library: electronics - common electronic system components
Note: The following is one of a series of articles covering electronic and electrical system basics.
It appeared in a variety of industry trade publications and has been updated from time to time.
Author: Paul M. Allen

Electrical and electronic products are constructed on a building block principle. The lowest level building block is an individual component. A complete system is built up from a collection of components and circuits.

In a radio, for instance, the system includes receiving (tuning), amplifying, and various other sub-systems that typically are built using standard electronic components such as transistors, resistors, capacitors, and diodes mounted on a printed circuit board.

The various components serve a single purpose - to control the flow of current through circuit.

Only two form of control are available:

1. Switch the flow of current on and off.

1. Increase or reduce the amount of current that flows.

By selecting the right combination of components, the designer can control where the current will flow, when it will flow, and how much of it will flow.

the difference between electrical and electronic

The primary difference between electrical and electronic components is their size.

Electrical components carry relatively high volumes of current and must be large enough to handle the requirements. They will typically be constructed using mechanical sub-components.

Electronic components carry small amounts of current and can therefore be much smaller and will be built using technology unique to the electronics industry.

The thing that does not change is function. Regardless of the amount of power involved, the component will be used to either switch power on and off or regulate the amount of current that flows.

Most products use electrical and electronic components in combination with the electronic components providing logic and control, and the electrical components doing the "heavy lifting".

In an automotive ignition system, for instance, the electronic components tell the system when to fire a given spark plug while the electrical components carry the high voltage necessary to ignite the fuel in the combustion chamber.

basic electronic components

While electronic systems are typically built from a limited number of kinds of basic components, those components will be combined in different ways to perform any number of functions.

In a wiring diagram, those components are represented by standard symbols. The most common components are:


Resistors do exactly what their name implies. They limit the flow of electric current at a given point in the circuit, performing their task by absorbing a portion of the energy and turning it into heat.

In an electrical system blower motor circuit, for instance, A resistor block would include a different wire resistor coils for each fan speed with the voltage going to the motor determined by the resistor through which the current is passed. At high speed, any resistors would be bypassed allowing full system voltage to flow to the motor. Selection of a lower speed would result in the voltage being passed through a resistor to reduce the voltage to the motor.

In an electronic circuit, a resistor would determine the amount of voltage flowing to another circuit component which would then respond based on the amount of voltage received.  A resistor might also reduce the amount of voltage to the voltage required by the component.

Resistors in electronic circuits typically take the form of ceramic rods coated with carbon or metal and covered by a protective coating of resin. silicone, or glass with wire leads at both ends.

They are rated in ohms of resistance, tolerance, and watts. The color-coded rings on the shell of the resistor indicate the amount of resistance and tolerance.

Tolerance is the possible variation in performance from the resistor's stated rating in ohms. It is expressed as a percentage of the ohm value. The more critical the function becomes, the more the allowable tolerance will be reduced by the system designer.

While fixed value resistors are likely to be found in significant numbers on any electronic circuit board, variable resistors add the possibility of human input to the operation of the circuit. 

Variable Resistorss

They are available in slightly different configurations with all varieties sharing the common functionality of providing a means of adjusting the amount of resistance.

A Rheotsat would typically be used to control the flow of current for purposes of increasing or decreasing the brightness of a lamp or varying the speed of electrical motor.

A Potentiometer is similar but where a rheostat provides two connections with one leading directly to the component being controlled, the potentiometer provides a third connection that leads to the component being controlled as would be the case with a radio volume control knob.

Presets adjust the voltage output using an adjustment screw and would be used in instances where "fine tuning" the output would be required but would not be under the control of the system user.

Capacitors  ;

Capacitors are devices used to store an electrical charge. They can be used to collect it and then release it once it has built up to the desired, higher level or simply store it for later use.

A capacitor is made up of two metal plates or electrodes that are separated by an insulating material.

