[From the last episode: We saw that lower powerThe rate of energy consumption. For electricity, it’s measured in watts (W). doesn’t always mean lower energy consumption.]
We’re going to start winding down our coverage of energy concepts in the IoTThe Internet of Things. A broad term covering many different applications where "things" are interconnected through the internet. by looking at sources of energy. And in order to do that, we have to address the two main categories of energy source that IoT devices will use: AC and DC. For those of you with some electrical background, this will probably be review.
Mains Power
AC stands for alternating currentAlternating current (which is also alternating voltage). This is how power is delivered to homes and factories.. But you could think of it as alternating voltage instead. In fact, it’s both. The power oscillates up and down as a sine wave from positive to negative to positive. How fast it goes depends on where you live. For example, North America uses 60 Hz – that is, 60 cycles per second, or 60 oscillations per second. Europe uses 50 Hz.
This is the type of supply that we get from plugs in the wall. Long ago, it was decided to use AC as a way of transferring electricity over long distances between where it’s generated and where used. Not only that, generators naturally tend to create AC. Motors can naturally work well with AC. So, when you plug something into your wall at home, it will be AC coming out of the wall. It will often be called wall power or mains power.
The voltage that comes out of the wall also varies by region. In North America, it’s 110 volts (plus or minus). In Europe, it’s 220 volts. But here’s an important thing: the voltage changes as the power is being delivered. Transformers can change the voltage, so, for any random power line you see, you won’t really know what the voltage it uses. It’s almost guaranteed that it will be higher than what you get out of the wall.
Household gadgets that will plug into the wall will need to be able to handle AC power. But, as we’ll see, many of the gadgets can’t actually use AC power. AC is great for motors, but not usually for computer chipsAn electronic device made on a piece of silicon. These days, it could also involve a mechanical chip, but, to the outside world, everything looks electronic. The chip is usually in some kind of package; that package might contain multiple chips. "Integrated circuit," and "IC" mean the same thing, but refer only to electronic chips, not mechanical chips.. So that means converting from one to another – a story for another day. The “bricks” we plug into the wall to power our gadgets usually convert AC to DC.
Battery Power
So what is DC then? It stands for direct currentDirect current (which has constant voltage). This is what electronics require as an energy source.. But, again, you could think of it as direct voltage. It’s simply a constant voltage that powers something. Batteries work like this: they don’t have oscillating power; it’s just a simple voltage. Some alternative energy sources, like solar, also generate DC instead of AC.
So… why is there a difference? Why don’t we use just one and call it good? Part of it is historical. There was a longstanding battle as the US looked to build up its electric infrastructure. Edison lobbied for DC; others lobbied for AC. AC won out.
Many of the things that used to run on electricity also did well with AC. We’ve mentioned motors, but also old-fashioned light bulbs do just fine with AC. Back in the day, there were no computer chips.
Today’s computer chips run on DC. And the voltagesVoltage is what gets electrons to flow. It's analogous to water pressure, which gets water to flow. Voltage is measured in units of "volts." are usually far smaller than the 110 V we get from the wall. Think 5 volts or less. So times have changed. But it’s not practical to rethink all of our infrastructure.
Moving Power from Here to There
In addition, AC works way better for distribution to homes and factories. You can change the voltages very easily in ways that don’t work for DC. Why change voltages? To lower losses. “Loss” here means energy lost along the way from here to there. That lost energy doesn’t do anyone any good.
Where that loss comes from is easier to describe for DC. Let’s say we have a wire transporting DC from one place to another. The wire it travels through has some resistanceForces that tend to reduce the amount of flow or current. Measured in ohms (Ω).. Not a lot, since it’s a good conductorA material through which electricity can flow. Metals are a good, familiar example., but the resistance isn’t zero. So, as the currentThe amount of electrical flow. Measured in amperes or amps (A). flows along, that resistance makes the wire heat up a little bit, and that heat is electric energy converted to heat energy and wasted.
AC is more complicated, since there are other notions (like capacitanceCapacitance is the ability of an electron or hole to “feel” other electrons or holes on the other side of a small gap. The gap prevents actual current from flowing, but, if small enough, electrons and holes can pile up on either side of the gap. Specific devices that make use of this are called capacitors. and inductance) that contribute to loss. We won’t get into the details. All in all, the lower the current, the lower the loss.
Converting AC from one voltage to another comes with a trick: the power (ideally) stays the same. So if you double the voltage, you cut the current in half in order to keep the power the same. So the higher you boost the voltage, the lower the current becomes. And that helps lower losses. As we’ll see, AC is easier to convert between voltages.
While DC doesn’t work well for distributing power, it can be used for simply transporting power from one place to another. But it’s not easy or cheap, and so it’s done very selectively. The original AC from the generator is converted to DC at a high voltage. That goes from one point to another (usually over a long distance) before being converted back to AC for distribution. So it’s out there, but the familiar power lines you’re used to seeing are overwhelmingly AC.
This whole notion of converting from one to the other, or changing voltages, forms the basis for our next two blog episodes.
Leave a Reply