You lot know more about this than me

Is this a sensible idea?

The main cost to LED\’s is that they all have to have a transformer. LED\’s run at low voltages, like 5v, so house current has to be stepped down at every bulb. LED\’s in theory should run cool and be cheap, but they are expensive and run hot because of the transformers.

Which leads me to wonder whether we may start wiring houses for 12v in parallel to 110v. When I grew up, nearly everything I plugged into the wall — lights, motors, appliances — ran on 110V. Now, most everything (other than appliances) that I plug in the wall actually needs 5-12v (computers, cell phones, all my audio equipment except big amps). I don\’t know enough about power lines to know if this is feasible. I am pretty sure the resistance losses for 12V DC would be too high, so it would have to be 12V AC, but a diode bridge and some capacitors is a hell of a lot smaller and cheaper than a full blown transformer. I know my landscape lighting has long runs of 12V, that seems to work OK. It is also a hell of a lot safer to work with.

Obviously, would be a long term change. But if LEDs take off, we\’re all using 5 volt bulbs or whatever, would it be sensible to rewire rather than keep buying the transformer in each and every bulb?

89 comments on “You lot know more about this than me

  1. The sensible way is probably to have low power lights run from solar panel on the roof and a relatively small storage battery.

  2. The Greens will probably make it compulsory.

    Although actually I think my bathroom does have such a plug for electric razors. I didn’t put it there, it was there when I moved in. Never used it. So some people are doing it. Don’t know if it is AC or DC though. Never checked.

  3. I’m running 240v LEDs. rated at 5 watts, they don’t get very hot, produce excellent light. 80% of power consumed by incandescent lamps is wasted as heat. Transformers dump heat as a by product and so waste power.
    I have no idea if each and every LED has a transformer in it but, packed in a GU10 lamp, it must be pretty clever.

  4. I doubt they’ll be running 12v power lines, but a solution might be to run normal power to the house, which is then split into one 240v circuit and another 12v circuit. And it might be a good idea to make the sockets different for each circuit.

  5. Nick Luke // Nov 30, 2012 at 10:58 am
    ….. Transformers dump heat as a by product and so waste power.
    ==============================

    I use my lights most in the winter, when daylight hours are few. It’s also cold. Hence that heat is not wasted (and same applies to incandescents, but more so)

  6. I run all my LED lighting off a 24v track, it’s very efficient. 2.8 watts each, the equivalent of halogen lights at 10x the power use. I live on a boat though, so already have DC electrics and require very low power devices.

    Having a large capacity of batteries is a pain in the arse, expensive / high maintenance or both. Only really practical if you want to be able to live off grid.

  7. And yes, voltage drop in wiring is an issue. A 60 foot boat is pushing it. We have a very thick, expensive copper trunk wire to carry 24V.

  8. Don’t think it would really save anything. Electricity is transmitted at very high voltages, and it has to be transformed at substations for domestic consumption. If it wasn’t further reduced by a transformer in the home, it would have to be reduced at the substation. You might get some benefits from economy of scale, I suppose…

  9. Well, you can already buy low-voltage lighting kits that will power a long run of halogen bulbs, so it can’t be that difficult.

    But perhaps there’s a load or a length of wire issue? Would it have to be room-by-room for large houses?

  10. “80% of power consumed by incandescent bulbs is wasted as heat”

    Bollocks. None of it is wasted. It heats your house, reducing the load on your gas C/H. Incandescent bulbs also have the advantage of not containing mercury – for which our EU overlords didn’t think to introduce a safe disposal programme.

    The whole thing is a triumph of lobbyists conspiring against the wider interests of society.

  11. LEDs run at a voltage of roughly hc/e.wavelength. The shortest wavelength of visible light is about 380nm, corresponding to 3.26eV, which is about what you need for a white LED (which is usually a blue LED with a phosphor).

    There’s no significant difference in resistance losses between AC and DC. The advantage is in high voltage over low voltage, because higher voltage requires lower current for a given power, and it’s the current flow that creates losses.

    Power supplies nowadays are almost always switched mode, which allows for a much smaller, cheaper transformer than the traditional transformer+bridge rectifier+capacitor arrangement.

    The underlying point is good – nothing in my house outside the kitchen really needs mains voltage – all the lighting could well be done by LED. Apart from the immersion heater used for back-up water heating if the boiler fails. But a 12V a.c. supply would not obviate the need for power supplies for individual devices.

  12. Always liked Weetabix and I like what Sebastian Weetabix said.

    Do you know what they are doing with the long-life (ha,ha) mercury bulbs? Try and find out. It will shock you.

