What are Diodes?

A Diode is a semiconductor which can be used to change Electric Supply from Alternating Current to Direct Current. They allow current to pass through them in one direction.

Schematic Symbol

Rectifier diodes in a circuit

A Diode has two parts; the P-type semiconductor and the N-type semiconductor. The P-type semiconductor has holes (Positive charges) as the majority charge carriers and N-type has electrons (Negative charges) as the majority charge carriers.

Silicon or Germanium are the materials used to manufacture diodes. During manufacture, some impurity atoms are added to silicon. This is called doping. To produce the P-type semiconductor silicon is doped with Boron, for example, which introduces holes as majority charge carriers. Phosphorus can be added to silicon to produce N-type semiconductor. Phosphorus atoms offer their electrons to silicon in a covalent bond. There are also minority charge carriers in both semiconductors; the P-type has electrons as minority charge carriers and the N-type has holes as minority charge carriers.

P-type doping

N-type doping

Diode Bias

Diode bias is the condition of a diode.

No Bias

The No Bias condition, this is when there's no voltage applied. The layers of ions in the depletion region of the diode repel majority charge carriers, holes and electrons, of the P-type and N-type semiconductors and prevent them from crossing the junction.

Forward Bias (VD )
In the Forward Bias condition, also known as 'on' condition of the diode, a positive potential is applied to the P-type semiconductor and a negative potential is applied to N-type semiconductor. The ions in the depletion region get neutralized and eventually they will allow heavy flow of electrons. This is due to the pressurizing of electrons in the N-type semiconductor.


The forward bias voltage of a germanium diode is 0.3 volts and that of a silicon diode is 0.6 volts. It is important to note the maximum forward current which can be allowed to pass through the diode. If too large current is allowed to pass through a diode, it can  easily get damaged.

Reverse Bias
In the reverse bias, the depletion region enlarges. When a positive potential is applied to the N-type semiconductor and the negative potential to the P-type semiconductor, the uncovered positive and negative ions in the depletion region increases. This is because the negative terminal of the voltage supply repels electrons in the P-type semiconductor and the positive terminal attracts electrons in the N-type semiconductor. Therefore conduction is not possible.

How do Electromagnets work?

An Electromagnet is that magnet which attracts metals only when it is connected to an electric supply. It is made up of a solenoid (a coil of insulated copper wire wound on soft iron).  When an electric current flows through a wire, it produces magnetic flux around the wire.

Magnetic flux produced by current. Magnetic flux is clockwise 

when current is moving away through the wire (from right to left).

Since the coil has many turns, that is 500 or more, the magnetic fluxes in each turn join each other to form larger lengths of  magnetic fluxes.These magnetize the iron which also produces fluxes due to its ferromagnetic property. As a result, the soft iron and the solenoid wire together produce very strong magnetic fluxes. But it should be noted that the soft iron will quickly lose its magnetism if the electric current supply is switched off.

According to Ohm's law R = V/I. Therefore in order to harvest magnetic flux, resistance (R) must be greater than Current (I) and whenever resistance increases voltage also increases. This means if a solenoid of 500 turns is connected in series with a battery of 12 Volts, one will need to connect a solenoid of about 700 turns if the battery voltage is increased to 15 Volts. If the number of turns are not increased at 15 volts, then the solenoid will just heat up and it will not produce magnetic flux.

You might also need to read about Relationship between the Resistance and Dimensions of a Conductor

Who invented Electromagnets?
When William Sturgeon, a British scientist, was trying to magnetize soft iron permanently he found out that the iron could only be strongly magnetized when current was flowing through the solenoid wire. Therefore he discovered/invented an Electromagnet.

History of Magnets

In 600 B.C, the lodestone also known as Magnetite was already known to the Greek. It is an iron ore which has the property of attracting metals especially small pieces of iron. Chemically, a lodestone is made up of iron oxide with the formula Fe3O4. The place where magnetic iron ore was first discovered is called Magnesia.

