed LED and light-sensitive transistor.   When the LED is powered up, it switches the transistor on.   The big advantage of this device is that the LED can be in a low voltage, low power sensing circuit, while the transistor can be in a completely separate, high voltage, high power circuit.   The opto-isolator isolates the two circuits completely from each other.   It is a very useful, and very popular, low-cost device.

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