What is a Resistor?
A resistor is a device in which electricity cannot pass through it easily. When certain amount electricity is allowed to pass through a resistor, the electrical energy is changed into another form, usually light or heat. The working principle of a bulb is that electricity is passed through the filament usually tungsten, which is a resistor. The energy is converted to and released as light and heat.
The resistor is an electrical component which creates a resistance in the flow of electric current. It is a passive component as it consumes energy from a source (active component).
Although resistors are generally used to reduce the flow of current or lower the levels of voltage in a circuit, they are used in many electronic circuits for many purposes. Some of them are the basic current flow limitation ability, to provide a biasing condition to some of the active elements like transistors or to act as a terminating device in transmission lines.
Practically resistors are discrete components of various forms but are also implemented on integrated circuits.
ResistorSymbols
Generally there are two standards that are used to denote the symbol of a resistor viz.Institute of Electrical and Electronics Engineers (IEEE) and International Electro Technical Commissions (IEC).
The IEEE symbol of resistor is a zigzag line .
Resistance
The definition of resistance can be derived from the Ohm’s law in its Electromagnetic theory form or Continuum form
J = σ E —-1
Here σ is the conductivity of the material i.e. conductor.
E is the electric field developed along the length of conductor due to flow of electrical energy through the conductor.
If ‘V’ is the voltage drop across the conductor and ‘L’ is the physical length of conductor then
E = V/L —-2
The current density J is resulted within the conductor due to the flow of electrical energy through the conductor.
If ‘I’ is the current flowing through the conductor and ‘A’ is the cross sectional area of conductor, then by the definition of current density
J = I/A —-3
Now combining equations 1, 2 and 3
I/A = σ V/L
V = (L/Aσ) I —-4
The term in parenthesis is constant and let us denotes it by ‘R’.
∴V = R I
Resistors are main components in electric and electronic circuits as they determine the amount of current that flows in a circuit and also the potential at different points in a circuit. Therefore it is important to make sure that the value of resistance is known for a given resistor which is placed in a circuit.
From Ohm’s law, it is easy to calculate the resistance. Ohm’s law relates the Voltage V, Current in the circuit I and the Resistance of the resistor R.
R = V/I
∴In terms of units, 1 Ohm (Ω) = 1 Volt (V) / 1 Ampere (A).
In other terms, a resistor is said to be having a resistance of 1Ω when 1A of current is passed through it for a supply voltage of 1V.
Ohmmeter is a device designed specifically for this purpose. A resistor is connected across the terminals of ohmmeter and the reading of the ohmmeter is the value of the resistance of that resistor along with the resistance offered by the wires used to connect the resistor to ohmmeter.
Even though the resistance of wire is very small, it can’t be neglected. Hence the values measured using an ohmmeter is not accurate.
The next best way to determine the resistance is to make use of both voltmeter and an ammeter in the circuit.
Resistivity
Electrical Conductivity of a material is the measure of its ability to conduct current.
Resistivity is the reciprocal of conductivity. Resistivity is the measure of a conductors’ ability to resist the flow of electric current.
Resistance Units
Resistance R = V/I
This results in the units of resistance as volts per ampere. This combination is given a special name called Ohm named after the physicist Georg Simon Ohm.
Carbon Resistors
Carbon Composition Resistors are commonly used resistors which are manufactured at low cost. This is because of the simpler construction process. They are generally called carbon resistors. The main composition is carbon clay which is covered in a plastic case and the leads are made of tinned copper. The main advantage of carbon resistors is that they are easily available at very low cost at all local vendors and the durability is good. The only disadvantage is that they are very sensitive to temperature.
Carbon resistors can be manufactured in wide range of values as low as 1 Ω value to a high value as 22 MΩ. Due to its low cost, they are used in circuits where cost is a criterion rather than the performance.
Variation of Resistance with Temperature
The effect of changes in temperature is a change in the value of resistance of the material. The reason for this change is not because of the variations in the dimensions of the material but rather the change in the resistivity of the material.
The flow of current in materials like conductors is the movement of electrons between atoms in the presence of an electric field. This is achieved by applying a potential difference across the conductor. This potential difference will cause the negatively charged electrons to move towards the positive terminal from atom to atom. These electrons which move freely between atoms are called free electrons. The conductivity of a material is dependent on the number of these free electrons in the atom of the material.
In mathematical terms, a fractional change in resistance is directly proportional to the change in the temperature.
∆R/R0∝∆T
Where ∆R is the small change in resistance
∆R = R – R0
R is resistance at temperature T
R0 is resistance at temperature T0
∆T is change in temperature
∆T = T – T0
If we denote the proportionality constant in the above equation as alpha (α)
Then ∆R/R0 = α∆T
Where α is the temperature coefficient of the resistance.
The temperature coefficient of resistance is used to describe the relative change in resistance in association with change in temperature.
The above equation can be written as
R = R0 [1+α (T-T0)]
If the resistance increases with increase in temperature, then the material is said to be having a positive temperature coefficient. These materials are conductors.
If the resistance decreases with increase in temperature, then the material is said to be having a negative temperature coefficient. These materials are insulators.
The units of temperature coefficient are /0K or K-1.
Resistor Packages
Surface Mount Technology is a technique where the components are directly placed on the surface of the circuit board. These components are called Surface Mount Devices. Resistors are also manufactured as SMDs. SMD Resistors are generally smaller with small leads or no leads.
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