The word lodestone is got from an old English word way, this refers to the property, of the stone, of being able to show the direction of the earth's North pole and South pole. During middle ages, navigational compasses were made by joining a piece of  lodestone to a wooden splint then this was made to float on water in a small container. These two could point in N-S direction.

Uses of Electromagnets

Electromagnets are due to the magnetic effect of current in a wire or a conductor.   There are quite a number of applications of magnetic effect of current in a conductor and among these are:

• Electric Bells
• Lifting magnets
• Solenoid switch for the car starter motor
• Magnetic circuits of generators and motors

Extension Cords are only intended for temporary use

Extension cords are used to bring power to electrical devices that need to be used in areas that are a bit far from the wall socket. But this should not be permanent. There are various kinds; Power strips, Surge protector and multi tap.

Power strip (extension cord)

Most of the time extension cords are improperly used:

Since an extension cord is a length of cable with a plug on one end and three or more sockets on the other end, considering the fact that the longer the conductor the bigger the resistance heat energy can develope in the cord (cable) whenever current goes against that resistance. This gradually makes its insulation weak and therefore it should not be used for more than 90 days otherwise it will turn a threat to its user.

Due to many electrical needs and few power outlets people overload extension cords, since they have more than one socket. This can lead to fire outbreak as more current is drawn through the extension cord. The fuse may not blow immediately but the cord can get hot enough to ignite its insulation, the nearby cloths or carpet and fire starts. In most cases even the devices connected to it will get damaged if they are not protected.


Safety Practices

• Extension cords are not meant for permanent use. Look for a licensed electrician to install more wall socket outlets.
• Avoid overloading the extension cord. Whenever adding another electrical device on the cord, first check the current rating or power rating and calculate the total current drawn through the extension cord. Most extension cords have fuses with 13A current rating so if you have one with 13A, make sure you don't exceed this current.
• Worn out and damaged cords should no longer be used and be destroyed to prevent reuse.
• When buying an Extension Cord, verify that it is tested and labelled by a recognised testing laboratory.
• Extension cords should be visually inspected for damage before use on any work.

Relationship between the Resistance and Dimensions of a Conductor

With a uniform wire/conductor of a given material and a given length, the resistance obtained by dividing Potential difference between any 2 points by Current is directly proportional to the distance between them. That is to say if the distance between the 2 points (on the conductor) increases the resistance also increases.

If 2 resistors each having resistance R ohms are connected in parallel the equivalent resistance Re is given by Product / Sum (Product over Sum)

Re = R*R/R+R
Re = R*R/2R
Re = R/2
Re = (1/2)*R
Each resistor will offer half of its resistance.

See the working in the slide. Click the previous button then after click on the slide screen.

Therefore, if two wires of the same material, having the same length and diameter are connected in parallel their resistance is half that of one wire. The effect of connecting two wires in parallel is similar to doubling the area of conductor. Connecting 7 wires in parallel is the same as increasing the cross section area of a wire 7 times and this reduces the resistance to a 1/7 (seventh) of one wire. Generally, the resistance of a given length of a conductor is inversely proportional to the cross section area. 


Other factors that influence resistance are; nature of the material (different materials have different resistances) and temperature (the resistance of some materials increase with an increase in temperature). So everything we have discussed above is true if temperature is constant.

Resistance of a wire = (Length of wire (L)/Cross Sectional Area (A))*a constant for a given material (ρ)

See the equation in the slide.

The constant of a material is called Resistivity of a material and it is represented by a Greek symbol rho (ρ). Resistivity is measured in ohm meter (Ωm)

Definition for Resistivity

This is the resistance of the specimen having one meter long and one meter square of cross sectional area.