A photographer's strobe flash is an example of the functionality can provide. Electric current from a battery is collected and builds up in the capacitor to produce the large amount of power needed to produce a brief, powerful burst of light. Thus, the capacitor's function is exactly the opposite of the resistor, storing and multiplying the current rather than reducing it.

In an automotive airbag application, a capacitor stores enough energy to deploy the airbags in the event that a crash disables the vehicle's electrical system before the airbag is deployed.

Some capacitors are marked with color-coded rings or dots to specify the voltage multiplication provided, tolerance, and maximum voltage permitted.

Variable Capacitors allow output to be adjusted by moving the capacitor's plates and are commonly used in devices such as radio tuners.

Trimmer Capacitors are set-screw adjustable devices that might be used in an application such as a circuit board to "fine tune" the voltage coming into the circuit.

While capable of storing energy, capacitors - unlike batteries - will typically "leak down" and lose their charge in a relatively short period of time.

Because they can increase voltage to very high levels, they can be the most dangerous components in an electronic system. For that reason, experienced service technicians will discharge large capacitors in the circuit prior to performing any repair procedure.


Diodes are inserted into a circuit to allow current to flow in one direction but not the other.

They are usually made of silicon or geranium wrapped in a protective material with two wire leads.

They are the electronic equivalent of a mechanical check valve and can be used to prevent unwanted voltage from another component to flow into another component, to convert AC current to DC current or for a variety of other purposes.


Transistors are the most versatile of the basic electronic building blocks and come in a variety of configurations. As illustrated by the above symbols, a transistor will have three "wires" - an input wire, an output wire, and a "control" wire that leads to the base material.

They can be used as switches or to regulate current flow within the limitations set by their extremely small size.

In integrated circuits such as the central processors used in computers, the circuit can include millions of microscopic transistors that function as switches. In such cases, the transistor's state - off or on - translates into a binary value - 0 or 1. The combined states of large number of transistors serve as the inputs a computer uses in making its decisions and performing its calculations or other operations.      

A transistor is made by two pieces of semiconductor material placed back to back and separated by a center layer referred to as a "base". Inputs to the base from an external source will excite the base material and cause it to complete or break the connection between the input and output sides.

The factors that determine the capabilities of transistors and uses to which they can be put can in some ways be illustrated by the specifications most frequently used to rate them. By selecting the right transistor, the system designer can determine how much it will amplify any input voltage, how it will perform any switching functions, or perform other operations.

The specifications include:

1. Current Gain (Hfe). The number of times the transistor will multiply the current input to produce the current output. If, for instance, the input current was one milliamp (0.001 amps) and the output gain was rated at 100, the output would rise to 0.1 amps.

2. Noise Figure (NF). Noise is any unwanted signal in an electrical circuit. It can be cause by random fluctuation in the current flow or voltage. The noise rating will determine how "ragged" the transistor output is. It is rated in decibels since it can produce audible "hissing" or "cracking" in an audio system and could be critical in systems dependent on highly precise measures of sensitivity.

3. Maximum Power Dissipation. The amount of power lost when resistance in the transistor turns that power into heat. It is measured in watts and could be compared to a fuse rating - too much heat and it will fail.

4. Conductivity. The measure of how easily the current flows when the transistor is switched on. The higher the conductivity, there more easily the power will flow.

5. Leakage Current. The amount of voltage that flows through the transistor when it should be blocked. Too much leakage could cause activity in another component to be accidentally triggered.

6. Breakdown Voltage. The highest voltage that the transistor can hold back when it is turned off.

7. Operating Speed. The measure of how quickly the transistor will turn on and off or change from its input voltage to its output voltage.

NPN Transistors are the most commonly used transistors and are typically specified to perform switching operations.

PNP Transistors are similar to but are mirror images of NPN transistors and can be used as amplifiers.

Photo transistors are a specialized form of transistor in which the base material responds to light. They would be used in applications such as the circuits that automatically turn lights on and off.

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