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Why do home appliances like a light, TV, and fan use AC power directly from the supply, but a phone and laptop use an adapter to convert AC to DC?

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    \$\begingroup\$ All modern TVs have internal ac->dc adapter. Same goes for most LED lights and modern CFL lights. \$\endgroup\$ – Agent_L Jan 5 '18 at 8:59
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    \$\begingroup\$ @Agent_L Please don't answer questions in a comment. \$\endgroup\$ – pipe Jan 5 '18 at 16:07
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    \$\begingroup\$ In other words, your question actually is: Why do some appliances use external power supplies while others use internal power supplies? \$\endgroup\$ – Dave Tweed Jan 5 '18 at 16:52
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This is mostly due to the line voltage being "high" compared to what normal electronics needs, and what is safe.

Devices that use significant power, like lights and fans, are built to run directly off the power line voltage. The cost of protecting the user from the higher voltage is offset by the more efficient use of the power and not having to convert it.

Devices with electronics in them need low voltage to run the electronics. This means a extra power supply, which converts the high line voltage to the low voltage needed by the electronics. It also converts to DC, because the electronics need DC to operate.

Given that there is this converter between the line voltage and the actual internals of the device, manufacturers have a choice about where to put it. In some cases, it makes sense to put this converter external to the device. That alleviates the need to protect the user from high voltage at the device. I go into more detail here.

Some electronic devices are large enough that it makes sense to put the power supply inside. Your TV example is in this category. It's still a electronic device that internally runs on low voltage DC, but it's big enough that the power supply is internal.

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    \$\begingroup\$ This seems a circular answer to the question, which in spirit seems to be why some devices can run off higher-voltage AC and other devices require lower-voltage DC. \$\endgroup\$ – Beanluc Jan 5 '18 at 0:27
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    \$\begingroup\$ Things like replacement LED lights for incandescent fittings tend to include circuitry for transforming to low voltage DC even though the incandescent lamps they replace worked directly from mains AC. \$\endgroup\$ – tom r. Jan 5 '18 at 0:49
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    \$\begingroup\$ There's plenty of small TVs that run off external power packs (line voltage => 12VDC <3A) \$\endgroup\$ – Zac Faragher Jan 5 '18 at 6:51
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You have to start from "why is the voltage of power network is 110VAC/220VAC". So it's AC because this way it's easy to convert voltage by transformers or to move motors (AC). Both reasons were there long before electronics came around.

For electronics, like TV, laptop, whatever, you will always need DC, and significantly lower than 160V/315V (rectified AC). This is because how semiconductors work- unlike vacuum tubes (which i have no idea about, i am not so old).

So why do we have sometime power supply inside a device and sometimes outside? It's because application considerations. Laptop- you don't want to have a power supply inside when you are on the move. So you use something external, that you can leave at home.

For TV it doesn't matter, so normally it's better for the customer to have it inside. Although sometimes for extremely slim devices the power supply will still be external.

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    \$\begingroup\$ A lot of appliances now use external power supplies just because it's easier to certify them that way. Buy an off-the-shelf adaptor from a well-known supplier, then everything inside your appliance is safe (extra) low voltage DC. \$\endgroup\$ – Simon B Jan 4 '18 at 17:01
  • \$\begingroup\$ Yes. I would fold that behind "depending on application". Size is just an example. It could also be cost or time to market. \$\endgroup\$ – Gregory Kornblum Jan 4 '18 at 17:25
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    \$\begingroup\$ FWIW, old tube sets worked several different ways. The cheaper, simpler ones had a rectifier tube to convert the 120VAC to DC, to power the "logic", such as it were, then had the filaments wired in series so that together they had enough resistance to draw the right heater current. Fancier ones used a transformer to produce 6.3V for filaments/heaters and anywhere up to 300V to be rectified to serve as the "B" supply. Sometimes there would be other voltages for "bias", et al. \$\endgroup\$ – Hot Licks Jan 5 '18 at 1:37
  • \$\begingroup\$ @HotLicks, In other words, tube sets run on DC too. Although, in some cases, the DC voltage was higher than the AC mains voltage: I used to play with a 900V "B" transformer that the ham radio operator who lived next door gave to me when I was a kid. \$\endgroup\$ – Solomon Slow Jan 5 '18 at 15:43
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I have a feeling that even though most of the explanations given here are mostly correct, they're not really answering what the OP asked. Which is funny, because I think the explanation should only be a few sentences long.

The main reason AC comes out of your outlet, is because it is very inefficiënt to transport DC over long distances (and especially low voltage DC). (Besides that: the electricity is also AC when it is generated, for most sources at least.) When it comes in your home, it is pretty much always converted to DC before use, because almost all modern devices need DC. The ones that need some kind of AC will generate their own, and not use the AC supplied to it from the outlet.