Calculating Current in Two Parallel Resistances

Like I have already stated in one of the previous posts, voltage is the same in parallel resistor connection.
Voltage for resistor 1 = Voltage for resistor 2

That is to say (Voltage supplied), V = (I1R1 = I2R2).
Not forgetting that V = IR
Also Is = I1 + I2. (Is is the current supplied)
I-I1 = I1-I1+I2
I2 = I-I1
Substitute I2 in the equation V = (I1R1 = I2R2)
I1R1 = R2(I-I1)
I1R1 = IR2 - I1R2
I1R1 + I1R2 = IR2
I1(R1+R2) = I*R2
I1 = I*R2 / R1+R2
Similarly
I2 = I*R1 / R1+R2

Wiring a Three Pin Plug

Wiring a three pin Plug

Three Pin Plugs (or 3-pin plugs) are designed to protect the electrical appliance and its user from electrical hazards. They vary in size; there are those whose pins are moderately sized, also known as UK plugs, and those with large pins known as non-UK plugs. Someday or today you might need to change the 3-pin plug of your electrical appliance for some varying reasons. Though some electrical appliances are manufactured with the 3-pin plug molded on the AC in put cable, you might still need to change it. Perhaps you bought your electrical appliance when the plug pins are larger than the socket outlets. You will need to cut off that plug and replace it with the suitable one. In this post we will look at how to wire a three pin plug.

To wire a 3-pin plug, one has to be familiar with the colour codes of the wires:

• the live wire is brown
• the neutral wire is blue
• the earth wire is green and yellow striped

If you can get a new Universal 3-pin plug like the one in the picture below, unscrew the top cover of the plug, the 3 terminals and the cable grip.

Universal 3-pin Plug

Cut away about 1 cm of the insulation from each wire using a cable insulation stripper or a razor blade if the former is not in your possession. Make sure not to sever the wire strands.

Put the flex under the cord grip and screw it tightly in order to hold it in place.
Note: Do not leave the flex outside the plug to prevent the wire strands from breaking off their respective terminals as a result of strain.

Do not leave flex outside plug

Twist the wire strands so that there are no strands straying.

Connect the wires to their respective terminals. 3-pin plugs have their pins marked E for earth, N for neutral and L for live. Screw tightly all the terminals.

The connections should be like this.

Put back the cover and screw it tightly.
Disclaimer

Sometimes Fluorescents can be repaired.

Compact Fluorescent glass tube

Fluorescent lights are often known to flicker, shut down (light) after a few seconds when they are switched on, give dim light or even no light at all. Sometimes these problems can be fixed, that is if the glass tube is not damaged and its connectors show a very good connectivity/continuity on the multimeter. So our hope to see our fluorescent working properly again is in examining the ballast circuit. The following explanation shows the components that are likely to be faulted when the lamp is not working properly.

Electronic ballast of a Compact Fluorescent Light

When you open the lamp, you will see the ballast circuit inside and you will most probably notice a blown fuse.
Note: fluorescent lights may contain Mercury which is very poisonous. Make sure you discard broken tubes safely. Work in a well ventilated area, use gloves and respirators if possible. You may also read about how to clean up a broken fluorescent.
In the ballast circuit: 

• the fuse often get blown when the rectifiers or switching transistors get shorted. So they all have to be checked on the digital multimeter to make sure they are working properly.  If not working, first replace them (the rectifiers and transistors) and then replace the blown fuse.

• Sometimes the fuse can blow due to a shorted inductor, replace the inductor if possible. 
• Dim light, light goes off after a few seconds or no light at all, this is caused by shorted capacitors (ceramic and electrolytic caps). So all these must be checked in order to repair the lamp.

Curiosity about Solar Energy

Solar Energy is energy which the sun radiates and reaches the earth. Solar Energy is a renewable energy.

There are two energy sources; Renewable energy and Non-renewable energy. Renewable energy are energy sources that occur naturally and continuously in the environment. Apart from Solar Energy, Wind, Hydro, Biomass are also examples of renewable energy sources. Non-renewable energy are energy source that cannot be made again in a short period of time. Examples include; fossil fuels, nuclear energy.

Solar Energy has two types of use:

1. Solar Heating; the heating effect of the sun is used to dry crops, bricks etc.
2. Solar electricity (use of electricity from solar energy). Solar energy is converted to electricity by either using Photovoltaic devices or Solar power plants.

