[From the last episode: We saw how ACAlternating current (which is also alternating voltage). This is how power is delivered to homes and factories. powerThe rate of energy consumption. For electricity, it’s measured in watts (W). from the wall can be turned into the DCDirect current (which has constant voltage). This is what electronics require as an energy source. power that typical IoTThe Internet of Things. A broad term covering many different applications where "things" are interconnected through the internet. devices need.]
The last post talked about the first thing we need when bringing power to our devices if we supply that power through the wall: converting AC to DC. Of course, if we use a battery, then batteries are automatically DC anyway, so we don’t need to do that conversion step.
But batteries and wall power don’t necessarily have the right 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." that we need. Wall power in particular, if just turned into DC, would still give way too high a voltage. Instead of something like 90 V, we need something closer to 24 or 12 or 9 V. Many of the circuits use a voltage even lower than that, but often those yet-lower voltages are generated in the systemThis is a very generic term for any collection of components that, all together, can do something. Systems can be built from subsystems. Examples are your cell phone; your computer; the radio in your car; anything that seems like a "whole.".
For most of us, the big change from AC to DC and to a voltage we need comes from those bricks we plug into the wall. That provides a much lower voltage, with the system taking care of the rest.
So How Do We Lower Voltage in General?
How we change the voltage depends on whether we’re working with AC or DC. It’s actually more straightforward with AC, since we can use transformers. With a transformer, you take the incoming AC and you wrap the wire around a metal core, which creates a magnet. You then place another wrapped metal core very near it – close enough to feel the magnetic field from the first one.
Source: By BillC at the English-language Wikipedia, CC BY-SA 3.0
Here’s the trick: you change the voltage by changing the number of windings around the cores. If you want to cut the voltage in half, then you have half the windings in the second core as compared to the first one. You can also boost the voltage. To double the voltage – yup, you got it. Have twice as many windings on the second core.
Are you getting something for nothing? Nope. If you double the voltage, you cut the currentThe amount of electrical flow. Measured in amperes or amps (A). in half. Since power is voltage times current, that keeps the power constant. Of course, these things aren’t perfect, so you do lose a little something in translation, but it works pretty well. As an everyday example, that transformer sitting on the power pole outside your house drops an even higher line voltage down to the 110 V (or 220 V, depending on where you live) that you use in your home.
Changing DC Voltage Efficiently Is Harder
If we’re trying to lower DC voltage, there’s actually a really simple way to do it. It’s called a “voltage divider.” In short, you pick a couple resistors and run current through, picking off the voltage at the point between the two resistors. The resistor values determine what that voltage will be. If the two resistors are the same, then you get half the voltage. If you want a quarter of the voltage, then you make the top resistor 3 times bigger than the bottom one. Simple, right?
(Note: the “Z” can be interpreted as a resistor for our purposes.)
Source: By Velociostrich – Own work, CC BY-SA 3.0
Well, that gives you a voltage, but you also need current to power your circuit, so that makes things more complicated. But you could figure it out, so that’s fine as far as it goes.
But here’s the problem: you’re wasting current that’s running through the resistors. It literally just flows to ground. That would be horrible for a battery-powered device. And it simply wastes energy in a wall-powered device.
So, to deal with this, designers have invented clever circuits that can convert voltages much more efficiently. They’re conceptually pretty complicated – way more so than the resistor divider. But they’re very common these days. Look for buck, boost, or buck/boost regulators if you want to do some googling.
Raising the DC Voltage
One other thing: the resistor divider only lets you reduce voltage. You can’t create a voltage that’s higher than the original. But with these other clever techniques, you can. There’s a circuit called a charge pump, and it literally pumps charge into parts of the circuit in a way that boosts the voltage above what you’re starting with.
How it does that is best explained in detail if you have some engineering background, but there is an analogy that can help to understand qualitatively how it works. The little video below shows how we could get some boxes on one conveyor belt to move onto a higher conveyor belt. It’s overly simplistic, but the bit you’ll see about tossing a box up and catching it before it comes completely back down is very much in line with how the charge pump works.
Leave a Reply