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Basic principal of diode and its working.

P-N Junction Diode
A diode is a very basic semiconductor component containing junction of P-Type and N-Type material. It consists of one block of P-Type material and one block of N-Type material. Base material of almost all semiconductors is Silicon and sometimes Germanium is also used. P-Type material is made by doping silicon with trivalent impurities such as Boron, Gallium, Indium etc are called acceptor impurity, because they have empty spaces to accept one more electron as silicon has four electrons in its outer shell and these impurities have only three in outer shell to make bond. So we say P-Type material consist of holes or these vacant spaces.

On the other hand N-Type material is made by doping base silicon material with impurities like phosphorus, arsenic, antimony, bismuth which are pentavalent materials, hence they make bond with four electrons in outer shell of Silicon and have one extra electron to conduct.

symbol of diode
Diode Symbol

The symbol show a triangle with a vertical bar, triangular side is Anode and bar side is cathode. That means current is allowed to flow from anode (A) to cathode (K), the triangle is actually resemble an arrow saying current can flow from A to K, but bar indicates a barrier saying current cannot flow from K to A. In this animated simulation video below, you can see that when I press switch S1, the lamp doesn’t glow as the diode is not allowing current from positive terminal of battery till lamp as it encounters the bar (i.e. cathode) 
AC to DC converter
of diode, but when I press switch S2, it allows the current from positive terminal of battery to flow from anode of diode to cathode and to the lamp hence the lamp glow up. This feature of conducting current in one direct helps us to use diode to convert AC to DC which we will see further. 

Diode image
Diode

A diode physically looks like a small cylindrical part with two wires coming out from both side with black body and a white/silver color ring made on one side of body indicating that is cathode (K).



So the condition when positive supply is applied to anode and negative toward cathode, the electrons from negative terminal of battery pushes the electrons in N-type material and they cross the barrier and move to hole or vacant spaces in P-type material, this causes a current opposite to flow of electron i.e. from A to K and hence this condition is called forward biasing the diode as we can see for D2 in video.

Hence a diode conducts current in only one direction when it is forward biased. The reverse condition of applying positive supply to K and negative to A is called reverse biasing and a diode does not conduct electricity in reverse biased condition as in case of D1 in video. But yes there is a breakdown voltage of the barrier and if the reverse voltage of more than that barrier voltage is applied to diode, barrier will break down and off course it will start conducting in reverse direction, but normally this reverse breakdown voltage is very high, hence that never reaches in a circuit.

In the second video I have removed one diode and connected a voltmeter to measure voltage drop across diode when it conducts in forward biased condition. We can see that the meter shows 0.77V when the current flow through it under forward biased condition. But in reverse bias condition, when there is no current through it, there is no voltage drop across it. So, whenever we are using one diode in any circuit, we are losing 0.7volt across it, sometimes it is useful and sometimes it is a loss to us. This is called forward voltage drop and different diodes have different diodes have slightly different voltage drop. Mostly silicon diodes have voltage drop starting from 0.7V can go to 1V also and for Germanium diodes it starts from around 0.3V. Watch the simulation video for more clarity. 



Zener Diode

what is zener diode
Zener Diode

Construction of zener diode is also similar to diode with small modification due to which it has a lower breakdown voltage; this is achieved by heavily doping the silicon base with P-type and N-Type impurities. It can work as a normal diode when it is connected in forward biased mode, but in reverse mode when the supplied voltage exceeds the breakdown voltage, the diode works as a voltage regulator because the voltage across it remains same over the breakdown voltage. Normal diodes designed to work in forward biased mode but not to work in reverse biased mode in breakdown voltage region as compared to zener diodes 
symbol of zener diode
Symbol
which are designed to work specially in breakdown voltage region. We can get zener diode operating in range of 2.4V to 200V. Physically looks similar to diode except transparent body with red color internal structure visible. Symbol is similar to diode with broken/bent bar showing it works in breakdown voltage range where the current flow from cathode to anode breaking this barrier. The black ring on body indicates the cathode.

zener diode circuit
Zener Diode Circuit

A typical zener diode based voltage regulator circuit consists of a DC voltage source greater than zener voltage, a current limiting resistor ‘R’ and a zener diode of required output voltage. As we can in the circuit we have used three cell of 1.5V in series to get a battery of 4.5V feeding supply to zener diode through resistor ‘R’. Here the anode of zener is connected to negative of battery and cathode is connected to positive terminal through resistor ‘R’, hence here the zener is working in reverse biased condition in the breakdown voltage which is 3.3V here as we are using 3.3V zener diode.

Now a days many simple voltage regulators are available in for of chip or IC which have internally this basic circuit using zener and some more components, hence these zener based voltage regulator circuits are rarely used now a days.

Further since we have learnt basics of transformer, diode, capacitor and zener diode, we will next learn basic transformer based AC to DC circuit and regulated DC source using zener to the AC to DC converter.



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