A Resistor is an electrical/electronic device that resists the flow of current. A resistor may be thought of as a current limiting device. A resistor is one of the most useful electrical/electronic components, it is very rare to see a working circuit without a resistor. A resistor can be used to set an operating current for an electronic component such as an Integrated circuit to prevent the component from burning due to excess current passing through it. A combination of resistors is called a resistor network. depending on the complexity of the network a theorem such as Thevenin, Norton, Kirchhoff, etc can be applied to determine the final output resistance of the network. Electronic circuits such as radio circuits, Television circuits, etc, contain many resistors. Understanding a resistor is important, being able to solve resistor networks is important in designing circuits. Sometimes a resistor is used to attenuate or reduce signal level, current flow, voltage level, etc.
There are some types of resistors that are called variable resistors. They are called variable resistors because their value can be set based on their range of values of resistivity eg a 50k variable resistor has its value ranging from 0 ohms to 50K ohm. Fixed resistors are discrete, their value is fixed. Most resistors have their value indicated in color code on their surface, some resistors (usually power resistors) have their value indicated in numbers. The tolerance and reliability of the resistor are also indicated. Tolerance is the deviation of the value of the resistor, a resistor of 1K ohm with a tolerance of ±20% ohm means it can be less than or greater than its' value by 2 ohms, eg 800 ohms or 1.2K ohm.
The figure below shows the different symbols for both fixed and variable resistor
Potentiometer image
Theory of a resistor
A resistor limits or completely block a current that is passing through it based on its value. A resistor does this by resisting the flow of electrons passing through it, as the electrons carry electric charges. The famous formula that relates current passing through a resistor and the voltage across it is:
Resistance generates heat, the heat generated is proportional to the amount of current resisted from passing through the resistor, therefore, because of this, a resistor with a given value is rated differently. A 1Kohm resistor may have a different power rating. The power rating of a resistor has to be determined before it is used, in order to know whether it is a power resistor for use in a high current application or small-signal application. Resistors are rated according to their maximum power dissipation, exceeding the rated power may alter the value of the resistor or the resistor could get damaged completely. Some power resistors are not color-coded there value is written in numbers on top of them.
The power of a resistor can be derived from general electric power formula: from V = IR, I = V/R Substituting either one in the P=IV Gives
The ohm is the symbol of electrical resistance Ω. 1 ohm is equal to one volt/ampere. Resistors are manufactured over a very large range of values from 1mΩ to 1KΩ to 1MΩ.
Value of most fixed resistors are indicated by color code, each color has its value.
Resistor color code
Color-Coded resistors
4 band
Structure of a variable resistor
The figure above shows a structure of a variable resistor, it has three leads, using lead 1 and 2 or 2 and 3 makes it possible to vary the value of the resistor. Using 1 and 3 makes it a fixed resistor with a value equal to the maximum value of the potentiometer.
Resistor circuits
Resistors in series
When two or more resistors are connected in series their resistance adds up. The resultant resistance is
Resistors in parallel
when two or more resistors are connected in parallel the total value of their resistance decreases. The resultant resistance can be calculated using the relation.
This is a combination of two or more resistors in series with the output voltage taking between the resistors
When two or more resistors are connected in series and then connected to another set of resistors in parallel, a series-parallel network is formed. Their value could be calculated by first solving the series or parallel network to find the equivalent resistor and then use the value of the network as if it is a value of a single resistor to solve the other part.
The figure below shows the solution of the above network. First, sum the resistors that are in series. this will give the equivalent resistor, then find the resistance of the parallel resistors.
Another example
Some of the applications of a resistor.
Voltage divider
This is a combination of two or more resistors connected in series to ground a tap between any point between the resistors is taken as the output voltage, the output voltage is less than the voltage across the resistors, a voltage divider is commonly used to bias an amplifier as would be seen later.
Current limitation
A resistor is placed in series with an electronic component so that the current passing through the component is limited based on the value of the resistor connected.
Linear attenuation
A resistor could be used to attenuate a signal.
Timing
A resistor could be connected in series with a capacitor or an inductor to create an RC/RL circuit, which can be used as a timer, as would be seen later.
