alternating current (ac) vs direct current (dc)

Keywords

Circuit, multimeter, oscilloscope, volts, potential difference, amp, amperes, alternating current, direct current. volts, hertz, ammeter, voltmeter, ohmmeter.

Introduction

"I was caught, In the middle of a railroad track (Thunder). I looked 'round, And I knew there was no turning back (Thunder)..." ~ AC/DC, Thunderstruck, 1990.

As an iconic band named themselves after the power types, in the world of physics, ac and dc sit right in the centre of the modern world. It powers the device you're using to read this article, and most modern devices like laptops and mobile phones use them both.

alternating current (ac)

Alternating current has it's history as far back as 1832, when Michael Faraday tested it using a Dynamo Electric Generator. This was one of the first uses of alternating current. Now, we use dynamos for all sorts - but their most common use is on a bicycle to light your lights at night.

When looking at electrical currents on an oscilloscope, if a current changes it's direction often, it is referred to as alternating current. Our electric supply is measured at 230V (volts) and it has a frequency of 50Hz (Hertz). This means that the current peaks 50 times a second. Your laptop screen (by default) is set up to refresh at 60Hz, so the peaks are 60 every second.

Interesting fact: although we are all used to it, we shouldn't capitalise the abbreviations for AC and DC. They should be ac and dc.

direct current (dc)

Direct current has it's history slightly earlier of that of alternating current. It was first properly used by Alessandro Volta, who created a voltaic pile that was

Direct current is the opposite of, if you like, of alternating current. It has it's ducks all lined in one basket, so to speak. By this, I mean that it's electrical flow is in one direction. Batteries and solar cells supply dc electricity. So, if you're on a mobile phone or laptop, that is powered by dc current.

A typical battery (AA, for example) provides just 1.5V of power. They are typically used in smaller devices like an alarm clock, or for your TV remote. It provides just enough power to source the device and enable its functions.

So, when does your laptop, or mobile phone, use alternating current? When you plug it in to charge. You see that little cord you use on your laptop? That is alternating current from the plug you put in the wall. How does it change to direct current then? As we know that batteries run on direct current, we need something that converts the power. You have a little black box along that laptop charging cable - it holds a converter, which convertes from ac to dc power.

Take a look at this YouTube video from Cognito about ac/dc:

Multimeter

Using a multimeter allows us to measure current, voltage and resistance, and some have other capabilities for testing other parameters. Allowing you to measure so much (some also allow for other components like temperature and infrared), enables you to have just one piece of apparatus with you for lots of different issues.

They also come in two formats - analog and digital. Analog display readings on a display with a needle, whereas a digital one will show you with an LCD or LED screen. Digital meters are usually more precise and can give exact readings to a decimal point.

End Note

We have learnt that there are two types of power current - alternating current (ac) and direct current (dc), and that they both have their uses. We can read them using multimeters that can test for issues when needed. We also learnt that oscilloscopes are a way to visually see the current within a wire (for example), and that the current can work at different frequencies.