# eli5 ac vs dc current and why appliances run on specific ones

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eli5 ac vs dc current and why appliances run on specific ones

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You know how when you blow into a straw, you create positive pressure, but when you suck, you create negative pressure? Voltage is a bit like the pressure, and it can also be positive or negative. The higher the pressure, the higher the voltage.

If it’s constantly positive (or negative), that voltage is just pushing electrical current in one direction. It’s a Direct Current (DC). But if it’s oscillating back and forth between positive and negative pressure, eg, alternating between sucking and blowing, then it becomes an Alternating Current (AC). The conductor (the wire) is kind of like the straw.

DC voltage becomes weaker over long distances, but AC can affect the voltage over very long distances. So, high voltage AC is used to move electrical power from your electricity provider to your home, where it is stepped down to 120 volts or 240 volts AC. This is also known as your “Mains Power.” It is the plug in your wall.

Many older appliances run directly on 120 volts AC. However, this can be dangerous. That 120 volt shock can hurt or burn you, or even kill you if it passes through your heart. Nowadays, many appliances (edit: Not necessarily for reasons I had previously stated. See /u/Nacomment’s comments below.) use a power supply to convert the electricity from Mains AC (120 or 240 volts) down to a much safer and easier to work with DC voltage, typically under 24 volts. This power supply might be a brick that plugs in to the wall, like your phone, tabler or laptop might have, or it might be inside of the device itself, like your TV probably has.

A very common voltage nowadays for devices that do not require a lot of power is 5 volts DC. This is what USB devices run on, and what many internal power supplies convert to. 12 volts is common, too, and is what your car battery and “cigarette lighter” power outlet use. Desktop PCs use a mix of 5 volts and 12 volts.

One more thing… your basic power generator creates AC electricity. This includes the generators at your power company’s power plants. So it starts off as AC, which is convenient for long distance, and eventually gets converted down to DC as it makes its way into parts of your appliances. Though some heavier duty stuff (industrial tools, washers and dryers, etc) I think still mostly operate directly on AC, for that extra bit of “oomf.”

I hope this makes sense or helps.

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Edit: The short answer of why some appliances want more voltage than others is, if a device needs more power, it’s easier to push more electrical power into it at higher voltages.

Edit again: I thank and defer to /u/NameUnavail to correct my inaccuracies here.

we often compare electricity flowing like water flowing in pipes. for DC, the water is just going in one direction. with AC, it’s going in one direction, then the other direction, and does this many times a second

most things actually need DC. computer processors for example, need DC. that’s what the power supply in it does (convert AC from the wall to DC for your computer parts). alarm clock. old filament lightbulbs used AC directly. some large appliances can have AC induction motors. but electronics specifically are all DC.

and you’ll probably want to know why we have both. to keep it simple, you can’t increase or decrease DC voltage directly. with AC, you can make the voltage higher or lower with a transformer. so now your question is probably why we don’t just have DC in power lines. great question! that’s because high voltages are more efficient for sending power long distances, so we crank the voltage of power lines up to hundreds of thousands of volts. closer to cities they get lowered to tens of thousands of volts, and in a neighborhood, transformers (those round tube things at the top of power poles) step it down to 240 volts, and that’s what goes into your house. computer processors and most electronics use low voltages, like less than 5 volts. but to make sending power across the country more efficient, we step the voltage up really high. but since we have to change the voltage a few times before it gets into your house, using AC makes more sense.

you CAN change DC to AC, but it requires complicated electronics to do it. AC just needs a transformer. in the simplest form, it’s a like a ring of iron with wires wrapped around opposite sides. if you have 50 loops of wire on both sides, the voltage coming out is the same as the voltage going in. if you have 100 loops on the input side, and 50 on the output side, and your input voltage is 10volts, your voltage coming out will be 5 volts. the voltage changes with the difference in turns (loops) of wire on the input/output side. for changing DC voltage, you need a chip that chops the voltage as much as half a million times a second, and uses a tiny inductor (it’s like a type of transformer) to change the voltage, and the chip constantly monitors the output voltage and adjusts the frequency to keep the output smooth and stable. they get converted back to DC through a thing called a rectifier, then it goes through a filtering stage that makes the output DC as smooth as possible. they’re much more complicated than a traditional transformer for AC. since the power changes voltages many times between the power plant and your house, it makes the most sense to use AC since a simple transformer is all you need to change the voltage.