Solar Power Plants. These indirectly generate electricity. They employ solar thermal collectors to collect heat from sunlight and this heat is used to heat the fluid which produces steam that is used to power a generator (turning turbine). This heat can also be put to other uses like in steam baths.

Photovoltaic devices. These change sunlight directly into electricity. Photovoltaic devices/cells make up a Solar Panel.

How do Solar Panels generate electricity?
A Silicon Solar Panel/module (also known as a Photovoltaic module) generates electricity when photons from sun excite electrons in it.
Let's first go through the construction  of a solar cell. Solar panels are made up of solar cells. Like semiconductor electronics such as diodes and transistors, silicon solar cells are doped that is to say some impurity atoms are added to the silicon crystals to form negative (n-type) crystal and positive (p-type) crystal. The n-type doping can be Phosphorus atoms added to silicon. Silicon atoms offer each other one electron in a covalent bond so this makes silicon unable to conduct electricity. But if a phosphorus atom is added, it will behave like silicon with a extra electron.

When a photon from sun strikes this electron, it gets excited (it becomes delocalized).

P-type doping can be Boron atoms added to silicon. Boron atoms have three valence electrons, each, to offer to silicon in a covalent bond and this will create a need for one more electron. Therefore a hole is created.

So in a silicon solar cell the two doped materials, made into plates, are joined together with a junction in between. In full sunlight; when you create an external circuit by attaching wires to the p-type and n-type doped materials, Boron in the P-type doping will create a positive potential to attract the excited electron in the n-type doping.
In a silicon solar panel, cells are connected to each other in series. A typical solar panel has:

• 33 cells in series
• the maximum useful voltage generated by a solar cell is about 0.58 volts dc.
• the total output voltage of the panel is 19.14 volts dc and maximum of about 16 volts is required from a solar module/panel in order for a 12 volts lead acid battery to be fully charged.

What do I need to do before installing a solar panel?

• The number of cells in a solar panel. If there are few cells in the panel the out put will be low.
• The type of silicon used. A monocrystalline produces the highest current compared to polycrystalline silicon.
• The surface area. Cells with a large surface area have high short circuit current than small cells.
• The angle of the module/panel with respect to the sun's radiation. It should be at 90 degrees.
• There should be enough air circulation around the panel.

Curious about Electric Motors?

Electric Motors work by following a rule called Fleming's Left Hand Rule. But before that rule, when an electric current flows through a conductor, it produces magnetic flux around the conductor.

Picture 1.

Right-hand grip rule, a rule concerned with current through a conductor. It says, Imagine  the wire to be held firmly in the right hand with the thumb pointing along the wire in the direction of the current. The direction of the fingers will give the direction of the magnetic flux. Like in the illustration above.

Note these symbols

Picture 2.

they mean...

Picture 3.

When a conductor currying current is placed in a magnetic field created by some other sources than its self, it experiences a force. The fields of the conductor carrying current tend to repel those of the permanent magnet and this produce a turning couple. See this in the picture below.

Picture 4. Magnetic field pattern of permanent magnet and coil of wire (conductor).

Picture 5. Simple direct current motor. Blocks labelled N and S are permanent magnets with a conductor carrying current (coil) placed between them.

Picture 6. A sketch to show Fleming's Left-hand rule

Fleming's Left-hand Rule states that , 'if the thumb (T) and the first two fingers of usually left hand are mutually at right angles with the first finger pointing in the direction of the field (B) and the second finger pointing in the direction of the current (I)  then the thumb (T) predicts the direction of the force or motion.

Picture 7. The coil of wire as shown in picture 5 above. Observe the direction of the flux when current is flowing through it.

By applying  Fleming's left-hand rule, the side where flux is anticlockwise will experience an upward force and the side where flux is clockwise will experience a downward force. These two forces cause the coil to rotate with its momentum supporting it.

The magnitude of the force acting on the conductor depends on:

1. The strength of magnetic field (B)
2. The length of the conductor cutting the magnetic flux (L),
3. The magnitude of current(I) flowing in the conductor
4. The angle between the magnetic field and the current

Picture 8. The length of the conductor as mentioned in the list above, that means the number of turns of the coil should be suitable enough for the motor to be powerful.

