Electronics 101: Lesson-03 AC Vs DC

AC stands for “Alternating Current” which describes voltage or current that changes polarity or direction respectively over time whereas DC stands for “Direct Current” that maintains constant polarity or direction respectively over time.

AC is the form in which electric power is delivered to households and businesses and it’s what consumers typically use when they plug their home electric appliances for-example television sets, electric lamps/bulbs, chargers into a wall socket. A common source of DC power is a battery cell.

Take a look at the previous Tutorials…

Lesson 1: Electricity and how it works

Lesson 2Circuits basics – Open Vs Closed

Waveforms:

The most common type of AC is the sine wave. The AC we use has an oscillating voltage that produces a sine wave. AC also comes in a number of other forms for as long as the voltage and current are alternating for-example if we hookup an oscilloscope to a circuit with AC and plot the voltage against time, we get to see a number of different waveforms and these include:

  1. The Sine Wave:

Other common forms of AC include the Square wave and the Triangular wave.

  1. The Square wave:

This is often used in digital and switching electronics to test their operation.

  1. The triangular wave:

This is applied in sound synthesis for testing linear electronics like Amplifiers

 

Describing a Sine Wave:

1 – Amplitude (Peak)

2 – Peak-to-Peak

3 – Period

Upon describing an AC waveform in a mathematical term, we use the common sine wave. There are three parts to a sine wave i.e. Amplitude, Phase & Frequency.

So looking at voltage only, we can describe a sine wave as the mathematical function.

V(t) = VpSine(2πft + Φ)    ……………… (i)

  ↑                     ↑

(Part 1)        (Part 2)

Where  2πf = ω         ………………….. (ii)

Part 1,

{V(t)} – represents the voltage as a function of time, which means that our voltage changes over time.

Part 2,

{VpSine(2πft + Φ) } – describes how voltage changes over time.

Vp – is the Amplitude and it describes the maximum voltage that our sine wave can reach in either direction i.e. voltage can be +Vp (volts) or –Vp (volts) or somewhere in between.

Sine() – This function indicates that our voltage will be in the form of a periodic sine wave which is a smooth oscillation around 0v.

ω  – This is the angular frequency (rad/sec). This is related to the physical frequency f (Hz) by the equation, .

t – This represents the time and it is the dependent variable measured in seconds. As time varies, the waveform also varies.

Φ This is the phase of the sine wave and it is the measure of how shifted the waveform is with-respect-to time. It is often given as a number between 0º and 360º. For this tutorial, we’ll assume that our phase is zero and therefore we’ll have our voltage equation as

V(t) = VpSine(2πft) …………………………… (iii)

For-example, the power provided in Uganda to our homes is AC with about 230V, zero to peak (Amplitude) and 60Hz frequency. So upon plugging these figures in our equation, we get

V(t) = 230Sine(2π60t) ……………………. (iv)

How to generate AC:

AC can be produced using a special type of electrical generator called an alternator that is designed to produce an alternating current.

Consider a simple alternator with a rotating magnetic core (rotor) and a stationery wire (stator) with current induced in the stator by the rotating magnetic field of the rotor as shown in the figure below:

A conductor moving relative to a magnetic field develops an electromotive force (EMF) in it. This EMF reverses its polarity when it moves under magnetic poles of opposite polarity.

The rotating magnet (rotor) turns within a stationery set of conductors wound in coils on an iron core (stator), the field cuts across the conductors, generating an induced EMF as the mechanical input causes the rotor to turn. The rotating magnetic field induces an AC voltage in the Stator windings.

Applications of AC:

  • Home and office outlets are always AC.
  • Powering electric motors.

Direct Current (DC):

This is the unidirectional flow of electric charge. DC may flow in a conductor such as a wire, but can also flow through semiconductors, insulators or even through a vacuum.

How to generate DC

Direct current can be generated in a number of ways:

A battery is a good example of DC power supply which is generated from a chemical reaction inside of a battery.

Use of a device called a Rectifier that converts AC to DC. This device contains electronic elements or electro-mechanical elements that allow current to flow only in one direction.

DC can be converted into an AC and vice versa with an inverter.

An AC generator equipped with a device called a commutator can produce DC.

Describing DC

Consider the water analogy below, DC is similar to a tank of water with a horse pipe at the end.

Here the tank can only push the water one way out the horse pipe and once the tank is empty, water will no longer flow through the pipe.

Similarly, a battery producing DC current, produces voltage and current in one direction. Here, voltage and current can vary over time so long as the direction of flow does not change.

For-example:

Consider a Duracell battery providing 85v, this can be described in mathematical terms as V(t) = 85v, assuming that voltage is constant.

If we plot this over time, we see a constant voltage.

What the graph above means is that, most DC sources provide a constant voltage over time, but in reality a battery will slowly lose its charge that is the voltage will drop as the battery is used. For the most part of this tutorial, we’ll assume that the voltage is constant.

Applications of DC:

At most, all electronic projects and parts we sale on www.sonalabs.org run on DC. That is everything powered by a USB cable, an AC adapter or a battery relies on DC. Examples of DC electronics include:

  • Cell phones
  • Hybrid & Electric vehicles
  • Laptop computer (Use an AC adapter that coverts AC to DC)
  • Flat screen Television (AC goes into TV which is converted to DC).

Conclusion:

At this point, you should have a good understanding of the differences between AC & DC. AC is easier to transform between voltage levels, which makes high voltage transmissions more feasible. DC on the other hand is found in almost all electronics.

One should also note that the two do not mix well and you’ll need to transform AC to DC if you wish to power your electronics off a wall outlet. With this understanding, you should be ready to tackle some more complex circuitry concepts.

In the next chapter we are going to learn about Voltage, Resistance, Current and Ohm’s Law.

 

 

Tum Kurtzman
Author: Tum Kurtzman

Computer Engineer, Ugandan Life Hacker, Tech Blogger, YouTuber, Founder & Lead Engineer @ SonaLabs.....

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