What is a resistor?
An electrical resistor is a two-terminal passive component used specifically to oppose and limit current. A resistor works on the principle of Ohm's Law, which states that the voltage across the terminals of a resistor is directly proportional to the current flowing through it.
Ohm's Law: V = IR
where V is the voltage applied to the resistor,
I am the current that flows through him,
and R is the constant called resistance.
The unit of resistance is ohms.
Types of resistors:
Resistors can be broadly classified based on the following criteria: the type of material used, the rated power and the resistance value.
1. Fixed resistors .
In some scenarios, an electrical circuit may need a smaller amount of current to flow through it than the input value. Fixed resistors are used in these situations to limit current flow.
1.1 Carbon Composition Resistors:
These resistors are cylindrical rods that are a mixture of carbon granules and ceramic powder. The value of the resistor depends on the composition of the ceramic material. A greater amount of ceramic content will result in more resistance. As the rod is coated with an insulating material, there are chances of damage due to excessive heat caused by welding.
High current and voltage can also damage the resistor. These factors bring irreversible changes in the resistance power of these resistors . This type of resistor is rarely used nowadays due to its high cost and is preferred only in power supply and welding circuits.
Fig. 1: Image of carbon composition resistors
1.2 Carbon film resistors:
This resistor is formed by depositing a layer of carbon film on an insulating substrate. Helical cuts are then made through the carbon film to trace a long, helical resistive path. The resistance can be varied by using carbon material of different resistivity and modifying the shape of the resistor. The helical resistive path makes these resistors highly inductive and of little use for RF applications.
They have a temperature coefficient between -100 and -900 ppm/°C. The carbon film is protected by a conformal epoxy coating or a ceramic tube. The operation of these resistors requires high pulse stability.
Fig. 2: Image of carbon film resistors
Metal film and wirewound resistors
1.3 Metal film resistor:
These resistors are made of small ceramic rods coated with metal (such as a nickel alloy) or metal oxide (such as tin oxide). The resistance value is mainly controlled by the thickness of the coating layer (the thicker the layer, the lower the resistance value). A thin spiral groove can be cut along the rod using a laser to effectively split the carbon or metal coating into a long, spiral strip, which forms the resistor.
Fig. 3: Image of metal film resistors
Metal film resistors can be obtained in a wide range of resistance values, from a few Ohms to tens of millions of Ohms, with a very small tolerance. For example, for a declared value of 100K Ohm, the actual value will be between 99K Ohm and 101K Ohm. Small carbon, metal, and oxide resistors come in many colors, such as dark red, brown, blue, green, gray, or white.
1.4 Wirewound resistor:
Wirewound resistors vary in size and physical appearance. Its resistive elements are usually lengths of wire, usually an alloy such as nickel/chromium or manganin, wrapped around a small ceramic or fiberglass rod and coated with a flameproof cement insulating film. They are typically available in low resistance values but are capable of dissipating large amounts of energy.
These resistors can become very hot during use. For this reason, these resistors are housed in a finned metal case that can be screwed to a metal chassis to dissipate the heat generated. Fire protection is important and fireproof enclosures or coverings are vital. The output wires are typically soldered rather than soldered to the resistor. Enamel resistors are used in scenarios where high power is involved and are encapsulated in heat-proof bases.
Because wirewound resistors are primarily coils, they have more undesirable inductance than other resistor types, although winding the wire in sections with alternately reversed directions can minimize inductance. Other techniques employ bifilar winding to reduce the cross-sectional area of the coil. For the most demanding circuits, resistors with Ayrton-Perry windings are used.
Fig. 4: Image of wirewound resistors
Thin-film and thick-film resistors
1.5 Thin film and thick film resistors:
The difference between thin film and thick film resistors is how the film is applied to the cylinder (axial resistors) or the surface (SMD resistors). Thin-film resistors are made by sputtering (a vacuum deposition method) resistive material onto an insulating substrate, while thick-film resistors are made using screen and stencil printing processes.
