Design Con 2015

DC distribution in your house and 42-V cars

-November 05, 2013

I spotted an article in Electronic Products about ac-to-dc converters that fit inside a wall plug. At least that was the intent of the article. Unfortunately it started with a comment about how so much of the stuff in houses run off dc, yet according the author, we waste energy by distributing ac and then having every gizmo make dc inside of it. He noted, “A far more efficient solution would be a central dc-grid supply that would power all of your home electronics appliances, as one large PSU wastes less energy than many separate ac/dc converters.”

It’s the old Edison versus Tesla/Steinmetz argument over a century later. Edison wanted to distribute dc, since he thought it safer. In fact the problem with ac is losses. Steinmetz and Tesla wanted to distribute ac, since it is easier to convert up and down. You can step it up to kilovolts to transport across long distances and then step it down to run your toaster or iPod. Now let's examine the argument that it would be simpler to make one big batch of dc, and wire it to all the gizmos in your house.

Recom power supply

First, lets pay the rent and give credit where credit is due. These RECOM power supplies are really neat. Rather than making you read 1000 words, here is the deal: 3 Watts, universal input, isolated, fits in a wall plug, CE, UL. So that is a few syllables more than a Haiku, but still most of what you need to know. Oh I forgot the most important spec, 20 bucks in single quantity at Digi-Key.

OK, back to dc versus ac power distribution. So Edison thought ac was more dangerous for electrocution, but that is not true. In fact the danger with electricity is in starting fires. That is why dc distribution is so tricky. When you get an arc in ac, it is self-quenching; 100 or 120 times a second, 50 and 60 Hz power goes to zero. That really helps extinguish the arc.

electric arc

Lesson 1: I designed a 48V 200W power supply. I was testing its short-circuit capability. I took the output and ran it across a metal file, like my mentor showed me. He maintains that dragging the wires across a bastard-cut file is even more effective at finding control loop problems than just touching the wires together. Then the wires did touch together, and when they parted, I got a nice ¼-inch arc that just stayed there, melting the copper wire strands. See, 48V is a nice arc-welding voltage. Once you start an arc it just burns and burns.

Lesson 2: When I worked at GMC Truck and Coach, we made trucks and buses with 24V dc power. All the relays and switches would fail much quicker. We could not use the dirt-cheap relays and switches used on 12V cars, they would fail within a year. As a note, the 24V headlamps and tail lamps failed more often too, since the filaments were twice as long and hence much more delicate and prone to breakage. They also sagged and were hard to aim or focus.

Lesson 3: There has been this MIT professor that has been pushing a 42-V system in cars for over two decades. At first it was supposed to save cost because you make the wires thinner. But we used 18ga wire in cars even for milliamp signals since 18ga wires did not break when dragged through a hole in the body during assembly. So then the rational was because 42-V systems could run electrically-operated intake and exhaust valves in the engine. Well we still don’t have electrical valves, although I think they use them in Formula 1 racing. And it turns out you can operate them with 12V if you have to.

The real reason 42-V cars are not here gets back to that arcing in relays and switches. With 42-V cars, every single load has to be switched with transistors, you just can’t use relays or contacts. That might still pay out, many loads these days are handled with FETs anyway. But the deal is, you can use 30V FETs with a 12V car, but you need 200V FETs to handle 36V cars. (The charging voltage is 42, the system just uses three 12-V batteries, so the uncharged voltage is 36.) But the die size of FETs goes up as the square of the voltage. So tripling the voltage makes the FET die nine times bigger and you don’t get any real cost savings with 42-V cars, if you still need 18ga wires and can’t use relays or switches. And worse yet, all the loads you control with FETs have to be 9 times the cost. Sorry, engineering is science crossed with economics, and college professors never appreciate that cost is king to an engineer.

So now think about distributing dc in your house. If you use 12V you need copper 10 times thicker and more expensive than the wires that carry 120V. If you up the voltage, even a little, you can’t use any mechanical switches or relays anywhere. On top of that, you still have this incredibly lethal wiring that can kill you in a flash, or will arc like crazy if it gets shorted. Heck even with ac distribution, the NSA data center keeps exploding since they can’t quench the arcs.

Put on the headphones for this little video:


It has taken a while to kill Edison’s dc distribution. The last bit of it was only decommissioned in 2007. The 120-V ac power in your house is darn-near perfect. Our good-ol-American power is much safer than 240V European power. I have read that 60Hz has less chance to screw up your heart when you get shocked than 50Hz. And old time CRT (cathode ray tube) televisions were much brighter since the American TV refreshed 60 times a second instead of 50. And the frame rates of our video are 60, which is smoother. No, give me 120V 60Hz. It is not perfect for any one thing, but it is a darn near perfect compromise for everything it has to do. If you want to improve power in the home lets go to 400Hz like the airplane people.

Now don’t think that dc is all bad. It makes sense to distribute dc if you have to send power to an island or through a single cable. With the wires so close together, the ac losses go way up, and it makes sense to distribute dc. I hear that semiconductors are almost cheap enough that it might make sense to transmit dc over long distances over land, but you will have to run it into an inverter so that you end up with ac that you can distribute to homes.

And I saw a Fairchild presentation where they claim their SuperFET has broken the square law relationship between die size and breakdown voltage. So with that and a way to reliably run thin-gauge wires in automobiles, maybe it does make sense to go to 42-V cars. But remember, you now need to have 42-V bulbs and such. Maybe with LEDs the bulbs last forever, and who cares what voltage they are. The economics could all change in a few years. But it is economics, not “neat” that determines what will happen.

So please don’t take any of this as a fixed absolute statement. After all, the trucks and buses I worked on were 24-V because we needed that much to run a starter motor that could turn over a Diesel engine. But that higher voltage was a pain in every other part of the truck, including when the circuit breakers would arc, catch fire, and burn down the truck, killing some poor guy in the sleeper cab. The world is full of specialists that only think about one small aspect of a problem. To be a good systems engineer you have to look at the whole picture, all while keeping cost, service, and reliability in mind.


Paul Rako is an engineer that writes and a writer that engineers at Atmel. This content was originally posted to the company's Bits & Pieces from the Embedded Design World blog.


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