The article provides an overview of electrical circuit components, covering power sources, switches, and passive elements such as resistors, capacitors, and inductors. It explains their functions, characteristics, symbols, and measurement standards, including resistor color codes, capacitor labeling, and inductor properties.
Electrical circuit elements include power sources (such as a power supply or a battery), switches to open and close the circuit, and circuit components (such as resistors and capacitors). A schematic of an electrical circuit is shown in Figure 1. The figure shows a closed loop where a conducting element (such as a copper wire) connects a voltage source (or a current source) to load elements on the electrical circuit.
Figure 1 A schematic of an electrical circuit
Figure 1 also shows the direction of conventional current flow from the anode (positive terminal) of the power source to the cathode (negative terminal). The conventional current flow direction is the opposite of the direction of actual electron flow in the electrical circuit but was kept due to Benjamin Franklin, who thought that electrical current is due to the motion of positively charged particles.
Passive Electrical Circuit Components
Electrical circuit components can be of the passive type, which require no external power to operate (such as resistors and capacitors), or active components which require power to operate (such as operational amplifiers). Three basic passive electrical circuit components are the resistor, the capacitor, and the inductor. Table 1 shows the electrical symbols for these elements. The table also shows the symbols for two energy sources that are normally represented in circuits. These include an ideal voltage source and an ideal current source. The sources are considered ideal because they do not have any internal resistance, capacitance, or inductance.
Table 1 Symbols of Electrical Circuit Components
Resistor
The resistor is an element that dissipates energy. The constitutive relation for an ideal resistor is given by Ohm’s law, in which the voltage drop across the resistor is linearly related to the current through the resistor, or
V = IR
The resistance is measured in units of ohms (Ω). Resistors can be either fixed type or variable. Fixed-type resistors are made in a variety of forms including surface mount, wire wound, thick film, and carbon composition (see Figure 2).
Figure 2 Resistor types (a) surface mount, (b) wire wound, (c) thick film, and (d) carbon composition
Typical fixed-type, low-wattage resistors are made of molded carbon composition and have a resistance that ranges from a few ohms to about 20 MΩ. These resistors have a cylindrical shape and have sizes that increase with the power rating of the resistor. The typical power rating is 1/4 to 1 watt (W). The resistance can be read from the color code printed on the resistors. Typically, four color bands are shown on the resistor, as shown in Figure 3, but resistors with five or six color bands are also available. For four color bands, the left three bands give the resistor value, while the fourth band gives the resistance tolerance (Tol). The resistance is given by the formula:
$$R\ =\ ab\times{10}^c\left(\pm%Tol\right)$$
where a band is the value of the tens digit, the b band is the value of the ones digit, the c band is the base-10 exponent power value, and the Tol band gives the tolerance or expected percentage variation in the resistor value. Table 2 gives these values.
Figure 3 Resistor color bands
Table 2 Resistor Bands Color Code
|
a, b, c Band Color |
Value |
Tol Band Color |
Value (+/−) |
|
Black |
0 |
Silver |
10% |
|
Brown |
1 |
Gold |
5% |
|
Red |
2 |
Brown |
1% |
|
Orange |
3 |
Red |
2% |
|
Yellow |
4 |
Green |
0.5% |
|
Green |
5 |
Blue |
0.25% |
|
Blue |
6 |
Violet |
0.1% |
|
Violet |
7 |
Gray |
0.05% |
|
Gray |
8 | ||
|
White |
9 |
As an example, a resistor whose bands are colored brown, black, orange, and silver has a resistance of 10k +/−10% ohms. The resistance of a real resistor is not constant, but it increases with temperature.
Commercial resistors are typically available in a set of standard values known as the E-series. These values are organized according to the tolerance of the resistors. The three most common E-series are E12, E24, and E96, which are used for resistors with tolerance values of ±10%, ±5%, and ±1%, respectively. For each series, the number represents the number of distinct values within each decade of resistance. For instance, in the E12 series, there are 12 distinct values ranging from 10 to 100 ohms (10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, and 82), and the pattern repeats for each subsequent decade (100 to 1000, 1000 to 10,000, etc.). Similarly, E24 has 24 distinct values per decade, providing a greater choice for more precision applications.
Variable-type resistors include rheostats and potentiometers. Rheostats are two-terminal resistors, while potentiometers are three-terminal resistors. They can be of the linear or rotary type, and the resistance between the terminals is changed as the position of the wiper terminal is changed.
Capacitor
Unlike a resistor, a capacitor is an energy storage element. The constitutive relation for a capacitor is
$$\frac{dV}{dt}=\frac{1}{C}I$$
where C is the capacitance in Farads (F). Small capacitors are typically of the ceramic type, which can be used in both AC and DC circuits. These capacitors have a capacitance that is less than 0.1 micro-Farads (μF).
Ceramic capacitors often have a numeric code printed on them to indicate their capacitance value. This code typically consists of two or three digits, followed by a letter. The first two digits represent the base value of the capacitance, and the third digit represents the multiplier or the number of zeros to follow the base value. This code represents the capacitance value in picofarads (pF). For instance, a ceramic capacitor with the code ‘104’ would have a capacitance of 100,000 pF or 0.1 μF, because the ‘4’ adds four zeroes to the base value ‘10’.
Sometimes, there may only be two digits, which means there are no additional zeros. For example, a capacitor labeled ‘47’ would have a capacitance of 47 pF. The letter following the number code often indicates the tolerance of the capacitor. Common tolerance letters are ‘M’ (±20%), ‘K’ (±10%), and ‘J’ (±5%), among others. For example, a capacitor labeled ‘104J’ would have a capacitance of 0.1 μF with a tolerance of ±5%.
Capacitors with large capacitance (up to several thousand micro-Farads) are of the electrolytic type. These are used only in DC circuits, and their leads are polarized. One characteristic of capacitors is the leakage current, which is the current that flows between the capacitor plates when a voltage is applied across the plates of the capacitor. This current leads to the loss of charge over time from the capacitor. This current, however, is typically small, unless the capacitor is of the electrolytic type. Similar to resistors, capacitors are also available as fixed or variable types.
Inductor
An inductor is also an energy storage element. Inductive elements in practice include solenoids and motors. The constitutive relation for an inductor is
$$\frac{dI}{dt}=\frac{1}{L}V$$
where L is the inductance, and it is measured in units of Henry (H). Small-sized inductors are of the molded type, and they have inductance that varies from sub-micro to several thousand micro-Henry (μH).
Electrical Circuit key Takeaways
The components of an electrical circuit power sources, switches, resistors, capacitors, and inductors—play crucial roles in various applications across industries. Resistors regulate current flow, capacitors store and release energy, and inductors manage magnetic fields, making them essential in power systems, communication devices, and signal processing. These components ensure efficient circuit operation, enabling the functionality of consumer electronics, automotive systems, medical devices, and industrial automation. Understanding their properties and applications helps engineers and designers create reliable, optimized electrical circuits for modern technological advancements.
