Why are Current Signals preferred over voltage Signals in instrumentation ?

Current signals are preferred over voltage signals as they are less prone to distortion due to interference and coupling.  Current Signals are also less prone to be affected to loop resistance.  

Any break in the circuit will be immediately noticed, if the live-zero provision is available.  

What is Power Conditioning ?

Power Conditioning refers to the process of improving the quality of AC power supply.  AC power supply is often affected by problems such as low or high voltages, harmonics, brownouts and spikes and transients. Such disturbances can damage loads connected to the supply.  

Power Conditioning Equipment eliminate these problems by filtering harmonics, maintaining voltage within limit (such as +/-1%). They also provide protection against noise.  

They also provide galvanic protection.  Power Conditioning Equipment work on a combination of technologies such as Harmonic Filters, Ferroresonant Transformers and Power Electronics to modify and smoothen the AC waveform.

Power Conditioning leads to improved life of components, protection from surges and spikes and and better performance and efficiency.

Doping in Semi-Conductors

Doping in Semiconductors refers to the addition of donor atoms into the crystal structure of intrinsic semiconductors material to modify their conducting property.  In doping, an impurity or foreign atom is introduced into the lattice structure of a semiconductor material.  This makes free electrons or holes available in the lattice structure of the material.  

A semiconductor with an excess of free electrons is called an n-type semi-conductor while a semiconductor with an excess of holes is called a p-type semi-conductor.   N type semiconductors are made when the semi-conductor material is doped with a pentavalent impurity.  A pentavalent impurity is an impurity whose atom has 5 electrons in its outer most shell. Examples of pentavalent impurity are Antimony, Arsenic and Phosphorus.

A p-type semiconductor is obtained when the semiconductor material is doped with a trivalent impurity i.e. an element whose atom has three electrons in its outermost shell.  Examples of trivalent impurity are Boron, Aluminium and Gallium.  

PNP and NPN transistors

A PNP transistor consists of two P-type materials with a N-type material sandwiched in between.  A NPN transistor consists of two N-type materials with a P-type material sandwiched in between.  

For a PNP transistor to conduct, the base which is made of N-type material has to be connected to the negative voltage.  The connection should look like this.  The transistor switches off when connected to the positive voltage.

In the NPN transistor, the base which is made of P-type material should be connected to the positive voltage.  The transistor will switch off when connected to the negative voltage.

PNP and NPN transistors can be identified by checking the polarity of the PN junction.  An PNP transistor has on PN junction and one NP junction.  Thus a simple check to identify the polarity of a diode can be used here.  Connect a multimeter positive to the base of the transistor and the multimeter negative to the emitter of the transistor.

If it shows low resistance the junction is in forward bias.  The transistor is a NPN transistor.  If it shows high resistance, the junction is in reverse bias and the transistor is PNP transistor.

Laser Diode - An Overview

A Laser Diode is an electronic device which produces coherent light.  All the waves of the radiation are of the same phase and frequency.  The radiation may be in the visible or the infrared regions of the electromagnetic spectrum.

Laser Diodes find applications in burglar alarms, optical storage system such as compact discs, optical fibre communication, etc.  

The Laser generated by diodes is of lower power.  It is portable and handy.  It produces a conical beam which can be focused using convex lenses.

A laser diode is a P-i-N Diode in which the P type material and the N type material enclose an intrinsic semiconductor.  In the P type material, the majority carriers are holes while in the N type material, the majority charge carriers are the electrons.  The diode is designed such that the holes and the electrons recombine in the I region i.e. the region with the intrinsic semiconductor.

When an electron and a hole recombine, they get annihilated.  This results in the emission of a photon. 

The light thus emitted needs to be of the frequency and phase to become a laser.  This is done by means of a waveguide on the crystal surface. The photons emitted will travel back and forth in the waveguide and be reflected several times. When the   The light is amplified and the laser emerges.

Gallium Arsenide is a material which is widely used in the construction of Laser Diodes.

Leak Off Current in Transistors

Leak Off Current in Transistors refers to the curent  which passes through the transistor, even when it is switched off and the PN regions are not in forward bias.

Leak-off current in modern transistors is very small.  The leak off current varies inversely with the thickness of the semiconductor layer.  

Leak-off Current is caused by design imperfections in the transistors.  the Leak-off Current is of the order of micro amperes or nano amperes.  The leak-off current is considered to be significant only when the transistor is designed to be of very low power rating.

Causes of Transistor Failure

Transistor failure can occur due to a variety of reason.  The following are some of them.

Age

Aging of transistor due to temperature variations inside the components due to carrying current can cause failure.  The electrical properties of the materials inside can drift due to age.

External Causes

External causes such as spikes in the power supply, heat, mechanical damage can also result in transistor failures.  Hence, all transistors should be adequately protected against overvoltage.

Poor Circuit design.

Improperly chosen components and wrong circuit design can also result in transistor failure. Hence, all components in the circuit should be properly rated with sufficient allowance for overloading and temperature rise.  The failure of one component can lead to cascade failures of other components.

Overheating

Overheating is the most common cause of transistor failure.  Hence, it is important that transistors be provided with cooling mechanism.  Devices which contain temperature-sensitive electronic components should be kept in air conditioned environments.

Electrostatic Discharge

Electrostatic discharge can also damage transistors.  Hence proper precautions against Electrostatic Discharge Should be taken.