> Would be nice if they could just accept direct DC input.
The thing is, lots of appliances do accept DC input (most electronics and probably all inverter-based stuff like washing machines and aircos). If only there was a standard for DC they could adhere to. Then they could add yet another adhesive to proudly proclaim the feat.
>> If only there was a standard for DC they could adhere to.
There are. 12v automotive (13.9v) is one. Then comes 24v, a standard in aviation. There was once a push for 48v in cars so that air conditioners and braking systems could be made all-electric, but it never became widespread.
In any case, neither system appears to have defined a plug standard, which IMO is the real issue as far as supporting end user applications, and where USB and the 12V cigarette lighter plug have been such winners. It's only ever going to be useful for installed applications (RV fridges and the like) if it's something you have to wire in.
You still wouldn’t want to, especially if your house is fairly large. DC transmission line losses are actually pretty big over distances you might think are short.
I'm not sure that's correct. My understanding is that for a given voltage the transmission losses are basically the same (actually possibly slightly better for DC since they don't suffer from the skin effect). The problem comes when you try to get the same power down a low voltage DC circuit which naturally requires high currents.
My understanding is AC is safer at higher voltages than DC - and if you have a choice between 12v DC and 120v AC, the latter needs radically smaller cables.
I haven't seen any truly high quality data on voltage safety, due to the obvious ethical issues with performing properly controlled experiments. It's possible the reason you see 120v in every home but 120v DC almost nowhere is inertia rather than safety.
So, there is a lot of bad, anecdotal information out there about which is worse, AC or DC. The reality is that it _depends on the situation_.
In the case of shock and electrocution DC is _less dangerous_ than low frequency AC (sub ~1kHz). The "let go" currents for DC are several times higher than that of low freq AC, meaning it requires a higher DC voltage to prevent someone from being able to let go. The same is true for the currents where danger of injury and death can occur. DC is still safer than low freq AC.
This has been scientifically tested numerous times in both ethical and non-ethical ways. Here is a paper that shows actual numbers for "let go" currents and dangerous currents vs frequency (from DC up to 10kHz): http://www.wright.edu/~guy.vandegrift/wikifiles/Electric%20s...
In particular look at Fig 3 on page 3 of the above PDF. (One really interesting thing to note in this paper is that women have lower "let go" and dangerous current levels!)
However! There is another factor here where AC can be safer than DC. Fire safety! It is much more difficult to prevent DC from arcing and potentially caused fires than AC. This is because the zero crossover of AC which you mentioned generally causes any arcs to quickly extinguish at lower voltages. DC doesn't cross zero volts and will produce far more arcing at the same voltage.
This is why if you look at the ratings for switches, relays, plugs, etc the DC rating is always much lower than the AC rating.
Why woud AC be safer than DC? AC has a higher peak to peak voltage (factor sqrt(2)), so if it's breakdown voltage related DC would be safer. but in cases dielectric breakdown happens for both AC and DC, I can imagine AC being safer since it regularly crosses zero current, potentially allowing the breakdown plasma to extinguish...
the 12V DC vs 120V AC is an apples oranges comparison, with 12V AC and 120V DC it's the other way around. both would be false comparisons.
why would voltage safety tests be unethical? why would one actually test flesh, instead of the theoretical safety models?
The reason we have AC everywhere is simple, historically it was easier to step up and down with transformers (which don't work with DC), but nowadays DC-DC converters are a solved problem.
I don't know if it's only anecdotal or if there is some actual study to verify it, but the typical reasoning on why AC is "safer" than DC is that AC has a "zero-crossing" point, whereas DC (obviously) does not. Why is this considered "safer" (again, possibly anecdotal)?
Because if you accidentally contact DC at a high enough voltage to shock you, your muscles contract - and stay contracted. AC, on the other hand - at least at the relatively low frequencies typically used (50/60 Hz) - crosses a "zero point" where the voltage is "zero" - and lets your muscles relax - briefly - long enough to be able to move away from the current (or in worst case - ungrip your hands).
Again, I don't know if any study has been done on this potential "mythological" reasoning (I would be surprised if there hasn't) - but that's usually the reasoning given.
> why would voltage safety tests be unethical? why would one actually test flesh, instead of the theoretical safety models?
Well, Edison did it that way because it made for better PR for people to watch criminals or elephants being killed by deadly ac, than to have them read papers on theoretical safety models.
Stepping AC up/down is relatively easy, all you need is a transformer. I'd say the invention of the transistor / IC made it possible for DC. But it took a while to perfect those designs - wall warts changed from transformers in the 1990s?
hmm, mostly mass manufacture, but also very low ON resistance mosfets, very emcheapened "microcontrollers" (yesteryear's microprocessors) with more performance, dedicated SMPS chips, and of course ... china
As others have already noted, cheaper and high-power low-resistance MOSFETs are likely the reason - and that did mostly happen in the 1990s.
