A resistor is a component or device designed to have a known value of resistance. OR,
Those components and devices which are specially designed to have a certain amount of resistance and used to oppose or limit the electric current flowing through it are called resistors.
Good to know: Resistance of a resistor depends on their length (l), resistivity (ρ) and its cross sectional area (a) which is also known as laws of resistance … R = ρ (l/a).
IEEE & IEC Symbols of Different Types of Resistors.
Types of Resistors:
Resistors are available in different size, Shapes and materials. We will discuss all possible resistor types one by one in detail with pro and cons and application as follow.
Different Types of Resistor Chart/Tree.
There are two basic types of resistors.
- Linear Resistors
- Non Linear Resistors
Those resistors, which values change with the applied voltage and temperature, are called linear resistors. In other words, a resistor, which current value is directly proportional to the applied voltage is known as linear resistors.
Generally, there are two types of resistors which have linear properties.
- Fixed Resistors
- Variable Resistors
As the name tells everything, fixed resistor is a resistor which has a specific value and we can’t change the value of fixed resistors.
Types of Fixed resistors.
- Carbon Composition Resistors
- Wire Wound Resistors
- Thin Film Resistors
- Thick Film Resistor
Carbon Composition Resistors
A typical fixed resistor is made from the mixture of granulated or powdered carbon or graphite, insulation filler, or a resin binder. The ratio of the insulation material determines the actual resistance of the resistor. The insulating powder (binder) made in the shape of rods and there are two metal caps on the both ends of the rod.
There are two conductor wires on the both ends of the resistor for easy connectivity in the circuit via soldering. A plastic coat covers the rods with different color codes (printed) which denote the resistance value. They are available in 1 ohm to 25 mega ohms and in power rating from ¼ watt to up to 5 Watts.
Construction and Wattage Rating of Carbon Composition Resistors.
Wire wound Resistors
Wire wound resistor is made from the insulating core or rod by wrapping around a resistive wire. The resistance wire is generally Tungsten, manganin, Nichrome or nickel or nickel chromium alloy and the insulating core is made of porcelain, Bakelite, press bond paper or ceramic clay material.
The manganin wire wound resistors are very costly and used with the sensitive test equipments e.g. Wheatstone bridge, etc. They are available in the range of 2 watts up to 100 watt power rating or more. The ohmic value of these types of resistors is 1 ohm up to 200k ohms or more and can be operated safely up to 350°C.
in addition, the power rating of a high power wire wound resistor is 500 Watts and the available resistance value of these resistors are is 0.1 ohm – 100k Ohms.
Thin Film Resistors
Basically, all thin film resistors are made of from high grid ceramic rod and a resistive material. A very thin conducting material layer overlaid on insulating rod, plate or tube which is made from high quality ceramic material or glass. There are two further types of thin film resistors.
- Carbon Film Resistors
- Metal Film Resistors
Carbon Film Resistors
Carbon Film resistors contains on an insulating material rod or core made of high grade ceramic material which is called the substrate. A very thin resistive carbon layer or film overlaid around the rod. These kinds of resistors are widely used in electronic circuits because of negligible noise and wide operating range and the stability as compared to solid carbon resistors.
Construction of Carbon Film Resistors & Its labels.
Metal Film Resistors
Metal film resistors are same in construction like Carbon film resistors, but the main difference is that there is metal (or a mixture of the metal oxides, Nickel Chromium or mixture of metals and glass which is called metal glaze which is used as resistive film) instead of carbon. Metal film resistors are very tiny, cheap and reliable in operation. Their temperature coefficient is very low (±2 ppm/°C) and used where stability and low noise level is important.
Cermet Oxide Resistors (Network Resistors)
In the cermet oxide resistors, the internal area contains on ceramic insulation materials. And then a carbon or metal alloy film or layer wrapped around the resistor and then fix it in a ceramic metal (which is known as Cermet). They are made in the square or rectangular shape and leads and pins are under the resistors for easy installation in printed circuit boards. They provide a stable operation in high temperature because their values do not change with change in temperature.
