Bring 12V DC Home: An Idea to Reduce Transformer Clutter

[WarrickF]

Another interesting point on home electronics raised was - why does every device you purchase have it's own transformer. Would it not have made sense to implement a series of 12V DC plugs in modern homes?

That way minor electronic internals could shape it as they want and we would not all be carrying these clunky transformers with us.

Just a thought.

Thanks
Warrick

 

[chroot]

It would be a brilliant idea, in fact. Maybe a bit higher than 12V, though -- 18V would be better.

Microelectronics didn't exist when the 60 Hz AC system was invented; back then homes mostly had only motors and resistive-heating devices.

- Warren

 

[DaveC426913]

Nope.

There is an excellent reason why they use AC in homes. It is way safer. Electrocution from DC is much more deadly than from AC.

 

[chroot]

Uh, Dave... 12 VDC won't do anything to you at all.

Voltages lower than about 50 V are not harmful to people.

- Warren

 

[Delta]

Are you infering that the house is supplied with 12V dc?, or that plugs be adapted to carry transformers.

On the 1st issue, kettles, cookers, etc. would not be able to function, and the power supply has become an international standard. This change would cause a lot of agro.

If you are suggesting just changing the plug itself, isn't this just as simple as having a transformer in the appliance?

 

[chroot]

I assumed he meant an additional plug, above the 120VAC plug, which supplied 12VDC from a large transformer which supplied the entire house.

- Warren

 

[russ_watters]

Are we also assuming that the myriad of DC devices that exist and use about 20 different voltages would all start coming at 12v...?

 

[govinda]

dave,
why is ac safer than dc?

 

[Integral]

It is current which is fatal, not voltage, so if you get ~50ma through your heart your life will be in danger, whether the current is generated by 120VAC or 12VDC. So the question is can 12VDC generate 50ma through the heart. Certinally not through dry or even wet skin, but what if your skin is broken, if you were to get 12VDC under the skin and into the blood stream I would think that there exists a good chance that the blood would carry sufficient current to cause some troubles. This seems pretty unlikely, so I do not think safety is a reason not to do this.

The best reason I can come up with is cost. Specialized wiring is not cheap. Losses, It would be difficult to maintain a consistent voltage thought the system. As was suggested above, a higher voltage would be better. I would think that 24VDC would be even better, it is a industry standard so there are many 24VDC devices on the market and that voltage gives some head room for regulation to the needed voltage. This of course is the key problem. There is no industry standard Voltage for running small electronics devices.
So you would need a regulator for each device you plugged in, where is the gain? You still need a special plug for each device, and until every house hold in the developed world has a DC distribution you would still need to supply a
AC-DC conversion.

Seems simplist just to assume a AC source.

 

[Integral]

dave,
why is ac safer than dc?

AC goes to 0V twice every cycle. so it "lets go" giving you an opportunity to free yourself , DC never let's go.

 

[chroot]

The advantage to DC is that there are myriad tiny surface-mount regulators available with efficiencies reaching 99%. These regulators also work with a wide variety of input voltages. By contrast, an AC-to-DC converter is clunky, has a transformer, and ends up being relegated into a wall-wart. A DC-to-DC converter is miniscule and could be integrated directly into the device without even changing its external appearance.

- Warren

 

[zoobyshoe]

AC goes to 0V twice every cycle. so it "lets go" giving you an opportunity to free yourself , DC never let's go.

A guy was electrocuted in downtown San Diego a couple years ago from the AC current that went to the lighting in a bus shelter. The wiring was shorted to some metal in the shelter frame, and he was grounded through a rain puddle on the sidewalk. Once the circuit was complete he could not let go. A witness tried to pull him free, first insulating his own hands with his jacket, but could not. If you're grounded 110 v AC is deadly. You can't let go.

Theoretically, since the point of 0 voltage is there, you have the opportunity to let go, but in practise the human muscle cannot unclench in 1/60 of a second.

 

[DaveC426913]

Uh, Dave... 12 VDC won't do anything to you at all.

Voltages lower than about 50 V are not harmful to people.

- Warren

Really? Like to test that next time you open the hood of your car? I dare ya.

 

[DaveC426913]

A guy was electrocuted in downtown San Diego a couple years ago from the AC current that went to the lighting in a bus shelter. The wiring was shorted to some metal in the shelter frame, and he was grounded through a rain puddle on the sidewalk. Once the circuit was complete he could not let go. A witness tried to pull him free, first insulating his own hands with his jacket, but could not. If you're grounded 110 v AC is deadly. You can't let go.

Theoretically, since the point of 0 voltage is there, you have the opportunity to let go, but in practise the human muscle cannot unclench in 1/60 of a second.

1] You sure it was AC? Fluorescent lights convert the power to DC (though that's internal). Don't know if they were fluor. or not.

2] The idea is not so much that you have time to let go, as your muscles alterantely flex and distend, throwing you off.

 

[DaveC426913]

Are we also assuming that the myriad of DC devices that exist and use about 20 different voltages would all start coming at 12v...?

Please.


The phrase "...the myriad of..." is redundant.

Correct usage is "...we also assuming that myriad DC devices ..."

(See chroot's correct usage.)



Apologies. Nothing personal. Pet peeve.

 

[chroot]

Really? Like to test that next time you open the hood of your car? I dare ya.

