As an engineering student or practising engineer, it's important to have a solid understanding of resistors and their role in electrical circuits. This article will give you a comprehensive overview of resistors, including what they are, important calculations and equations, and key applications.
What are Resistors?
A resistor is a passive electrical component that resists the flow of electrical current in a circuit. It's essentially a device that converts electrical energy into heat, which is then dissipated into the surrounding environment. The amount of resistance provided by a resistor is measured in ohms, represented by the symbol Ω.
Resistor Equations and Calculations
When working with resistors, there are several important equations and calculations that you need to be aware of. The key resister formulas are:
Ohm's Law
Where:
V = Voltage (V)
I = Current (A)
R = Resistance (Ω)
Power Equation
Where:
P = Power (W)
Worked Example & Calculation tools
For the given circuit, find the total voltage of the circuit as well as the total power absorbed by the system.
First Step
Find the voltage of the system using the total resistance and given the current
For our system, we can sum the total resistance as (600+400) Ω as they are connected in series.
Using this new value, we can calculate the voltage:
Inputs
R_total
:1,000ohmsOutput
Voltage
:200volts
I
:0ampere
Second Step
Using voltage and current, we can find the total power of the system.
Inputs
I
:0ampereOutput
P
:40W
V
:200
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Conclusion
For the given circuit, we found the voltage to be 200V. Using this value, we could determine the total power dissipated by the system, which was 40W.
Have a play with the above parameters to understand the relationship more.
Concept Summary
Ohm's law states that the current flowing through a resistor is proportional to the voltage across it. The power equation relates the power dissipation in a resistor to the current flowing through it and the voltage across it.
Applications of Resistors
Resistors are used in various applications, from basic electrical circuits to complex electronic systems. Some key applications include:
- Voltage division: Resistors can be used to divide voltage in a circuit, for example, in a voltage divider circuit.
- Current limiting: Resistors can limit the current flowing through a circuit, protecting other components from damage.
- Impedance matching: Resistors can be used to match the impedance of one part of a circuit to another, for example, in a radio circuit.
- Signal processing: Resistors can be used in analog circuits to shape or filter signals, for example, in a low-pass filter.
Types of Resistors
There are two main branches of resistors, linear and non-linear, and varying types of resistors within these:
Linear Resistors
Fixed Resistors
Thick Film Resistor
Thick film resistors are characterised by a resistive special paste that is fired onto a substrate. This paste is typically a mix of glass and metal oxide.
They are also classified by having a thickness greater than 0.1 um.
Thick Film resistors lack high power rating properties, yet are much cheaper for general applications.
Types of Thick Film Resistors:
- Fusible Resistor
A fusible resistor is a type of resistor with a low power rating that is designed to melt at a circuit current value.
It is the only type of resistor to quickly burn up when overloaded.
They are used in household appliances and electronic devices as safety mechanisms to prevent electrical fires and other electrically induced damage.
- Cermet Film
Cermet Film resistors use a thicker conductive paste. This paste is made from ceramic and metal and hence its name.
They possess qualities of low noise, temperature stability and good voltage rating.
They are utilised in radar, x-rays and power supplies.
- Metal Oxide
Metal-oxide film resistors use metal-oxide film as the resistive element.
They exceed the performance of metal film and carbon film resistors by means of the following metrics: power rating, surge capacity and temperature operation.
They are often employed in high-endurance applications.
Thin Film Resistor
Thin film resistors are characterised by their metallic film that is vacuum deposited onto the substrate.
Thin film resistors have a film thickness of <0.1 um.
They typically exhibit better temperature coefficient properties.
They are utilised in high-precision applications.
Types of Thin Film Resistors:
- Metal Film
Metal film resistors have a resistive layer comprised of a thin metal layer as per the name. The body is non-conductive (such as ceramic).
They typically exhibit properties of low noise and low voltage coefficient.
They are utilised in low-temperature coefficient applications such as active filters and bridge circuits.
- Carbon Film
Carbon film resistors are comprised of a ceramic base with a thin pure carbon film surrounding it
Generally advantageous over carbon-composed resistors. However, they have a limited resistive range in the conventional electronics market due to its cost.
This type of resistor has a capacitance of 0.5 pF.
They are typically used in high-voltage and high-temperature systems. They have seen utilisation in radars, x-rays and lasers.
Carbon Composition Resistor
This type of resistor is made out of fine carbon particles mixed with a binder (such as clay). This mix is then baled giving a solid form.
Carbon composition resistors have a high energy pulse tolerance. They can also be made with higher resistance values. Their disadvantages include inferior temperature coefficient, noise, voltage dependence and load.
Applications of this resistor include protection circuits (such as surge protection), current limiting, strobe lighting and welding. It is utilised in medical defibrillators due to their high energy pulse tolerance.
Wire-Wound Resistor
This resistor type is created using a conductive wound coil around a non-conductive core.
This design allows for a range of different properties and characteristics, as many different conductive materials and possible cores exist.
They are typically utilised in high-power applications such as circuit breakers.
Variable Resistors
Potentiometer
A potentiometer is a manually adjustable resistor. There exist 3 terminals in this device, 2 of which are connected to a circuit, and a third which acts as a sliding contact (or wiper) that manually increases the resistive value.
Potentiometers have simple construction, a wide range of resistance values, and accuracy and have been proven as an essential component of both old and new electrical engineering applications.
They are used in a range of different applications, from audio control and television to motion control and transducers.
Rheostat
A rheostat is a variable resistor used to control the current in a circuit. They share a similarity to potentiometers. They also utilise a wire-wound design to accommodate large magnitudes of current.
Rheostats possess notable inefficiency as a property, making them unsuitable in power control applications.
They are often used in control circuits where alteration of the current is necessary. They are also utilised in calibration and tuning circuits.
Trimmer Resistor
A trimmer resistor is a type of potentiometer (also known as a trimpot). Generally, they operate on the same principle as potentiometers, namely that the resistance can be varied based on the turning of a screw.
This has applications in audio electronics, such as turning the knob on a stereo system.
Trimmer resistors are extremely cheap and simple in construction.
Non-linear Resistors
Thermistor
Photoresistor
Varistor
Conclusion
Resistors are a fundamental component in electrical circuits, playing a critical role in power control and regulation. Understanding the basic equations and calculations associated with resistors, as well as their key applications, is essential for anyone working in electrical engineering or any other engineering discipline. Whether you're a university student or a practising engineer, this knowledge will be invaluable in your work and help you design and build more effective electrical circuits.