Diodes allow electricity to flow in only one direction.The arrow of the circuit symbol shows the direction in which the current can flow.Diodes are the electrical version of a valve and early diodes were actually called valves.
Symbole :
Forward Voltage Drop:
Electricity uses up a little energy pushing its way through the diode,rather like a person pushing through a door with a spring.This means that tere is a small voltage across a conducting diode,it is called the FORWARD VOLTAGE DROP and is about 0.7v for all normal diodes which are made from silicon.The forward voltage drop of a diode is almost constant whatever the current passing through the diode so they have a very steep characteristic ( current-voltage graph).
Reverse Voltage :
When a reverse voltage is applied a perfect diode does not conduct,but all real diodes leak a very tiny current of a few µA or less.This can be ignored in most circuits because it will be very much smaller than the current flowing in the forward direction.However,all diodes have a maximum reverse voltage (usually 50V or more ) and if this is exceeded the diode will fail and pass a large current in the reverse direction,this is called breakdown.
The other variant of diodes have different construction;characteristics and applications.The diffrent types of diodes are :
- Small signal or small current diode-These diodes assumes that the operating point is not affected because the signal is small .
- large signal diodes-The operating point in these diodes get affected as the signal is large.
- Zener diodes-This diode runs in reverse bias consition when the voltage reaches the breakdown point.A stable voltage can be achieved by placing a resistor across it to limit the current .This diode is used to provide reference voltage in power supply circuits.
- Light emitting diodes(LED)-This is the most popular kind of diode.When it works in the forward bias condition,the current flows through the junction to produce the light.
- Photodiodes-The electrons and holes are generated as light strikes across the p-n junction causing the current to flow.Theses doides can work as photodetector and are used to generate electricity.
- Constant current diodes-This diode keeps the current constant even when the voltage applied keeps changing.It consists of JFET (junction-field effect transistor)with the source shorted to the gate in order to function like a two-terminal current limiter or current source.
- Schottky diode-This is a four layer diode which is also know as PNPN diode.This diode is smilar to thyristor where the gate is discconnected.
- Step recovery diodes-This semiconductor diode has the ability to generate short pulses and hence it is used in microwave applications as pluse generator.
- Tunnel diodes-This diode is heavily doped in the forward bias condition that has a negative resistance at extremely low voltage and a short circuit in the negative bias direction.This diode is useful as a microwave ampilifer and in oscillators.
- Varactor diodes-This diode works in revers bias condition and restricts the flow of current through the junction.Depending on the amount of biasing,the with of the depletion region keeps varying.This diode comprises of two plates of a capacitor with the depletion region amidst them.The variation in capacitance depends upon the depletion region and this can varied by altering the reverse biase on the diode.
- Pin diodes-This diode has intrinsic semiconductor sandwiched between P-type and N-type region.Doping does not occur in this type of diode and thereby the intrinsic semiconductor increses the width of the depletion region.They are used as ohtodiodes and radio frequency switches.
- Laser diode-This diode is used to protect the electronics that are sensitive against voltage spikes.
- Glod doped diodes-These diodes use gold as dopant and can operate at signal frequencies even if the forward voltage drop increases.
- Super barrier diodes-These are also called as the rectifier diodes .This diodes have the property of low reverse leakage current as that of normal p-n junction diode and low forward voltage drop as that od schottky diode with surge handling ability.
- Point contact diodes-The construction of this diode is simpler and are used in analog applications and as a detector in radio receivers.This diode is built of n-type semiconductor and few conducting metals placed to be in contact with the semiconductor.Some metals move from towards the semiconductor to from small region of p-type semiconductor near the contact.
- Peltier diodes-This diode is used as heat engine and sensor for thermoelectric cooling.
- Gunn diode-This diode is made of materials like GaAs or InP that exhibit a negative differential resistance region.
- Crystal diode-These are type of point contact diodes which are also called as cat's whisker diode.This diode comprise of a thin sharpened metal wire which is pressed against the semiconducting crystal.The metal wire is the anode and the semiconducting crystal is the cathode.These diodes are obsolete.
- Avalanche diode-This diode conducts in reverse bias condition where the reverse bias voltage applied across the p-n junction creates a wave of ionization leading to the flow of large current.These diodes are designed to brealkdown at specific reverse voltage in order to avoid any damage.
- Silicon controlled rectifier-As the name implies this diode can be controlled or triggered to the ON condition due to the application of small voltage.They belong to the family of tyristors and is used in various fields of DC motor control;generator field regulation,lighting system control and variable frequency drive.This is three terminal device with anode,cathode and third controled lead or gate.
- Vaccum diodes-This diode is two electrode vacuum tube which can tolerate high inverse voltages.
Forward Voltage Drop :
Electricity uses up a little energy pushing its way through the diode,rather like a person pushing through a door with spring.This means that there is a small voltage across a conducting diode,it is called the FORWARD VOLTAGE DROP and is about 0.7V for all normal diodes which are made from silicon.The forward voltage drop of a didoe is almost constant whatever the current passing through the diode so they have a very steep characteristic (current -voltage ).
Reverse Voltage :
When a reverse voltage is applied a perfect diode does not conduct,but all real diodes leak a very tiny current a few µA or less.This can be ignored in most circuits because it will be very much smaller than the current flowing in the forward direction.However ,all diodes have a MAXIMUM REVERSE VOLTAGE (usually 50V or more ) and if this is exceeded the diode will fail and pass a large current in the reverse direction,this is called BREAKDOWN.
Ordinary dioes can be split into two types:Signal diodes which pass small currents of 100mA or less and Rectifier diodes which can pass large currents.
Connecting and soldering :
Diodes must be connected the correct way round,the diagram may be labelled a or + for anode and k or - for cathode.The cathode is marked by a line painted on the body.Diodes are labelled with their code in small print,you may need a magnifying glass to read this on small signal diodes!
Small signal diodes can be damaged by heat when soldering,but the risk is small unless you are using a germaniu diode in which case you should use a heat sink clipped to the lead between the joint and the diode body.A standard crocodile cip can be used as a heat sink.
Rectifier diodes are quite robust and no special precautions are needed for soldering them.
Diodes are used in various applications like rectification,clipper,clamper,voltage multiplier,comparator,sampling gates and filters.
- Rectification :The rectification means converting AC voltage into DC voltage.The common rectification circuits are half wave rectifier (HWR), full wave rectifier(FWR) and bridge rectifier.
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