Thunderstruck: AC/DC

ByMike Moffit

I feel like I’m definitely on a roll with these articles about electricity, and I think people have found them very informative. So until I run out of topics that can be discussed in a short format, I think I’ll keep going. One thing that came up in my first three articles was that I was mostly talking about things from the perspective of direct current (abbreviated DC). Most people are (or should be) pretty familiar with DC since it’s what powers all of our little electronic gizmos. iPhones, MacBooks, WiFi access point hardware and more…it all runs on DC.

This of course is in contrast to alternating current (abbreviated AC). We’re more familiar with AC in our homes where it powers pretty much everything including refrigerators, fans, toasters, most lights and much more. When we need to charge a device like an iPhone or MacBook, we always need to convert the AC from the wall into the DC that’s used by the device. To do that, two things need to happen. First, we have to drop the voltage from 120VAC to the 5VDC that’s used by most small portable electronics that charge via USB (MacBooks will take more). Second, we need to make the alternating current direct. This is accomplished most commonly by transformers and rectifiers.

But why do we even use AC at all? First of all, for most power generation techniques that use motion (steam turbines, hydro turbines, etc) the current they generate will be alternating by definition. The mechanics of this are sort of complicated but it’s based on the fact that the generator is spinning. Couldn’t we simply generate as AC and then convert to DC before sending it off to homes for use? We could, and in the early days of electricity generation and transmission, that was a big debate. We don’t do that though, and there’s a very good reason why.

As we’ve learned I = V / R. Power transmission over distances is not exempt from this rule. Even wires used for transmission have some resistance, and over long distances, that resistance can be substantial. At this point I have to introduce the equation P = I*V. This means that power (usually measured in watts) is equal to the current times the voltage. What this means is that the same amount of power can be delivered at a lower current if we increase the voltage. Less current flowing in the transmission lines will mean less voltage drop from resistance when the power gets to its destination.

Still with me? The key thing here is that using a higher voltage to transmit the power means voltage will have to be stepped up and down during the transmission process. Changing DC voltage is very difficult and often not very efficient. Changing AC voltage is almost trivial using transformers. Transformers have an input and an output, usually called primary and secondary windings. The windings are connected by some type of metal core but do not actually touch each other. When an alternating current is passed through the primary winding, this creates a changing magnetic field that is captured and concentrated by the core. This changing magnetic field is then carried via the core to the secondary winding where it induces a current. How much the voltage is stepped up or down depends on how many times the windings wrap around the core. It was a popular “hack” on the internet a while ago to disassemble microwaves and rewire their transformers. By rewinding the secondary winding with thick, heavy gauge wire, you can create very high currents (with very little volts) from regular 120VAC household power.

It’s because AC voltage is so easy to transform that we use it for power transmission. Though I should note that more recently, technology has improved such that high voltage DC transmission (HVDC) is sometimes a viable solution. This of course requires a complicated facility on either end to step the voltage up and down. I hope you’ll join me again next week. As you can see, discussing AC significantly complicates things, but I haven’t even gotten into the basics about phases, sine waves and root-means squares. Hopefully I’ll be able to condense some of that stuff out and make it accessible.

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