 

Picture 9. Electronic DC motors

Picture 10. Electromagnet

For better results, a number of coils should be wound on a soft iron armature made up of soft iron discs with slots. The coils are wound in slots. The armature, when magnetized, adds its magnetic flux to that of the coils. Also the commutator is multi-segmented. Electronic DC motors are often constructed this way.

Picture 11.

Picture 12. Permanent magnets inside

In larger motors, the magnetic field in which the armature rotates is produced by an electromagnet. Now let's get the difference, the coils of the electromagnet are called field coils and the coils of the armature are called armature coils. An example of an electromagnet is shown in picture 10.

OHM'S LAW

Ohm's law states that for a given current, the Potential difference (P.d) between two points is directly proportional to the resistance of the circuit between those points.

V = IR

Calculating Resistances in a Circuit

If current is to flow through a circuit, it must go against resistance with majority of it taking paths of least resistance and if resistance is great (in a certain path), current will not flow.
Circuits are designed with resistances both in parallel and in series.

Series Connection of Resistances
The figure below shows three resistors having resistances of R1, R2, R3, connected in series across the supply voltage labelled Vs.

Note. When resistors are connected in series the total current flowing through the circuit is the same (current supplied (Is)) while voltage drop depends on the value of the resistance of the individual resistor and total voltage of the circuit is the sum of the voltage drops of the individual resistors.
Vs = V1+V2+V3
From ohm's law
V=IR
Substitute V for IR in the equation Vs = V1+V2+V3
IsRT=IsR1+IsR2+IsR3 (Is being current supplied and RT being total resistance)
Divide both sides by Is
IsRT/Is=Is(R1+R2+R3)/Is (Is dies on both sides)
RT=R1+R2+R3

Parallel Connection of Resistances
For resistors in parallel connection, the supply voltage is the same while current flowing depends on the value of the resistance of the individual resistor. The total supply current is the sum of all the currents flowing  through the circuit.

In the figure above three resistors having resistances of R1, R2 and R3 respectively, are connected across supply voltage Vs.
From total current
Is = I1+I2+I3
And from Ohm's law
I = V/R
Substituting I for V/R in the equation Is = I1+I2+I3
Vs/RT = Vs/R1+Vs/R2+Vs/R3 (Vs being supply voltage and RT being total resistance)
Vs/RT *Vs = Vs(1/R1+1/R2+1/R3)/Vs
1/RT = 1/R1+1/R2+1/R3
If there are two resistors connected in parallel, the total resistance RT is given by product/sum (Product over Sum)
1/RT= 1/R1+1/R2
1/RT = R1+R2/R1*R2
RT = R1*R2 /R1+R2

Current, Potential difference and Resistance

In an electric circuit, there are three things to measure; Current, Potential difference and Resistance.

Current (I)
An electric current in a wire is a drift of electrons, but according to the convention adopted in 1800, current is regarded as the flow of positive electricity.
Current can also be defined as charge flowing in a circuit per unit time. Current (I) = Charge (Q)/time (t).
Current is measured in Amperes.
An Ampere is the Current which, if flowing in two straight parallel and infinitely long wires, placed one meter apart in a vacuum with a negligible cross sectional area, will produce on each of the wires a Force of 0.0000002 N per meter length of the wire.

Potential difference

Potential difference between 2 points x and y, in a circuit, is the work done when moving a unit of charge from x to y.

The SI unit is the Volt. A volt is the potential difference between 2 points of a circuit carrying a constant current of one ampere when the power dissipated between these points is one Watt.

The circuit consists of resistors R1 and R2 each having a potential difference, in volts, V1 and V2 depending on the value of their resistances. The arrow shows the path of convention current.

Resistance

This is the opposition to the flow of Current through a conductor. The SI unit of Resistance is the Ohm. An Ohm is the resistance in which current of one ampere flowing for one second generates one joule of thermal energy.