In 1827, Georg Simon Ohm noticed that different electrically conductive materials did not allow an electric current to flow with the same ease. He described this behavior as “electrical resistance” and stated that this resistance primarily depends on the type of material.
Resistors are passive electronic devices designed to do precisely what Ohm’s finding is about: they restrict current flow while producing voltage drops between their terminals. However, while seemingly simple, many factors and considerations go into the proper specification of resistors and resistor circuits.
This article will get back to the basics of resistors by explaining how resistors work, the different types of resistor circuit connections, and their suitability for different application requirements.
Resistors work by limiting the flow of electric current in a circuit. Electrical resistance describes the capability of a resistor to limit electric current flow, and it can be estimated using:
Where:
R = resistance of the resistor (Ω)
p = resistivity of the material (Ω.m)
L = length of the resistor (m)
A = cross-sectional area of the resistor (m2)
To better understand the resistance of a resistor, consider a system consisting of a pipe connected to an open water valve. The volume flow rate of water from the pipe depends on the size of the pipe. For instance, pipes with a larger cross-section area will provide less water flow resistance than those with a smaller cross-section area. Likewise, longer pipes cause greater resistance to water flow. Finally, if a stone is placed in the pipe, it causes obstruction and resistance to water flow. As a result, it can be concluded that the cross-sectional area, length and obstacle placed in the pipe affect the water flow rate.
Similarly, the electrical resistance is affected by the length, area and obstruction of the resistor, which is related to the type of material the resistor is made from. This obstruction is called the resistivity of the material. The resistivity of conductors is generally lower than insulators since conductors allow electrons to flow through them more easily.
Resistors can be connected in series and parallel. Each type of connection has different outputs and suitability for different applications.
Figure 1 shows a typical electrical circuit with resistors connected in series with an applied voltage, V.
Because there is only a single path for electric current to flow, the current flowing through each resistor is the same. However, there is a corresponding voltage drop as electric current flows through each resistor. The total voltage in the circuit equals the sum of the voltage drop across each resistor, according to the equation below:
The equivalent resistance of the resistors in this circuit is given by:
For n resistors in a series connection, the equivalent resistance is given by:
This equation shows that the equivalent resistance of resistors in a series connection is the sum of the resistances of the resistors in the circuit. This type of connection is ideal in applications where there is a need for higher electrical resistance than the resistance of the commercially available resistor. Resistors in series are also valuable for voltage divider circuits.
However, when resistors are connected in series, a fault in one of the resistors affects the functionality of the entire circuit since the faulty resistor will prevent current from flowing through the other resistors.
Figure 2 shows two resistors connected in parallel to a voltage source.
Unlike in the series connection of resistors, the voltage across each resistor is the same as the voltage source. However, the electric current through each resistor depends on the resistor’s electrical resistance. For instance, the resistor with a lesser resistance will allow more current flow than resistors with higher resistance, but the total current in the circuit is the sum of current flowing through all the resistors.
The equivalent resistance of this connection is given by:
For n resistors in parallel connection, the equivalent resistance is given by:
Circuits with resistors connected in parallel are ideal in applications where appliances are required to work independently. This is because the failure of one of the resistors (or appliances) does not affect the functionality of others. For instance, automobile headlights are typically wired in parallel.
Resistor power rating is among the essential parameters for specifying resistors for a particular application. It describes the maximum power (measured in Watt) that the resistor is designed to withstand without damage.
When electrical current passes through a resistor, electrical energy is lost by the resistor in the form of heat. The heat energy generated depends on the magnitude of current flowing through the resistor. However, suppose the amount of heat energy generated by the resistor (or resistor power) is greater than the resistor's power rating. In that case, the resistor melts and causes a short circuit.
The resistor power varies with the voltage and electric current according to the equation below:
While this article discusses the basics of resistors and resistor circuits, there are still several basic concepts that must be understood about resistors before specifying one for a particular application. For instance, there is still the need to understand color coding and types of resistors, and resistance measurement, among others.
Learn more about resistors on Globalspec.com.
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