Magnetic Pick-up sensors

Magnetic pickup sensors are used to measure the speed of rotating objects such as shafts, flywheels, etc.  These sensors generate a sinusoidal waveform with a variable frequency.  The frequency of the signal depends on the speed of the rotating object. 

The magnetic pickup sensor works by the changing magnetic field when a metallic gear passes through it. The sensor consists of a permanent magnet and a coil mounted on the same axis.  Under static condition, the flux from the magnet flows from the north pole to the south pole.  The flux passes through the coil.  However, no voltage is produced as there is no relative motion between the magnet and the coil as they are fixed.  When the tooth of a gear to be measured passes through the sensor.

The magnetic flux lines are distorted as the tooth comes in front of the sensor. And when the tooth passes, the flux lines return to their original position.  They change again when the next tooth comes in front of the sensor.  This change in the flux induces a voltage in the coil placed in the sensor.  The frequency of this emf is dependent on the speed of the gear teeth. 

Thus by measuring the frequency, the speed of the rotating object is measured.   Magnetic pickups can  

Why can't we use aluminium or copper vessels on an induction stove?

Induction stoves work on the principle of induction.  The stove contains a coil which is excited by a high frequency AC supply.  Above this coil is a plate.  The vessel to be heated is placed on this plate.  The alternating magnetic field set up by the coil induces currents in the vessel surface.  This current generates heat as it circulates in the vessel. 

In theory, this principle should work for all metals.  However, the heating is not efficient in non-magnetic materials such as aluminium, copper, etc.  This is because non-magnetic materials such as aluminium and copper have a lesser skin effect.  It means that the current can circulate for a greater depth in the vessel walls.  This reduces the surface resistance which is important for heat to be generated.  Hence, induction heating is not efficient in non-magnetic materials such as copper and aluminium.

All metal induction stoves are in the process of development which can heat vessels of all  metals.  Non-magnetic metals can also be heated, though, at a lesser efficiency. 

Anti-reflective coating in solar cells

The light which falls on a solar cell first makes contact with the shiny surface of the cell.  This results in the light getting reflected.  Thus, this the energy of the light rays are also lost along with the reflected rays.  Hence, it is essential that all photovoltaic cell have an effective anti-reflective coating which prevents the light from getting reflected. 

A reflective coating ensures that the energy from all the rays incident on the photo voltaic cell is made available for conversion into electricity thereby increasing the efficiency of the cell.

Anti reflective coatings are similar to the coatings used on camera lenses.  They interfere with the process of reflection by creating another light wave which is out of phase with the reflected wave.  This causes the reflected waves to cancel each other out and no energy is lost through reflection. This phenomenon is called destructive interference.

The thickness of the anti-reflective coating is critical.  It should be perfectly selected so that the light from the surface of the anti-reflective coating is out of phase with the light reflected from the surface of the photocell. 

Another method of preventing light from getting reflected off the surface is by texturing the surface of the photocell.  This causes the light to get reflected multiple times by the textured surface and a greater amount of the energy in light is captured by the photo cell.  This is called "light trapping"

Adding water to Battery Electrolyte

The Electrolyte in a lead acid battery is sulphuric acid mixed in water.  During the discharging process, the sulphuric acid dissociates into water and sulphate which is deposited in the electrodes. 

When the battery is charged, the sulphate recombines with the water to form sulphuric acid.  The specific gravity of the electrolyte thus varies from 1.1 to 1.3 depending on the level of charge. Over a period of time, there is a chance of the water in the electrolyte evaporating. 

This may lead to a change in the level of the electrolyte along with a variation in the specific gravity.   This may result in a deterioration in the battery performance.   In such situations, it may be necessary to add water to the electrolyte.

 Procedure 
Only distilled and demineralized water should be used, as ordinary water contains minerals such as calcium, magnesium, etc which can cause deposits in the electrodes and affect the battery's life and performance. The vent caps of the batteries need to be removed.  Once, the vent caps are removed, the plates will be visible.  See if the plates are completely covered by the electrolyte.  Add water if the level of the electrolyte is low and the plates are exposed.  Take care that excess water is not added this will result in an "overflow".  The acid can overflow and cause damage.

Safety Precautions
Follow the manufacturer's instructions before topping the electrolyte with water.  Always wear protection goggles for your eyes.  The acidic electrolyte is corrosive.  Take care that you do not accidentally come in contact with it.  The sulphurous gases which may escape the electrolyte are explosive.  Do not smoke near the battery as a spark can trigger an explosion.

Product Review - Fluke CNX 3000 Wireless Test Tools

Fluke CNX 3000 Wireless Test Tools is a set of measuring instruments which can be used for making remote measurements of electrical parameters.  The set consists of a central multimeter which can communicate with measuring probes wirelessly.  Fluke says that this enables simultaneous measurements of currents and voltages.  The multimeter can collect signals within a radius of 20 metres. 

The data can be read on a computer screen using a wireless PC adapter.  The data is also recordable.  This measuring tool set would be ideal for process industries which require simultaneous measurement of current at different points in the system. 

Video from Fluke

Solar Power - PhotoVoltaic Cells

Photo-Voltaic energy is an ideal choice of power for locations which are far from the electric grid. 

In many countries, it is not feasible to lay power lines to certain far-flung areas as there may be a small no of consumers.  Investing huge amounts of money in laying powerlines and maintaining them may not be economically viable.  Providing power by means of diesel generators may be expensive.

A practical and environmentally friendly source of power are Solar Cells.  Solar cells convert the sunlight into electricity.

Solar Cells work on the principle of the photo-voltaic effect.  When light falls on certain materials, the atoms absorb the photons and release an electron.  These electrons create an electric potential.  When an external circuit is connected, a current flows.  The direct conversion of sunlight to electricity makes the modules compact without any emissions or residue.  Photovoltaic cells are thus popular for powering small electronic devices  and lighting.

Photovoltaic modules have been used for powering aircraft, cars and even the international space station.

Photovoltaic modules are made of materials such as silicon, cadmium telluride, gallium arsenide, etc.  The basic silicon cell consists of a NP junction. When light is incident on the PN junction, electron-hole pairs are produced.  This creates a voltage across the junction.  When an external load is connected, current flows.

Photovoltaic cells are usually covered by an anti-reflective coating to prevent the incident light from being reflected away.

Current Photovoltaic cells can achieve effiency of around 30%.  Cells with concentrated sunlight focused on them can achieve still higher efficiencies.

It would be interesting to note that a solar cells is just an LED in reverse.  An LED (A light emitting diode) functions by emiting light when a voltage is applied, while the solar cell generates a voltage when light is incident.  A solar cell is specially designed to have a wide PN junction which can collect more light.   

The solar cell technology needs further development before it can be widely used for base power generation.  It is hoped that further research with new materials will improve efficiency and lower the high initial capital costs.