{"id":840,"date":"2017-07-27T14:15:04","date_gmt":"2017-07-27T18:15:04","guid":{"rendered":"http:\/\/www.circuitcrush.com\/?p=840"},"modified":"2023-11-19T02:52:57","modified_gmt":"2023-11-19T07:52:57","slug":"ac-vs-dc","status":"publish","type":"post","link":"https:\/\/www.circuitcrush.com\/ac-vs-dc\/","title":{"rendered":"AC vs DC: What’s the Difference?"},"content":{"rendered":"

One the most basic things an electronics enthusiast should know is the difference between AC vs DC.<\/p>\n

No, we\u2019re not talking about a hard rock band, we\u2019re talking AC vs DC with regards to electricity.<\/p>\n

Some of you may already know this. Even if you\u2019ve been dabbling in electronics for a while and think you have this figured out, I highly recommend reading this whole post as there are some things that may surprise you.<\/p>\n

One of the easiest ways to explain the differences between AC vs DC is to treat each one separately and delve into some details about each.<\/p>\n

Toward that end, we\u2019ll start with DC first because it\u2019s a bit simpler to explain and most of your creations will use it.<\/p>\n

<\/p>\n

AC vs DC: Direct Current
\n<\/strong><\/h1>\n

Electric current (or a voltage, as voltage and current are proportional) can be classified as direct current (DC) or alternating current (AC).<\/p>\n

Direct current flows in one direction only, while alternating current changes direction periodically.<\/p>\n

In the land of electricity, another word for direction is polarity.<\/p>\n

But what do we really mean by this?<\/p>\n

If we were to plot the voltage of an ideal battery versus time, we\u2019d have a flat line, as you probably already know. With a given DC source, this line can either lie on the positive end of the y-axis or the negative end, but not both.<\/p>\n

To see what I mean, take a gander at the simple pictures below.<\/p>\n

\"Positive<\/p>\n

Figure 1: positive and negative DC voltages<\/em><\/strong><\/p>\n

The red horizontal lines represent DC voltage. On the left, we see a positive DC voltage (it could also represent a current if we labeled the y-axis current instead of voltage).<\/p>\n

Notice how the line lies above the x-axis, which represents time, and how it will never dip below the x-axis — if we assume a steady-state voltage \u2013 even if we drag the x-axis out to infinity.<\/p>\n

However, part (b<\/em>) of the figure shows the voltage lying below the x-axis, which makes it a negative DC voltage.<\/p>\n

In both parts, the line either dwells above or below the x-axis, but never actually crosses the x-axis, even if we watch this very exciting \u201cwaveform\u201d for an infinite amount of time.<\/p>\n

AC vs DC: a DC Surprise
\n<\/strong><\/h2>\n

To most of us, it\u2019s plainly obvious that a flat horizontal line on a graph represents a DC voltage.<\/p>\n

Become the Maker you were born to be. Try Arduino Academy<\/a> for FREE!<\/h3>\n

\"\"<\/p>\n

But what about the figure below? Is this AC or is it DC?<\/p>\n

\"Pulsating<\/p>\n

Figure 2: AC vs DC: is it AC or DC?<\/em><\/strong>\u00a0<\/em><\/strong><\/p>\n

If you said DC, you\u2019re right.<\/p>\n

Figure 2 depicts what\u2019s known as pulsating DC.<\/p>\n

Even though this graph does not depict a flat horizontal line, it does not go negative at any point (assuming it remains steady-state and periodic).<\/p>\n

Pulsating DC varies periodically from some minimal voltage (zero in this case) to some maximum voltage, back to minimum and then repeats.<\/p>\n

By periodic, we mean that the waveform repeats itself at regular, predictable intervals. I\u2019ll spare you the mathematical definition of periodic.<\/p>\n

Remember, pulsating DC does not<\/em> vary from a positive to a negative voltage or vice versa. It stays either positive or negative along the whole x-axis, regardless of the amplitude or wave shape.<\/p>\n

This type of voltage\/current occurs at the output of certain battery chargers, and also at the output of a full-wave bridge rectifier which is part of a circuit that converts AC into DC.<\/p>\n

