2.0 BASICS OF ELECTRICITY
Electrical supply and characteristics
Current generation:
Alternating current (AC)
A simple alternator consists of a stator with magnets of opposing polarity and a rotor or armature wound with many turns of copper conductors. When the armature rotates cutting through the magnetic field a voltage is induced. The higher the speed of rotation, the greater the induced voltage
Direct current (DC)
Another method of current generation is by electrolysis. If two plates (electrodes) of different materials, such as Zinc and copper are immersed in a saline solution (i.e. salt water) galvanic action takes place, the plates are charged and a voltage is produced.
In electro pneumatic controls, batteries are generally used for back up purposes only. That is in the event of primary supply failure, power remains available to operate key safety functions, alarms, lamps etc
A system using AC power may require the use of a power supply consisting of a transformer only. Where a dc power source is required, the AC power must be rectified, filtered and regulated to provide a DC output with little or no AC component (AC ripple) remaining
DC POWER SUPPLY
The power supply should be able to comfortably manage the total system current requirement without overloading. Consideration should also be given to the impact of future expansion of the system and the resultant power supply requirement such as current rating and output voltages
Where the power supply is mounted in an enclosed space ,adequate ventilation is primary requirement. The power supply should be placed so that heat is not transferred to other heat sensitive elements in the area such as printed circuit boards and control elements. Some systems employ fan forced cooling for the power supply
2.2 Elementary electrical circuit
An electric circuit consists always of a closed loop, with three essential elements
- Power supply
- Load
- Switching element
The power supply can be a battery, an accumulator or a generator which is the origin of the power supply to a wall plug. Without this component there is no electromotive force to move electrons.
The load can be a lamp, coil or heating element etc.Without a load the connection between the two poles of the power supply with a wire would be a short circuit. Under this condition the current possibly can be so high that the connecting wire would heat up and melt. Fuses in the circuit prevent this; a very thin wire melts to disconnect the supply, avoiding the danger of fire or destruction of expensive equipment.
A switching element or contact is required to interrupt the working of the load .It can be any where in the circuit. It opens or closes the circuit. A contact is called open when it makes no connection between its two terminals and closed when this connection is made
Note: That the opposite concept of what is used for pneumatic valves: a valve is called open when it makes a connection from the input to the output and closed when the supply is shutoff
Ohm’s law
There are four main terms used in electrical calculations; Voltage, current, Resistance and power. Ohms law however describes the relationship between voltage, current and resistance. We can compare
voltage with pressure
Both are potentials and can be described as the electrical force or pressure. It is thus applied force that is used to move the electrons and can be called the electro-motive force or simply emf.It is measured in volts
Electric current with an air flow
Both are created by a difference in potential. It is a term used to describe the flow of electrons in a circuit. It is measured in amperes
Resistance with flow capacity
This is the term given to that property of a circuit that opposes the flow of current in a circuit and is usually the combined effect of cables and individual load in the circuit and it is measured in ohms
Certain materials like copper, aluminum etc allow the current easily through them .Such materials are called conductors. Where as materials like ceramics, glass do not allow the easy flow of current .Their resistance is very large. These materials are called insulators. The resistance of a material depends on the kind of material, its length, and it’s cross sectional area. A thick and short wire has less resistance than a thin and long one.
Ohms law defines:
Voltage equals the product of current and resistance
V=IR
Ohm is the unit of resistance and is denoted as Ω
An easy way to remember the relationship of Ohms law is as shown in the three triangles below
Series and parallel connection:
In series connected elements the current through them is the same but the voltage across element depends on the resistance. The total resistance of a number of resistances connected in series is the sum of their individual resistances
SERIES CONNECTION OF RESISTORS
In the above figure the total resistance is 3.5 Ohm. The current through each resistance is the same where as the voltage drop across each resistance varies depending on the value of the resistance
Parallel Connection
In parallel connection of elements voltage across each element is the same where as the current through each element varies depending on the value of the resistance. In parallel connection the total resistance is less than that of the lowest resistance in the connection.
1/R(Total) = 1/R1+1/R2 + 1/R3 + ……
PARALLEL CONNECTION OF RESISTORS
In the above circuit voltage across each resistance is the same and the current through each resistance depends on the value of the resistance. We can find this current using ohm’s law as follows
Current through 10 Ohms = 10/10 = 1 Amp
Current through 20 Ohm = 10/20 = 0.5 Amp
Current through 50 Ohm = 10/50 = 0.2 Amp
The total resistance = _______1_______ = 5.88 Ohms
1/10+1/20+1/50
The total current is = V/R(Total) = 10/5.88 = 1.7 Amp
2.5 Kirchoff’s Law
Current law: This says that sum of the currents entering a junction must be equal to the sum of the currents leaving the junction
In the parallel circuit shown in figure total current entering the junction is 1.7 Amperes.
The sum of the currents leaving the junction = 1 + 0.5 + 0.2 = 1.7 Amperes
Voltage law: This says in a closed loop circuit voltage rise is equal to voltage drop
In the series circuit of the figure voltage across the series combination is
12 - 5 = 7 V
Sum of the voltage drops across resistances = (12-10) + (10-6) + (6-5)
= 7Volt
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