There are only a few exceptions. (I can think of only 3 for the moment, but there's probably more):

  • motors: most (large) elektromotors run on AC, so they can use it straight away from the outlet.

  • incandescant lamps: they require heat to make the lamp glow. Wheter that heat comes from AC or DC doesn't matter, so there's no reason to convert it.

  • heating elements: same as for incandescant lamps

Your TV is not one of the exceptions, because it converts to DC internally, same as your laptop does with an external adapter.

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    \$\begingroup\$ No. DC is more efficient to transfer over long distances, that's why world's longest lines are HVDC. DC is difficult to convert between high and low, that's the reason why lines are AC. \$\endgroup\$ – Agent_L Jan 5 '18 at 9:03
  • \$\begingroup\$ @Agent_L Good Point! Like I said: there are many exceptions in the world of electricity. We use AC for transport of middle and high voltages. We try to make the voltage as high as possible, so there will be as little current as possible to cause heatlosses. However, if we want to make a very long line (i.e. across several nations), and thus crank up the voltage even more, we will start to notice the capacitive and inductive properties of the cable causing significant losses. To avoid this, they use HVDC. But for short distances, AC is the clear winner. (Typed on mobile, sorry for typos) \$\endgroup\$ – Opifex Jan 5 '18 at 11:53
  • \$\begingroup\$ Capacitive and inductive properties are felt even on relatively short distances. \$\endgroup\$ – Gregory Kornblum Jan 5 '18 at 12:13
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    \$\begingroup\$ @Agent_L "transfer" doesn't just mean from one end of a power line to the other. It means, all the way from the generating station to the outlets in your home. And you might note that only the worlds longest transmission lines use DC. The line must be very long before the efficiency of transmitting DC matters enough to outweigh the enormous costs of the two end stations that must convert between high voltage DC and lower voltage AC. \$\endgroup\$ – Solomon Slow Jan 5 '18 at 15:49
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Another reason for having a separate AC to DC converter is Safety. Devices like laptops and mobiles are typically meant to be handled by the user with close contact to the skin. A small fault with an internal AC to DC converter can be very dangerous.

The easiest solution is to have the line voltage handling outside and far away from the device with proper isolation in the adapter. The device only sees the low voltage DC.

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    \$\begingroup\$ It's not safety. Building a safe power supply inside an electronic device is no harder than building it in a separate box of its own. The real problem is regulatory approval. Most countries prohibit sales of any device that accepts mains power unless the design has been extensively tested. That's the whole design: Change the design of a TV with a built-in power supply, and you have to re-do all the safety testing. But if it uses an external power brick, then only the power brick needs to be tested. You can change the design of the TV at will. \$\endgroup\$ – Solomon Slow Jan 5 '18 at 15:59
  • \$\begingroup\$ @jameslarge: But the regulatory approvals are all about safety, so Tejas is not wrong about that. \$\endgroup\$ – Dave Tweed Jan 5 '18 at 17:45
  • \$\begingroup\$ @DaveTweed, My point is, having an external power brick does not, in and of itself, make a product any safer than an equivalent product with a built-in power supply. \$\endgroup\$ – Solomon Slow Jan 5 '18 at 18:18
  • \$\begingroup\$ @jameslarge: But it makes it easier to get those safety approvals. \$\endgroup\$ – Dave Tweed Jan 5 '18 at 18:39
  • \$\begingroup\$ @DaveTweed Think in terms of having 240VAC on your lap vs 12VDC. Also consider that many mordern laptops(Macs!) have conductive Aluminium bodies. \$\endgroup\$ – Tejas Kale Jan 5 '18 at 21:39
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The home is supplied with AC because regular pole transformers and their bigger cousins require AC, and most early home electrical devices didn't care whether they got AC or DC. And for a number of other reasons it's easier to deal with AC in the distribution process.

Pretty much all devices that process audio or video (including old tube-type radios and TVs) use DC internally for most of their operations. But converting from AC to DC on a small scale is not particularly difficult.

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  • \$\begingroup\$ "and most early home electrical devices didn't care whether they got AC or DC" Dangerous statement. What did you have in mind? \$\endgroup\$ – Mast Jan 5 '18 at 9:27
  • \$\begingroup\$ @Mast - I have in mind the "120V AC/DC" rating stamped on the devices. An incandescent light doesn't care. A toaster doesn't care. A commuter-style motor doesn't care. Even many old tube radios don't care. Induction motors were probably the biggest exception, but they arrived fairly late in the game. \$\endgroup\$ – Hot Licks Jan 5 '18 at 13:24
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Part of the reason is historical. Voltage conversion used to involve much larger and more expensive hardware than it does today, and many electrical gadgets used to require higher voltage than they do today. White LEDs, which are the first general-purpose electric lamp design that doesn't need a high voltage (either for efficiency or to make it work at all), only came on the market circa 2010. Linear "wall warts" were still commonplace in 2000, and so were cathode-ray-tube TVs. Radios and TVs that used vacuum tubes internally (perhaps alongside transistors) were still commonplace in the early 1980s.