Ceramic conductors such as tantalum nitride (TaN), ruthenium dioxide (RuO 2 ), lead oxide (PbO), bismuth ruthenate (Bi 2 ru 2 Ó 7 ), nickel-chromium (NiCr) and bismuth iridate (Bi 2 Ir 2 Ó 7 ) are the materials commonly used to make thin film resistors. Thick film resistors are usually made by mixing ceramic with powdered glass. Thick films have tolerances ranging from 1 to 2% and a temperature coefficient between ±200 or ±250 ppm/K.
Thin film resistors are generally more expensive than thick film resistors. Thin-film resistors are preferred for passive and active microwave power components, such as microwave power resistors, microwave power terminations, microwave resistive power dividers, microwave power attenuators. waves.
Fig. 5: Image of Thin Film and Thanks Film Resistors
Surface Mount and Network Resistors
1.6 Surface Mount Resistor (SMT):
This type of resistor helps to achieve very low power dissipation along with very high component density. Most modern circuits use small SMT resistors. These are made by depositing a film of resistive material, such as tin oxide, on a tiny ceramic chip. The edges of the resistor are then precisely ground or cut with a laser to provide accurate resistance throughout the device. Tolerances can be as low as 0.02%. Contacts are provided at each end, which are soldered directly to the circuit board's conductive print, usually by self-assembly methods. They are mainly used where space is an important factor.
Fig. 6: Image of surface mount resistors
1.7 Network resistors:
These resistors are the combination of resistances that can provide identical values on all pins with one pin acting as common terminal. These resistors are available in single in-line package and double in-line package and can be surface mounted or through hole. They are used in applications like pull up/pull down, DAC etc.
Figure 7: Network resistor symbols
Figure 8: Network Resistors Circuit Diagram
Variable Resistors
two. Variable resistors.
Presets and potentiometers are commonly used types of variable resistors. They are mainly used for voltage division and sensitivity setting of sensors. These have a sliding or wiper contact that can be turned with the help of a screwdriver to change the resistance value. In the linear type, the change in resistance is linear as the wiper rotates. In the logarithmic type, the resistance changes exponentially as the wiper slides. The value must be set correctly when installed on a device and is not adjusted by the device user.
Fig. 9: Variable Resistor Diagram
The variable can have three tabs where the middle tab is the wiper. If all three tabs are used, it behaves like a voltage divider. If only the wiper tab is used together with another tab, it becomes a variable resistor or rheostat. If only the side tabs are used, it behaves like a fixed resistor. They are mainly used for tuning, voltage division and sensitivity adjustment of sensors.
The variable may have one or two built-in switches where the resistor operates to the ON state of the switch(es). These resistors were primarily used for volume control in older TV and radio circuits. There can also be four-tab variables where the fourth terminal is for feedback signal and placed next to the first tab. Wire-wound variable resistors are used for very precise control of resistance.
The wiper can also be rotating (as with most presets), sliding, or disc-shaped (as used on pocket radios for volume control).
Figure 10: Image showing variable resistor applications
Fig. 11: Image showing different types of variable resistors
Semivariable Resistors
3. Semi-variable resistors
These are two terminal variable resistors designed to handle higher voltages and currents. These are constructed by resistive wire wound to form a toroid coil with the wiper moving over the top surface of the toroid, sliding from one turn of wire to the next. A rheostat is also made of resistance wire wound around a heat-resistant cylinder with the slider made of several metal fingers. The fingers can be moved along the coil of resistance wire via a slider, thereby changing the touch point.
Fig. 12: Image of Semivariable Resistors
Special Resistors
4. Special resistors
4.1 Thermistors:
Thermistors are special resistors whose resistance changes with temperature . If the resistance increases with increase in temperature then it is called positive temperature coefficient (PTC) or posistors. If the resistance decreases with increasing temperature, this is called negative temperature coefficient (NTC).
An NTC can be replaced by a transistor with a trimmer potentiometer. PTCs are mainly used as current limiters for circuit protection. As heat dissipation from the resistor increases, the resistance increases, thus limiting the current. NTCs are mainly used for temperature detection, fuse replacement in power supply protection, and for measurements of low temperatures up to 10K. These are constructed using metal oxides sintered in a ceramic matrix.
