Blog 1: Diodes

Diode is a two-terminal electrical device that has its own resistance and conductance.

Diode has many different purposes, it can be used as voltage regulators, switching applications, light emitting, steering diodes, tuning devices, frequency multiplying devices, mixing devices or to make logic decisions in digital circuits.

A semicondutor diode has the following types:

* p-n junction diode

* Light emitting diode (LED)

* Zener diode

* Avalanche diode

* Laser diode

* Thermal diode

* etc...

I will only be talking about two semiconductor diode here which are p-n junction diode and Zener diode due to there are too many different types.

These diodes normally have two terminals which are Anode "A" and Cathode "K".

In most of the diodes, a white or black  painted band is always a Cathode terminal. This is how we identify the terminals of the diodes.

A semiconductor diode is a two terminal component that electron will only be able to flow in one direction: Anode"A" to Cathode "K".

P-N junction diode:

When a p-n junction diode is connected in the forward bias arrangement which is positive to Anode and negative to Cathode, if the voltage exceeds 0.7V the electron will cross the junction and combine with holes so the current will flow. However when the diode is connected in the reverse bias arrangement which is negative to Cathode and positive to Anode, holes get attracted away from the junction so the current will not flow

The Cathode needs to connected towards negative side of the circuit if we want the diode conducting.

This photo shown below is an actual 1N4007 p-n junction diode:

The action of the diode takes place in p-n junction where is the boundary between the two terminals

This photo shown below is the data sheet of the diode:

As we can see, a diode has many characteristics which the main things that we need to notice are:

* Average rectified current

* Power dissipation

* Thermal resistance

* Storage and operating temperature range

* Peak reverse voltage

* etc..

These parameter will change depends on the ambient temperature.

Zener diode:

Zener diode is a kind of semiconductor diode that can be operated to conduct backward which we call zener breakdown. 

When a zener diode is connected in forward bias region, it works as a normal diode. However it works totally different than the normal diode when it connected in the reverse bias region. 

If a normal diode is connected in forward bias region, when the breakdown voltage exceeds, the diode will be destroyed due to overheating, but the zener diode is designed to have a greatly reduced breakdown voltage which will allow current to keep the voltage across the zener diode close to it.

This photo shown below is an actual Zener diode:

The zener diode can be used as voltage regulation or protecting the circuit and etc…

How to check if a diode is bad?

First of all , we will need a multitmeter.Set the multimeter to diode test mode,place one probe from the multimeter on to one of the diodes legs, and the other probe on to the other leg. Write down the reading obtained, and then swap the positions of the probes and write down the new reading.
If we get a reading in one direction indicating the current is bocked or open circuit and a low resistance reading in other direction,then the diode is good.

If we get a open circuit reading from both direction or a low resistance reading in both direction,then the diode is broken and needs to be replaced.

A faulty diode or the diode connected in the wrong direction will cause the circuit either not flowing current which the circuit will not be working or the diode is not working properly as it should which this could blow other components in the circuit and causes huge problems.

References:

auto101.com

wikipedia

Unitec Moodle

Vijay Naidu

Blog 2: Bipolar Junction Transistor

Transistor is a semiconductor device,which can be used for switching and amplifying signal and power.

Transistor is a alterable switch controlling output current based on input voltage,so it can be used as a current switch.It is different than the normal mechanical switch and relay as the transistor is using signals to control and the switching frequency is way faster

When a voltage or current apply to the transistor,the controlled output power can be much more than the controlling input power,so a transistor can be using to amplify a signal.


There are two main types of transistors,one is BJT(Bipolar Junction Transistor) and the other one is FET(Field-effect Transistor)
A BJT normally has three terminals which are Collector(C),Base(B) and Emitter(E).A BJT is a combination of two junction diodes,when a p-type semiconductor is in the middle of two n-type semiconductor it is a NPN transistor,or when a n-type semiconductor is in the middle of two p-type semiconductor it is a PNP transistor.

This picture shown below is a three terminal bipolar transistor.

How to identify the terminals and NPN or PNP of a transistor?
First,get a multimeter and set to diode test, here we label the 3 terminals as leg1,leg2 and leg3 from left to right.Now test the continuity between legs in order of :
1 to 2
1 to 3
2 to 1
2 to 3
3 to 1
3 to 2

Whichever leg makes a reading on the multimeter with the other two is the base,if the Base is connected with the negative probe it is a PNP transistor,if the Base is connected with the positive probe it is a NPN transistor and whichever has more reading is Collector and the other one is Emitter.

The most BJT used today is NPN transistor as it produce greater current and faster operation so I am only going to take about NPN Bipolar Junction Transistor.


In a NPN transistor,the current path from the Collector to Base is in fact a reverse biased diode,and the current path from the Base to Emitter is in fact a forward biased diode.For the current to pass from Base to Emitter the voltage needs to exceeds 0.7V and when this voltage reaches more than 0.7V, the Collector current will flow.

A bipolar transistor normally has three distinct regions of operation which are Active, Saturation and Cut off as photos shown below.

Active: is an operation region when transistor is amplifying small signals
Saturation: is an operation region when both junction diode are forward biased so it will generates high current flow through Emitter from Collector,this corresponds a logical "on" or closed switch.
Cut off: is an operation region when both junction diode are reverse biased which it is the opposite of saturation.This is result in very little current which corresponds a logical "off " or opened switch


When amplification small signals the transistor is operating in the active region and it will make high power dissipation as it drops half the voltage across Vce.

The maximum power a transistor can dissipate is known as Pd which equals to I x V according to ohm's law.

The ratio between Collector current and Base current is know as the current gain Beta where Beta equals to Ic/Ib, this current gain is a major advantage of the transistor and can be used to all kinds of application.


The other ability of the transistor is using as a switch to turn things on and off which can often be seen in automotive applications such as Injectors and Ignition Coils

The Collector current is proportionally limited by its Bass current, so a switching transistor is actually a current-controlled switch. When there is a voltage supply which exceeds more than 0.7V to Base, the current will flow through from Collector to Emitter(Vce) and it's output voltage and speed changes proportionally when  the Base voltage and frequency changes.

What could cause a bad transistor?

* Wrong resister is been used in series with base

* Too much current going to Collector

* No current going to Base

* No supply voltage

* Circuit connected to different ternimals

This is just some typical reason that could cause a transistor go bad, there are more reason that we can trouble-shotting in specific case

Transistor is the most important component in any circuit.A bad transistor could cause the device that it is swithcing or amplifying for not working or working as it should be,sometimes it can destory the device which causes big money mistakes.










References:


auto101.com
wikipedia
Unitec Moodle
Vijay Naidu