Basic Electronics: Alternating Current(AC) vs Direct Current(DC)
Current, a flow of electrical charge carriers, is one of the major basic concepts that form the foundations of electronics. In daily life, we notice that there are two ways of current flow, that’s Alternating Current(AC) and Direct Current(DC). Have you ever wondered what are the differences between Alternating Current(AC) and Direct Current(DC)? You think it is very straight forward, is it? DC is the straight line and AC is a periodical line? Of Course no! Let’s see what exactly is alternating current and direct current.
In this blog, I will cover the following topics:
- Basic concepts of AC and DC
- Generating AC and DC
- Generating and Analyzing tools for AC and DC
- Differences between AC and DC
- Ohm’s Law with AC and DC
- Example applications of AC and DC
What is Alternating Current(AC) vs Direct Current(DC)
Alternating Current(AC):
Definition
Alternating current means the flow of electric charge periodically reverses direction.
Generating AC
AC can be generated by a generator consisting of magnets and a loop of wire. The wire is rotated inside of a magnetic field and induces a current along the wire. Then as the loop rotates through 180 degrees the force reverses to give an electric current in the opposite direction along the wire.
Power Sources: Generators in power plants, wind turbines, etc.
AC Waveforms
AC can come in a number of forms, as long as the voltage and current are alternating. There are three common forms of AC including sine wave, square wave and triangle wave. Sine wave AC signal is the most commonly used one. 
Measurements of the AC signal
- Peak value
- Peak-to-peak value
- Average value
- Root mean square(RMS)
In general, we say that American standard household circuits have an effective voltage of about 120 Volts, however, the peak to peak voltage is from -170Volt to +170 Volt. How does this happen? Since the voltage of an AC signal is changing constantly, we use a simpler method called root mean square(RMS) to count it. RMS value can make the calculation of electrical power much easier for the AC signal. It is the square root of the time average of the voltage squared. The value of Vrms of a sinusoidal waveform is V0/√2, equivalently, 0.707*V0. In this example, V0 is 170V thus the RMS value is 120V.
Example Applications of AC
- AC signals are usually used for long-distance transmission to deliver power to houses and offices.
- Less energy is lost in electrical power transmission for high voltages (> 110kV).
- It also uses power electric motors and generators.
- It provides a power source to large appliances like fridge, dishwasher, etc.
- AC can be easily converted from high voltage to low voltage and vice versa with the help of transformers.
Direct Current(DC):
Definition
Unlike the flow in the river, the current can flow steadily without any change, it’s called DC or direct current. Direct current is the unidirectional flow of an electric charge.
Generating DC
DC can be generated in many ways:
- Use a commutator with an electrical generator can produce a DC signal.
- The rectifier is the AC-to-DC converter where it transforms AC input into DC output through reversing the directional flow of current.
- Batteries provide DC, which is generated from a chemical reaction inside the battery.
DC Waveform
Unlike the AC signal, DC is the unidirectional flow of an electric charge, which means the current only flows in one direction. For a DC signal, as long as the direction of the flow remains unchanged, the values of voltage and current can vary. To make it simple, we assume the voltage level is fixed. Therefore, the DC voltage can be stated as :
V(t) = x Volt
where,
x – the amplitude of the voltage, e.g. 9
Note, in reality, if we use batteries as the DC source, the voltage level will decrease with the usage.
Example Applications of DC
Most of the electronic sensors, actuators and the computational device we find on Seeed website use DC as the power source. For instance, Seeeduino Xiao and its accessories. To make it more clear, all devices powered by batteries or USB cables rely on DC power, including:
- Electrical appliances: sensors, displays, actuators
- Cell phones
- Tablets
- Camera
- DC motor — For more information, please refer to Helen’s blog: Choosing the Right Motor for Your Project – DC vs Stepper vs Servo Motors
Comparison between Alternating Current(AC) and Direct Current(DC)
| Alternating Current(AC) | Direct Current(DC) | |
|---|---|---|
| Direction | Bidirectional | Unidirectional |
| The cause of direction of flows of electrons | Rotating magnet along the wire | steady magnetism along the wire |
| Current | Varies with time | constant |
| Long Distance Power Transmission | Yes, transmission loss is small | No, transmission loss is large & hard to create a high voltage signal |
| Passive Parameters | Impedance | Resistance only |
| Frequency | varies | 0 |
| Obtained from | AC generator and mains | Cell or battery, AC-to-DC converter |
| Types | Varies e.g. Sine wave, Square wave and Triangle wave | Pure and pulsating |
| Simplicity | Easier to amplify | Easier to measure |
Ohm’s Law
Ohm’s law is the most essential law of electricity. It states the relationship between the three fundamental electronics quantities: current, voltage and resistance.
Ohm’s Law is defined as:
I = V / R
where:
I = Electrical Current (Amperes, A)
V = Voltage(Volt, V)
R = Resistance(Ohms)
As defined in Ohm’s Law, the electric current(I) is proportional to voltage(V) and inversely proportional to resistance(R). Therefore, if the voltage is increased, the current would increase with the unchanged circuit resistance.
Ohm’s law is valid for both DC and AC circuit, but it is usually applied in the DC circuit. Please note that in an AC circuit composed purely resistive elements, the current and voltage are always in phase with each other.
Generating and Analyzing tools for AC and DC
Oscilloscope
The oscilloscope is one of the most important tools you will use in electronic labs. Oscilloscopes allow you to inspect circuit voltages as they vary over time, and measure about everything you want to know about them, including frequency, peak voltage, average voltage, waveform, etc. DSO Nano v3 is a pocket-size compatible 32bit digital storage oscilloscope with a built-in signal generator. Another option is MiniDSO DS213 Nano 4 Channel 100MSa/s, a 5-track 4-wire universal electronic engineering task digital storage oscilloscope based on the ARM Cortex M3 core.
Power Supply
The power supply aims to provide the electric power to a load. Otii Standard Power Supply and Measure is a small, portable power supply, a current and voltage measurement unit, and a data-acquisition module. Built specifically for developers, Otii solves the ultimate problem of optimizing for low energy consumption in device and application design.
Summary
This tutorial introduces one of the electronics basic concepts, alternating current and direct current. Each of these two currents has its own advantages. To achieve better results, you should consider the purpose and requirements of your project when choosing between using either DC or AC.
Do you have any other basic electronics knowledge that you are interested in, please feel free to let us know in the comments section below!
Recommended reading
- [wiki] Direct Current via Wikipedia
- [wiki] Alternating current via Wikipedia
- [blog] Work from Home: What Do You Need for An Electrical Engineer’s Home Lab by Elaine Wu
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