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9 Everything about Diode - What is Diode - How Diode works - Diode Rectifier - Diode Application


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Diodes
I was read alot of information about Diodes to understand it completely and i Collected the best i read ,, to understand it very easy .







What is Diode ?
How it works ?



Example:
Circuit symbol:


Function


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.









Forward Voltage Drop - Forward Bias
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 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 diode is almost constant whatever the current passing through the diode so they have a very steep characteristic (current-voltage graph).



Reverse Voltage - Reverse Bias
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. Ordinary diodes can be split into two types: Signal diodes which pass small currents of 100mA or less and Rectifier diodes which can pass large currents. In addition there are LED and Zener diodes .



Types of semiconductor diode :




?How Diodes works



When placed in a simple battery-lamp circuit, the diode will either allow or prevent current through the lamp, de


pending on the polarity of the applied voltage. (Figure below)






Diode operation: (a) Current flow is permitted; the diode is forward biased. (b) Current flow is prohibited; the diode is reversed biased.


When the polarity of the battery is such that electrons are allowed to flow through the diode, the diode is said to be forward-biased. Conversely, when the battery is “backward” and the diode blocks current, the diode is said to be reverse-biased. A diode may be thought of as like a switch: “closed” when forward-biased and “open” when reverse-biased.



A forward-biased diode conducts current and drops a small voltage across it, leaving most of the battery voltage dropped across the lamp. If the battery's polarity is reversed, the diode becomes reverse-biased, and drops all of the battery's voltage leaving none for the lamp. If we consider the diode to be a self-actuating switch (closed in the forward-bias mode and open in the reverse-bias mode), this behavior makes sense. The most substantial difference is that the diode drops a lot more voltage when conducting than the average mechanical switch (0.7 volts versus tens of millivolts).


This forward-bias voltage drop exhibited by the diode is due to the action of the depletion region formed by the P-N junction under the influence of an applied voltage. If no voltage applied is across a semiconductor diode, a thin depletion region exists around the region of the P-N junction, preventing current flow. (Figure below (a)) The depletion region is almost devoid of available charge carriers, and acts as an insulator:






Diode representations: PN-junction model, schematic symbol, physical part.

The schematic symbol of the diode is shown in Figure above (b) such that the anode (pointing end) corresponds to the P-type semiconductor at (a). The cathode bar, non-pointing end, at (b) corresponds to the N-type material at (a). Also note that the cathode stripe on the physical part (c) corresponds to the cathode on the symbol.
If a reverse-biasing voltage is applied across the P-N junction, this depletion region expands, further resisting any current through it. (Figure below)




Depletion region expands with reverse bias.
Conversely, if a forward-biasing voltage is applied across the P-N junction, the depletion region collapses becoming thinner. The diode becomes less resistive to current through it. In order for a sustained current to go through the diode; though, the depletion region must be fully collapsed by the applied voltage. This takes a certain minimum voltage to accomplish, called the forward voltage below. as illustrated in Figure




Inceasing forward bias from (a) to (b) decreases depletion region thickness.
For silicon diodes, the typical forward voltage is 0.7 volts, nominal. For germanium diodes, the forward voltage is only 0.3 volts. The chemical constituency of the P-N junction comprising the diode accounts for its nominal forward voltage figure, which is why silicon and germanium diodes have such different forward voltages. Forward voltage drop remains approximately constant for a wide range of diode currents, meaning that diode voltage drop is not like that of a resistor or even a normal (closed) switch. For most simplified circuit analysis, the voltage drop across a conducting diode may be considered constant at the nominal figure and not related to the amount of current.



Connecting and soldering




Diodes must be connected the correct way round, the diagram may be labeled a or +k or - for cathode (yes, it really is k, not c, for cathode!). The cathode is marked by a line painted on the body. Diodes are labeled with their code in small print, you may need a magnifying glass to read this on small signal diodes! for anode and Small signal diodes can be damaged by heat when soldering, but the risk is small unless you are using a germanium diode in which case you should use a heat sink clipped to the lead between the joint and the diode body. A standard crocodile clip can be used as a heat sink.
Rectifier diodes are quite robust and no special precautions are needed for soldering them.



Testing diodes


You can use a multimeter or a simple tester (battery, resistor and LED) to check that a diode conducts in one direction but not the other. A lamp may be used to test a rectifier diode, but do NOT use a lamp to test a signal diode because the large current passed by the lamp will destroy the diode !




Testing a diode with a multimeter

The techniques used for each type of meter are very different so they are treated separately:



Diodes
a = anode
k = cathode

Testing a diode with a DIGITAL multimeter

  • Digital multimeters have a special setting for testing a diode, usually labelled with the diode symbol.
  • Connect the red (+) lead to the anode and the black (-) to the cathode. The diode should conduct and the meter will display a value (usually the voltage across the diode in mV, 1000mV = 1V).
  • Reverse the connections. The diode should NOT conduct this way so the meter will display "off the scale" (usually blank except for a 1 on the left).
Testing a diode with an ANALOGUE multimeter




  • Set the analogue multimeter to a low value resistance range such as × 10.
It is essential to note that the polarity of analogue multimeter leads is reversed on the resistance ranges, so the black lead is positive (+) and the red lead is negative (-)! This is unfortunate, but it is due to the way the meter works.
Connect the black (+) lead to anode and the red (-) to the cathode. The diode should conduct and the meter will display a low resistance (the exact value is not relevant).
Reverse the connections. The diode should NOT conduct this way so the meter will show infinite resistance (on the left of the scale).