Fig. 13: Image of thermistors
4.2 Light dependent resistors (LDR):
LDRs have a zigzag adhesion of cadmium sulfide whose resistance decreases as the intensity of the light incident on it increases. In the absence of light its resistance is in mega ohms but with the application of light the resistance drops drastically. These resistors are used in many consumer items such as light meters for cameras, street lights, clock radios, alarms, and outdoor clocks.
Fig. 14: Image of the Light Dependent Resistor (LDR)
Resistance measurement
Resistance measurement:
By color codes
Fig. 15 : Image showing the color coding of the resistors
Chip resistors have a three-digit numerical representation, where the first two digits represent the number and the third digit is the multiplier. For example, on a chip resistor, the number 103 means that its resistance is 10K, with 3 being the multiplication factor.
Measurement using multimeter
Resistance measurement using multimeter:
There are a few simple steps required to take a resistance measurement with a digital multimeter:
1. Select the resistance that needs to be measured and estimate what the resistance might be.
two. Insert the probes into the required sockets. The digital multimeter will have several sockets for the test leads. Insert them or check that they are already in the correct sockets.
3. Turn on the multimeter.
4. Select the required range. When the digital multimeter is turned on, the required amount of voltage, current or resistance and its range can be selected. The range selected should be such that the best reading is obtained. Typically, the multimeter's function switch will be labeled with the maximum resistance reading. Choose the one where the estimated resistance value will be below, but close to the maximum of the range. In this way, more accurate resistance measurement can be made.
5. Take the measurement. The probes can be applied to both terminals of the resistor. The interval can be adjusted if necessary. The resistor value is shown on the multimeter display.
6. Turn off the multimeter: After the resistance measurement has been made, the multimeter can be turned off to save batteries. It is also advisable to place the function switch in a high voltage range. This way, if the multimeter is used again for another type of reading, no damage will be caused if it is inadvertently used without selecting the correct range and function.
General precautions
General precautions when measuring resistance
· Measure resistance when components are not connected in a circuit.
· Remember to ensure that the circuit under test is not turned on.
· Make sure the capacitors in a circuit under test are discharged.
· Remember that diodes in a circuit will cause different readings in either direction.
· The leakage path through the fingers may alter readings in some cases.
Circuit Analysis
Circuit Analysis:
The AC and DC behavior of resistors is the same. In series combination, the equivalent resistance is the sum of the resistances and is given by:
R = R1 + R2 + R3 +……
The current through the branch remains constant while the voltage drops across different resistors are different and are given by the product of the current and the individual resistances.
In the shunt combination, the equivalent resistance is given by:
1/R = 1/R1 + 1/R2 +1/R3+…..
The voltage between the branches remains constant while the currents in the different branches are different and are given by the supply voltage divided by the individual resistances.
Figure 16: Circuit diagram of series and parallel connections of resistors
Through analysis, we can conclude that in the case of circuits with two branches, the current in one branch is the product of the supply current and the resistance in the other branch divided by the sum of the resistances. This is called 'derivation formulas'
Figure 17: Equivalent Circuit for Shunt Formulas
EU R1 = I*R 2 / (R 1 +R 2 )
EU R2 = I*R 1 / (R 1 +R 2 )
There may also be combinations of star (Y and T) and delta (delta and crooked) resistances.
Figure 18: Circuit diagram of star and delta resistor connections
A delta network can be converted into a star network using the formulas:
R A =R AB *R AC / (R AB +R AC +R AC )
R B =R AB *R AC / (R AB +R AC +R AC )
R C =R AC *R AC / (R AB +R AC +R AC )
Mnemonics – For delta to star conversion, the resistor at a node is the product of the resistances in the adjacent branches connected to that node divided by the sum of all three delta resistances.
A star network can be converted into a delta network using the formulas:
R AB =R A +R B +R A *R B /R C
R CA = R A +R C +R A *R C /R B
R AC = R C +R B +R C *R B /R A
Mnemonics – For star to delta conversion, the resistance in a branch is the sum of the resistances maintained by the two nodes of the branch with the product of these resistances divided by the opposite resistance.