Common Units Used in Electronics
Understand the common units used in electronics from voltage, current, resistance and power on volts, amps, ohms and watts through capacitance in Farads, inductance in Henries, and charge in coulombs to energy and frequency.
SI, International System of Units Includes:
SI base units
SI units & symbols
SI / metric prefixes
Unit definitions
SI (metric) / Imperial conversion
Summary of common units for electronics
For anyone involved in electronics it very important to have a good understand ing of the various units that are commonly used.
The various important units for potential difference or voltage, current, resistance and power are all essential basics, with their units of volts, amps, Ohms and watts.
But there are other units that are also widely used including capacitance (Farads), inductance (Henries) and charge measured in coulombs or amp hours, etc - these are all important measurement parameters with their associated units.
We'll take a look at the main parameters that need to be measured and specified in many electronic circuits, and explain in simple terms what they are and how they are used.
Main electronics parameters and their units
There are several difference units, each of which will be described in turn.
• Voltage V
The voltage is a key parameter for any electronic circuit. The voltage is the electrical pressure at a point in the circuit and it is measured in volts. There is a potential difference across a point in a circuit when there is a difference in electrical potential.
Often we refer to the voltage at a given point and this refers to the electrical potential at that point. It can be across a specific component, or between two points in a circuit.
However if no reference point is given it is often implied that it is the potential of that point above ground.
There is also the electromotive force, this is the electric potential provided by a generator such as a battery or other form of generator and it is the no-load potential, i.e. the force that is within the generator before any resistive losses are considered.
Volt definition:
One volt is defined as the electric potential between two points of a conductor when an electric current of one ampere dissipates a power equal to one watt between those points.
• Current I
Current is another very important parameter within any electronic circuit design. The current the rate of electron flow within a conductor and it can be analogous to the level of water flowing in a pipe.
The current is measured in amperes and this is normally shortened to amps, e.g. a current of 5 amps flowing in a circuit.
Ampere definition:
The ampere was defined as the constant current which, if it was maintained in two straight parallel conductors of infinite length of negligible circular cross section and placed one metre apart in a vacuum, would produce a force equal to 2 × 10−7 newton per metre of length between these conductors.
A new definition was introduced on May 20, 2019, the ampere would henceforth be defined such that the elementary charge would be equal to 1.602176634 × 10−19 coulomb.
• Power W
The power parameter indicates the rate at which electrical energy is being transferred or consumed. It's the product of voltage and current, measured in watts (W). It can be thought of as the rate at which electrical work is being done.
It is important in electronic circuits because when power is dissipated in a component, heat is generated. This heat will cause the temperature to rise and in extreme cases the component could burn out.
Generally, though, temperature rises will be expected, but they should be kept within acceptable limits.
Accordingly, most components have a maximum power dissipation level stated in their specification, and an operating temperature range is given. Many semiconductor devices like transistors and diodes will have a maximum junction temperature as it is important to keep this area of the device within its range, and the case temperature will be lower because the heat is generally generated at the junction.
As with any component, it is best to keep it well within its heat dissipation specification as this will ensure better long term reliability.
Power dissipation will be measured in watts, or in many cases milliwatts (one thousandth of a watt).
Watt definition:
The watt is the SI unit of power, and it is equivalent to one joule per second. It corresponds to the rate of consumption of energy in an electric circuit component having a potential difference is one volt and with a current one ampere flowing.
• Resistance Ω
The resistance of a circuit can be considered to be an obstacle to the flow of the electrons, i.e. electric current in a circuit. It can be likened to the narrowing of the diameter of a pipe through which water flows.
The resistance is measured in ohms, named after the famous pioneer who discovered the relationship between the voltage and current in a circuit
Ohm definition:
The ohm is the SI unit of electrical resistance, and the resistance of a conductor is said to be one ohm when a potential difference of one volt causes a current of one amp to flow in it.
• Capacitance F
The capacitance describes the ability of an electronic component, etc to store electrical charge. In some ways it can be likened to a small battery.
The capacitance is measured in farads, F and the higher capacitance the more charge can be stored.
Farad definition:
The farad is the SI unit of electrical capacitance. A capacitance of one farad is one for which one coulomb of charge gives rise to a potential difference of one volt.
• Inductance H
Inductance represents an electronic component's ability to resist changes in current. It does this because a magnetic field is built up around the conductor, and this takes energy to set up, and once the energy is there, this energy needs to be dissipated if the current flowing is removed.
To increase the inductance, conductors are normally coiled as this gives a considerable increase to the level of inductance.
In terms of visualising the effect of inductance it can be imagined as being like a heavy flywheel in a machine that opposes sudden changes in speed.
Inductance is measured in henrys which are denoted H, and in view of the levels of used in electronic circuits the levels are more normally measured in terms of milliHenries, mH or microHenries, µH.
Henry definition:
The Henry the SI unit of inductance, and it is defined as the fact that an electromotive force of one volt is generated in a closed circuit by a uniform rate of change of current of one ampere per second.
• Charge C
The charge is the fundamental unit os the quantity of electrical quantity it represents the amount of electricity carried by an object.
It is used to understand the actual amount of electricity stored, normally in a capacitor.
The level of charge is measured in coulombs. As the charge of an electron is 1.6 x 10-19 C, i.e. very small, it takes 6,250,000,000,000,000,000 electrons to make up 1 coulomb of charge.
Coulomb definition:
The coulomb is the SI unit of electric charge, and it is the quantity of electricity conveyed in one second by a current of one ampere.
• Joules J
Joules are a unit that defines the energy level, and they are less commonly associated with electronic circuits, but nevertheless quite important.
When electrical power is used for a certain time, it delivers a specific amount of energy, measured in joules (J).
Joules definition:
The joules is the SI unit of work or energy. It is equal to the work done by a force of one newton when its point of application moves one metre in the direction of action of the force, equivalent to one 3600th of a watt-hour.
• Kilowatt kWh
This is the unit that is commonly used for electrical billing - electricity consumption is generally measured in terms of kilowatt hours.
It represents the energy consumed when 1 kilowatt (1000 watts) of power is used for one hour.
• Frequency f
The frequency describes how often a specific event repeats itself over a given time. In electronics, it is used to measure the number of cycles per second of an alternating waveform.
The unit of frequency is Hertz abbreviated to Hz and it is equal to one cycle per second, with the usual multipliers of kilo, Mega and Giga being used where needed.
Mains or line power typically has a frequency of 50 or 60 cycles per second, where as the normal audio limit extends up to 20 kHz, i.e. 20 000 Hz (some people can hear higher than this). Radio signals may be several MegaHertz, several millions of Hertz, and some may be a few GigaHertz, GHz.
By understanding these core units, you'll gain a solid foundation for comprehending electrical circuits and how electronic components interact. This knowledge is essential for anyone who wants to delve deeper into the fascinating world of electronics!
Written by Ian Poole .
Experienced electronics engineer and author.
More Basic Electronics Concepts & Tutorials:
Voltage
Current
Power
Resistance
Capacitance
Inductance
Transformers
Decibel, dB
Kirchoff's Laws
Q, quality factor
RF noise
Waveforms
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