Something to look at from that period are hobby-grade RC (radio control) cars. Most used NiCad battery packs, and had some extreme amounts of power behind the motors (which were all brushed DC - BLDC was in the future). These motors pulled a lot of amperage (550 and 750 can styles), which the battery packs could deliver, however, there weren't motor controllers small enough to control that much power.
So instead - pretty much up until the 1990s at some point - hobby RC cars used a "resistor speed controller" - something like this one (also known as a "mechanical speed controller":
Basically it was a multi-tapped high-power resistor (or multiple smaller value high-power resistors) that was tapped in a "variable rheostat" manner with a switch operated by a servo. You would usually have three speeds - high (direct to battery), medium, and low; the resistor would "bleed off" excess current as heat (boy, did they get hot!). Yes, it was inefficient, but it was small, robust, and simple to repair or replace.
Of course, there usually wasn't a "reverse gear" (though I am sure someone hacked something together back then). Most of the time, this wasn't a real issue in the hobby - you spent most of your time going forward.
Such controllers actually have a long history - the earliest electric cars used a similar system (just much larger resistors - usually open coil):
In both cases, switching was done either mechanically, or using large relays or contactors. While it is very inefficient, it is also fairly robust if designed right. Which is why it is still used in a lot of automobiles (though this is rapidly changing with newer models using electronic PWM control) - where?
The AC/heater blower motor! On many cars, there's a "resistor pack" that plugs into the control switch/knob for setting the speed of the blower, and it looks virtually the same as ever - here's one for an older vehicle:
About the only difference is the addition of a heat sink. Newer models from even more recent vehicles don't look much different, and they all work on the same principle. They are usually installed in the blower duct work, so that the air rushing by keeps them cool. Unfortunately, if they are designed improperly, or they don't get enough air cooling (or the fan motor dies) - they can heat up extremely hot and melt or catch the car (ductwork - which is usually plastic) on fire! This is especially true if the fan is on "medium" or "low" speeds and the motor seizes (maximum current draw); high speed wouldn't be a problem because the load would short things out and hopefully a fuse would blow (though - not always - sometimes the "fuse" is the wire itself!). This would cause the resistors to get extremely hot - glowing red even - and can cause a fire. I'm certain more than one automotive fire has started this way.
Today, though, thanks to low cost and highly efficient mosfets - and BLDC motors - more and more cars are implementing true variable speed blowers, and using more efficient motors as well. This comes at a cost of more complexity and (depending on how it's implemented) more difficult to repair/replace control and motor systems, but they tend to be safer, and more efficient (this isn't really an issue with ICE vehicles, but very important on electrics for obvious reasons).
strange that you would call a pair of wires carrying DC a "transmission line".
what makes you think identical lengths of identical cable with the same resistance R powering identical loads R_L will dissipate more heat when carrying DC than AC?
the total resistance of the pair of wires R and the load R_L form voltage divider
in the DC case: P_cable=RI^2
in the AC case: P_cable=R*(I_RMS)^2
they should dissipate the same heat, you may want to brush up:
>For alternating electric current, RMS is equal to the value of the direct current that would produce the same average power dissipation in a resistive load.
and
>Because of their usefulness in carrying out power calculations, listed voltages for power outlets (e.g., 120 V in the USA, or 230 V in Europe) are almost always quoted in RMS values, and not peak values.
The only association with higher resistance losses would be when using low voltages but high currents... and even then the resistance losses would be equally high with low voltage high current AC since the fraction of energy dissipated in the cable versus the load is the same in both cases I^2 R / R_L
correct, but I don't know how large these are typically, an anecdote: I visited a friend who had a lamp socket (with shade) above his desk, and he was complaining about the fluorescent bulb he put in, in comparison with the resistive filament bulbs he usually put in: it was ticking even though the switch in the wall was turned off. At first I suspected a bad contact, but that didn't really make sense. Then I realized what was probably happening: capacitive leakage, so I ask if the socket has 2 switches? sure enough. the distance between the switches was long enough so that the parallel wires embedded in the wall effectively formed a long capacitor, so in both of the off states AC current was flowing through this unintentional capacitor. AC will also have inductive losses yes.
It's ac that has transmission line losses, not dc. DC just has resistive losses, which are the same for ac (rms) and dc, but bigger for low-voltage systems. DC is safer, in most ways, at the same voltage as ac (rms).
So the issue isn't that running your house on dc is less efficient; it's that running your house on 12 volts is less efficient.
The thing is, lots of appliances do accept DC input (most electronics and probably all inverter-based stuff like washing machines and aircos). If only there was a standard for DC they could adhere to. Then they could add yet another adhesive to proudly proclaim the feat.