Remember, whether pulsating or steady (like the flat line) direct current is current that flows in only one direction. Figure 3 below shows this concept in a simple circuit.<\/p>\n

\"AC<\/p>\n

Figure 3: a simple circuit<\/em><\/strong><\/p>\n

Using electron flow (for more about electron flow vs conventional current flow<\/a> see the post), current would flow from the negative terminal of the battery through the resistance to the positive terminal.<\/p>\n

This direction does not change unless we change the polarity of the source. If we reverse the polarity of the battery by flipping it around (negative end now on top), current flows the opposite direction of the arrow.<\/p>\n

Either way, the current (and voltage) is DC because it will never alternate from positive to negative.<\/p>\n

AC vs DC: What is AC?
\n<\/strong><\/h1>\n

Alternating current, or AC, is electric current first in one direction for a period of time and then in the reverse direction for a period of time.<\/p>\n

Direct current can only change in magnitude, but alternating current can change in both magnitude and polarity.<\/p>\n

Most of us are familiar with the AC power supplied to our homes.<\/p>\n

Some of us may be familiar with the alternator in our vehicle that produces power. An alternator produces AC power (hence the name) before it is rectified and put to use to charge the battery and power the vehicle\u2019s accessories.<\/p>\n

The power at the receptacle in your home as well as the power created by the alternator in your vehicle (before rectification) comes in the form of a sine wave.<\/p>\n

Sine is short for sinusoidal which, as we can see from a graph of the mathematical\/trigonometric sine function, describes the shape of the wave.<\/p>\n

The figure below depicts a typical sine wave you\u2019d find at the receptacle in your home.<\/p>\n

 <\/p>\n

\"Typical<\/p>\n

Figure 4: a typical AC sine wave<\/em><\/strong><\/p>\n

 <\/p>\n

The AC power in your home is sinusoidal in nature because it is generated by rotating conductors in a magnetic field. If we take one cycle of the sine wave above and fold the negative half over to the left, we see that the resulting shape is somewhat circular in nature.<\/p>\n

By cycle, we mean one alternation from zero, up to some maximum positive value, back to zero where it crosses the x-axis, then down to some minimum negative, then back to zero again where the cycle ends.<\/p>\n

In figure 4, there is a small \u201cx\u201d where the first cycle ends and the second cycle begins.<\/p>\n

Since we are rotating something in a circular motion (with a generator) to produce AC power, it should be no surprise that the sine wave is shaped the way it is. And, since there are 360 degrees in a circle, each cycle of the sine wave also goes through 360 degrees.<\/p>\n

AC vs DC: More Detail on AC
\n<\/strong><\/h2>\n

We know that alternating current gets its name because it alternates from positive to negative, but what does that really mean?<\/p>\n

During one half-cycle electrons flow in one direction. Then, they switch and flow the opposite direction during the other half-cycle.<\/p>\n

Figure 5 from Introductory DC\/AC Electronics illustrates this.<\/p>\n

 <\/p>\n

\"How<\/p>\n

Figure 5: how alternating current moves<\/em><\/strong><\/p>\n

In part (b) of the figure, we can see a typical sine wave. Part (C) shows how electrons move during the positive alternation and part (d) shows how they move during the negative alternation.<\/p>\n

So, electrons in an AC circuit will move back and forth about a fixed point.<\/p>\n

To clarify, consider the simple circuit in figure 3, only there is an AC source there instead of a battery.<\/p>\n

Now, imagine that you\u2019re very small and can \u201csit\u201d in the middle of the resistor and see electrons flowing by.<\/p>\n

For one instant, they pass you in one direction, in another instant, the opposite direction.<\/p>\n

In fact, if you counted the number of electrons that slip by you during each alteration, you\u2019d see that not only is it the same number each direction, but if you could tell each individual one apart from others, the same electrons would keep passing you — first in one direction, then the other.<\/p>\n

Still want more clarification? This fantastic video from Khurram Tanvir should do the trick.<\/p>\n

 <\/p>\n