Signal diodes (small current)
Signal diodes are used to process information (electrical signals) in circuits, so they are only required to pass small currents of up to 100mA.
General purpose signal diodes such as the 1N4148 are made from silicon and have a forward voltage drop of 0.7V.

Germanium diodes such as the OA90 have a lower forward voltage drop of 0.2V and this makes them suitable to use in radio circuits as detectors which extract the audio signal from the weak radio signal.
For general use, where the size of the forward voltage drop is less important, silicon diodes are better because they are less easily damaged by heat when soldering, they have a lower resistance when conducting, and they have very low leakage currents when a reverse voltage is applied.







Rectifier diodes (large current)







Rectifier diodes are used in power supplies to convert alternating current (AC) to direct current (DC), a process called rectification. They are also used elsewhere in circuits where a large current must pass through the diode. All rectifier diodes are made from silicon and therefore have a forward voltage drop of 0.7V. The table shows maximum current and maximum reverse voltage for some popular rectifier diodes. The 1N4001 is suitable for most low voltage circuits with a current of less than 1A.




Bridge rectifiers





There are several ways of connecting diodes to make a rectifier to convert AC to DC. The bridge rectifier is one of them and it is available in special packages containing the four diodes required. Bridge rectifiers are rated by their maximum current and maximum reverse voltage. They have four leads or terminals: the two DC outputs are labelled + and -, the two AC inputs are labelled . The diagram shows the operation of a bridge rectifier as it converts AC to DC. Notice how alternate pairs of diodes conduct.





Note



  • Diodes have two leads like a resistor.
The current they pass depends upon the voltage between the leads.
Diodes do not obey Ohm's law!








How a PN-Junction Diode Works ?



To understand how a pn-junction diode works, begin by imagining two separate bits of semiconductor, one n-type, the other p-type.









Bring them together and join them to make one piece of semiconductor which is doped differently either side of the junction.








Free electrons on the n-side and free holes on the p-side can initially wander across the junction. When a free electron meets a free hole it can 'drop into it'. So far as charge movements are concerned this means the hole and electron cancel each other and vanish.







As a result, the free electrons and holes near the junction tend to eat each other, producing a region depleted of any moving charges. This creates what is called the depletion zone.







Now, any free charge which wanders into the depletion zone finds itself in a region with no other free charges. Locally it sees a lot of positive charges (the donor atoms) on the n-type side and a lot of negative charges (the acceptor atoms) on the p-type side. These exert a force on the free charge, driving it back to its 'own side' of the junction away from the depletion zone.







The acceptor and donor atoms are 'nailed down' in the solid and cannot move around. However, the negative charge of the acceptor's extra electron and the positive charge of the donor's extra proton (exposed by it's missing electron) tend to keep the depletion zone swept clean of free charges once the zone has formed. A free charge now requires some extra energy to overcome the forces from the donor/acceptor atoms to be able to cross the zone. The junction therefore acts like a barrier, blocking any charge flow (current) across the barrier.







Usually, we represent this barrier by 'bending' the conduction and valence bands as they cross the depletion zone. Now we can imagine the electrons having to 'get uphill' to move from the n-type side to the p-type side. For simplicity we tend to not bother with drawing the actual donor and acceptor atoms which are causing this effect!



The holes behave a bit like balloons bobbing up against a ceiling. On this kind of diagram you require energy to 'pull them down' before they can move from the p-type side to the n-type side. The energy required by the free holes and electrons can be supplied by a suitable voltage applied between the two ends of the pn-junction diode. Notice that this voltage must be supplied the correct way around, this pushes the charges over the barrier. However, applying the voltage the 'wrong' way around makes things worse by pulling what free charges there are away from the junction!



This is why diodes conduct in one direction but not the other.



Finally ...


This is a avideo to explain what is diode ?how it works?






Review :



A diode is an electrical component acting as a one-way valve for current.

When voltage is applied across a diode in such a way that the diode allows current, the diode is said to be forward-biased.
When voltage is applied across a diode in such a way that the diode prohibits current, the diode is said to be reverse-biased.
The voltage dropped across a conducting, forward-biased diode is called the forward voltage. Forward voltage for a diode varies only slightly for changes in forward current and temperature, and is fixed by the chemical composition of the P-N junction.
Silicon diodes have a forward voltage of approximately 0.7 volts.
Germanium diodes have a forward voltage of approximately 0.3 volts.

The maximum reverse-bias voltage that a diode can withstand without “breaking down” is called the Peak Inverse Voltage, or PIV rating.

 





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