Therefore, older uses of electricity, such as lamps and household appliances, tend to have been designed to run directly on the line voltage, both because availability of line voltage was more useful to them, and because it was cheaper to do it that way.

Any household device that you purchase today, that still runs directly on the line voltage, probably has a high-power motor or heater inside; those are the applications for which it's still more efficient to use line voltage directly. LED light bulbs are a special case: it would make a certain amount of sense to feed them 12VDC or so, but because houses already have all the wiring and lamp hardware set up for line-voltage lighting, they have embedded power converters instead.

(Question for the peanut gallery: What voltages are used internally inside current-generation consumer-grade audio gear? Those do also tend to take a line-voltage power input, but that might just be the "we can fit the converter inside the case" scenario mentioned in other answers. I don't know one way or the other.)

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    \$\begingroup\$ All vacuum tube radios and televisions still manufactured in 1980 ? Are you serious ? \$\endgroup\$ – Marla Jan 4 '18 at 19:18
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    \$\begingroup\$ Didn't misread anything. Radios and television didn't use tubes in 1980. Except for CRT.@jsotola \$\endgroup\$ – Marla Jan 4 '18 at 19:41
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    \$\begingroup\$ Tubes can still be bought. But manufacturing tube televisions and radios had long ended by 1980 \$\endgroup\$ – Marla Jan 4 '18 at 20:28
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    \$\begingroup\$ Just change your answer to 60 or 70, and all is well. videokarma.org/showthread.php?t=252066 \$\endgroup\$ – Marla Jan 4 '18 at 20:38
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    \$\begingroup\$ @LorenzoDonati: Televisions that used tubes (e.g., cathode ray tubes--CRTs for the display) remained common until around 2000 or so. \$\endgroup\$ – Jerry Coffin Jan 5 '18 at 0:50
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Fluorescent and incandescent lights can run off of AC directly basically because they're electrically simple (they can actually run off AC or DC without much modification) and they're designed to allow high voltages. You can take a slow-motion video of them and you'll see that they actually turn on and off as the current oscillates.

Motors like the one in your fan can also run off AC because it isn't much trouble to design a motor that can work this way.

LED lights and TVs actually convert to DC internally, using circuitry similar to the power brick for your laptop. The primary reasons your portable electronics have a separate AC-DC converter are heat being uncomfortable in something you're holding and portability, which aren't issues with the TV and LED light.

As to why designers choose to use DC instead of AC, there are many reasons. For applications like your phone and laptop that need to run off batteries; they'd need to add an inverter to convert from DC to AC that would add heat and weight to the product that is supposed to be portable. Also, essentially all digital circuitry runs off DC and it would be incredibly expensive if not impossible to design a phone or TV from scratch that only uses AC power. As with all design questions a primary motivator is cost: is it cheaper to design my motor to accept AC or to put in a transformer to convert from AC to DC?

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  • \$\begingroup\$ Actually, conventional fluorescent lights MUST be supplied with AC. DC would cause them to blow up. \$\endgroup\$ – Hot Licks Jan 5 '18 at 1:39
  • \$\begingroup\$ And pretty much all equipment (even tube equipment) that processes sound or images or similar signals runs off of DC. Always has. \$\endgroup\$ – Hot Licks Jan 5 '18 at 1:41
  • \$\begingroup\$ Modern fluorescent lamps convert 220V 50Hz to ~300VDC and then back to AC, this time in kHz range. It isn't much trouble to design some motors on AC, but it actually is trouble to design others. Eg. drills and vacuum cleaners have DC motors that run off AC. Not without problems. \$\endgroup\$ – Agent_L Jan 5 '18 at 9:18
  • \$\begingroup\$ Well, DC motor is actually an AC motor that is just making its own AC with commutation, though. \$\endgroup\$ – MrGerber Jan 5 '18 at 9:48
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The main reason is heat. Small devices/appliances have a harder time dissipating heat from AC to DC conversion. By placing AC/DC converter outside the chassis small devices/appliances move the source of heat away from the other components and reduce cost by not having to deal with it inside the chassis.

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  • \$\begingroup\$ Often the other main reason is space. A cell phone has an external charger because a cell phone is ultra dense with electronics plus a battery. Do a little thinking before posting a hasty answer, or expect few votes if any. \$\endgroup\$ – Sparky256 Jan 4 '18 at 22:34
  • \$\begingroup\$ I'd say that the main reasons for external power supplies on portable equipment would be space and weight, with heat being a minor concerrn (how hot does your iPhone charger get?). \$\endgroup\$ – Peter Bennett Jan 4 '18 at 22:42

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