I'd be quite happy to touch both terminals of a 12VDC battery with my hands. I have no idea why you think 12VDC is dangerous, because it isn't.

- Warren

 

[chroot]

1] You sure it was AC? Fluorescent lights convert the power to DC (though that's internal).

No, they don't.

- Warren

 

[zoobyshoe]

I'd be quite happy to touch both terminals of a 12VDC battery with my hands. I have no idea why you think 12VDC is dangerous, because it isn't.

I wouldn't do this. A car battery can deliver enormous current. My battery can deliver something like 800 cranking amps.

 

[chroot]

I wouldn't do this. A car battery can deliver enormous current. My battery can deliver something like 800 cranking amps.

I'll do it and sing a song at the same time, zooby.

Your car battery can deliver 800 amps to a very small load, but a human body is not a very small load.

12 VDC is utterly, completely, totally safe for humans. The only dangers are shorting things and causing sparks, catching things on fire, etc. -- not electrocution.

The automotive industry regards 50V as the threshold for safety. There is considerable incentive in the industry to move to 40V automotive electrical systems to improve efficiency.

- Warren

 

[zoobyshoe]

I'll do it and sing a song at the same time, zooby.

Your car battery can deliver 800 amps to a very small load, but a human body is not a very small load.

You will wet your hands with salt water and measure your resistance with an accurate meter, then work out how much current you'll receive BEFORE you try this. I just don't like the sound of it.

 

[chroot]

You will wet your hands with salt water and measure your resistance with an accurate meter, then work out how much current you'll receive BEFORE you try this. I just don't like the sound of it.

Sorry, zoob, you're out of your league. Trust me. I spent years building electric vehicles. You can't even -feel- anything less than about 60V. You can be a scaredy-cat if you want, but it's not reasonable.

- Warren

 

[zoobyshoe]

Please.


The phrase "...the myriad of..." is redundant.

myriad: n 1:ten thousand 2: a great number <a ~ of ideas>

Merriam-Webster's Collegiate Dictionary, Tenth Edition

Myriad has both a noun and an adjectival form. Russ' usage was perfectly correct.

 

[zoobyshoe]

Sorry, zoob, you're out of your league.

Not exactly. I do know I=E/R.

I just measured my own resistance with wet hands and got about 125 ohms. So 12v divided by 125 comes to .096 amps.
About a tenth of an amp.

Certainly not as dangerous as I thought, but I wouldn't stand there and sing a song.

 

[chroot]

Skin resistance (not soaked in saltwater) generally ranges from 2-50 kohms. The most current you're going to pull is a few milliamps at most.

Basically, you'd have to do some really stupid things (involving saltwater, open wounds, etc.) to get hurt by 12 VDC. It is, for nearly all purposes, harmless to people.

- Warren

 

[zoobyshoe]

Yeah, measuring it wet is for "worst case scenario" purposes. Working on vehicles, you're more likely to have oily hands and be well insulated.

edit: Plus, I see, in my exitment I see I set the meter to RX1K ohms not RX1 ohms.

 

[DaveC426913]

I stand corrected, and retracted. MW seems to have pretty much trounced my argument.

Apologies Russ and zoobyshoe.

Dagnabbit. That's one longstanding pet peeve shot all to blazes. How will I fill the void?



You know, I've always hated ...

um

people with 'zooby' in their handle. Yeah.

zoobyshoe et al < : hate :

 

[zoobyshoe]

zoobyshoe et al < : hate :

Don't hate me. I just had to conceed to Warren.

 

[krab]

Really? Like to test that next time you open the hood of your car? I dare ya.

What? You mean that you think the 12 volts from a car battery is dangerous? C'mon. I can hold on to my battery terminals no problem. I can even lick them. You really think your car can electrocute you?? Maybe you're thinking of the spark plug wires. I agree that you shouldn't touch them while the engine is running. But that is not 12 volts; it is because the high tension coil is essentially a stepup transformer.

edit: sorry, I responded after reading only one page. I see Dave's already convinced.

 

[zoobyshoe]

What? You mean that you think the 12 volts from a car battery is dangerous?

The main reason I suspected the 12 volt battery of being dangerous is from seeing the spark you get if you lay, say, a long screwdriver across the terminals. It is loud, bright and nasty sounding. When you see that you just jump to the conclusion it is dangerous.

Now, having worked out the true current you'd probably receive given how much resistance the human body has, I can see that there's probably no danger.

I don't understand, though, why it sparks at all.

 

[krab]

Fooling around with conductors near a high-capacity battery is definitely dangerous. The spark is not your usual type of high voltage thing. It results because if you lightly touch 2 conductors together, the contact area is a tiny fraction of a square millimeter. This area cannot support the large amount of current that comes because of the low resistance. So this little point melts and can glow white hot. That's what you see.

In this regard, I have an interesting anecdote. I once left my lights on in a parkade and could not start and had no jumper cables. There was another guy there with his car so I asked him to come and touch bumpers with my car. I found a piece of wire laying around and wanted him to hold it to his + terminal with a piece of wood. He refused, was in fact terrified of what I was doing. So I had to take the wire and hold it + terminal to + terminal, pressing it down directly with my thumbs, while my wife started the car. At the same time the car started, I of course burnt my thumbs but not too badly, and quickly I let go. The other guy of course thought from my reaction that I was electrocuted, exactly as he had predicted I would be.