As the name indicates, those resistors which values can be changed through a dial, knob, and screw or manually by a proper method. In these types of resistors, there is a sliding arm, which is connected to the shaft and the value of resistance can be changed by rotating the arm. They are used in the radio receiver for volume control and tone control resistance.
Following are the further types of Variable Resistors
Potentiometer is a three terminal device which is used for controlling the level of voltage in the circuit. The resistance between two external terminals is constant while the third terminal is connected with moving contact (Wiper) which is variable. The value of resistance can be changed by rotating the wiper which is connected to the control shaft.
This way, Potentiometers can be used as a voltage divider and these resistors are called variable composition resistors. They are available up to 10 Mega Ohms.
Rheostats are a two or three terminal device which is used for the current limiting purpose by hand or manual operation. Rheostats are also known as tapped resistors or variable wire wound resistors.
Variable wire wound resistors are available in the range of 1 ohm up to 150 Ohms. The available power rating of these resistors is 3 to 200 Watts. While the most used Rheostats according to power rating is between 5 to 50 Watts.
There is an additional screw with Potentiometer or variable resistors for better efficiency and operation and they are known as Trimmers. The value of resistance can be changed by changing the position of screw to rotate by a small screwdriver.
They are made from carbon composition, carbon film cermet and wire materials and available in the range of 50 Ohms up to 5 mega ohms. The power rating of Trimmers potentiometers are from 1/3 to ¾ Watts.
Non Linear Resistors
We know that, nonlinear resistors are those resistors, where the current flowing through it does not change according to Ohm’s Law but, changes with change in temperature or applied voltage.
In addition, if the flowing current through a resistor changes with change in body temperature, then these kinds of resistors are called Thermisters. If the flowing current through a resistor change with the applied voltages, then it is called a Varistors or VDR (Voltage Dependent Resistors).
Following are the additional types of Non Linear Resistors.
- Varisters (VDR)
- Photo Resistor or Photo Conductive Cell or LDR
Thermisters is a two terminal device which is very sensitive to temperature. In other words, Thermisters is a type of variable resistor which notices the change in temperature. Thermisters are made from the cobalt, Nickel, Strontium and the metal oxides of Manganese. The Resistance of a Thermister is inversely proportional to the temperature, i.e. resistance increases when temperature decrease and vice versa.
It means, Thermisteres has a negative temperature coefficient (NTC) but there is also a PTC (Positive Temperature Coefficient) which a made from pid barium titanate semiconductor materials and their resistance increases when increases in temperature.
Varisters are voltage dependent Resistors (VDR) which is used to eliminate the high voltage transients. In other words, a special type of variable resistors used to protect circuits from destructive voltage spikes is called Varisters.
When voltage increases (due to lighting or line faults) across a connected sensitive device or system, then it reduces the level of voltage to a secure level i.e. it changes the level of voltages.
Photo Resistor or Photo Conductive Cell or LDR (Light Dependent Resistors)
Photo Resistor or LDR (Light Dependent Resistors) is a resistor which terminal value of resistance changes with light intensity. In other words, those resistors, which resistance values changes with the falling light on their surface is called Photo Resistor or Photo Conductive Cell or LDR (Light Dependent Resistor). The material which is used to make these kinds of resistors is called photo conductors, e.g. cadmium sulfide, lead sulfide etc.
Construction of LDR (Light Dependent Resistor), Photo-resistor or photo conductive cell
When light falls on the photoconductive cells (LDR or Photo resistor), then there is an increase in the free carriers (electron hole pairs) due to light energy, which reduce the resistance of semiconductor material (i.e. the quantity of light energy is inversely proportional to the semiconductor material). It means photo resistors have a negative temperature coefficient.
For Current control and limiting
- To change electrical energy in the form of heat energy
- As a shunt in Ampere meters
- As a multiplier in a Voltmeter
- To control temperature
- To control voltage or Drop
- For protection purposes, e.g. Fusible Resistors
- In laboratories
- In home electrical appliances like heater, iron, immersion rod etc.
- Widely used in the electronics industries