    Oh and look at what the Kiwi gov. tell you if you accidentally break one!!!!!

  13. “And it might be a good idea to make the sockets different for each circuit.” Pity no-one thought of that for diesel and petrol at filling stations.

  14. @SMFS: your razor socket will be high-voltage AC, powered from a transformer, probably in the wall cavity. The transformer gives a certain amount of electrical isolation which is nice when the socket is in the bathroom.

    It will probably also feature a picture of a man eating a rat for some reason…

  15. “80% of power consumed by incandescent bulbs is wasted as heat”

    Bollocks. None of it is wasted. It heats your house, reducing the load on your gas C/H.

    Assuming you live in a climate where extra heat in the house is a good thing.

  16. Shaver sockets have an isolation transformer, so that the voltage is floating and you don’t get a shock by touching just one pin, however well earthed you are. Modern ones give mains or half mains voltage.

    MattyJ: are you sure that’s a rat and not a sanitary device?

  17. Seems like the idea is just shifting the losses from the switched mode supply in the LED bulb to distribution losses in low voltage cabling. Obviously the power consumption of LED bulbs takes into account the losses in the switched mode converter in the bulb. (I’m assuming the bulbs use a switched mode supply, I don’t have specific knowledge on this other than looking at the size of them).

    The main requirement for LED bulbs has been that they be plug-in replacements (form, fit, function) for conventional incandescent bubs, and you can take it that the current implementations of LED bulbs represent the best trade-off of properties that the designers can manage at the price point.

    Obviously, if we change the requirement to ‘the most efficient way of lighting a house with LEDs’, the distribution voltage might change, dunno. It might not, though. And we’d have to balance the benefit (a few percentage points of efficiency gained) versus the infrastructure cost (every other appliance still needs 240V, so we have to build two different power mains in the house, and ensure that they play well together).

    Summary: probably not a sensible idea; commercial competition will keep LED bulbs efficient, no need for the end user to worry about it.

    (…Which presumably is the answer the author and readers of this blog would expect…)

  18. These days transformers are not used much, most power supplies are switching mode (which DO contain a transformer but a much smaller one). This is more efficient.
    Wiring the house at 12v could certainly be done but the problem is, then your washing machine would have to draw an unfeasible amount of current to run at 12v.
    One thing I *COULD* see happening though, is houses being fitted with power-only USB sockets alongside 240v plugs. Given how many gadgets we have (phones etc) charging off USB, this could be a useful idea.
    However, I don’t think that the bit where the idea of LED lighting becomes too expensive is the transformer. As far as I know, it is simply a lot more expensive to produce a cluster of LED to produce the equivalent light of a light bulb, than a light bulb.

  19. LED lighting is crap. It’s actually worse than the compact fluorescent bulbs because the light is harsh and cold, even if you buy the “warm” colours. I did a mini-experiment and bought three LED bulbs and within a year one had died.

    Maybe LEDs will improve but for me the experiment was a failure and I will be continuing with compact fluorescent for the time being.

    Maybe over time (especially with local generation) it will make sense to have a 12V track in the house. Can’t see many people retro-fitting though.

  20. Wiring the house solely at 12v would mean that you could not use an electric kettle or vacuum cleaner or anything requiring non-trivial power. Power use is V^2/R so if you divide V by 20 you need to divide R by 400 to get the same power output.
    Retrofitting a 12v circuit in parallel to a 240v circuit in every house would be stupidly expensive and in many cases would consume more energy than is wasted by transformers in the remainder of the house’s lifetime. Wiring new houses with a 12v circuit in parallel might be worth considering, but only if we are going to have a lot more than just LED bulbs using 12v.

  21. My house has several separate electrical circuits running off the main fuse box; one for the cooker, two for sockets, one for lighting, etc.

    I thought that was fairly normal; it’s not a new or fancy wiring system.

    It doesn’t seem like a difficult job to put a transformer on the lighting circuit, just after it leaves the fuse box, so that the lighting is low voltage but the other stuff is still 240v.

    Or am I missing something?

  22. Blue Eyes, interesting that you say LEDs are crap. I’ve just bought one (prompted by this) to try it out. I thought it couldn’t be worse than the horrible mercury things, but perhaps that was optimistic.

  23. One problem with putting a switch mode power supply in the fuse box is EM interference radiating from the power lines inside your house.

  24. Simple answer: Volts X Amps = Watts

    Now one of the things I actually do is wire houses. Not in theory but in practice. Mostly, if I’m installing 12V halogen downlighters, they’ll be installed each with it’s own 240>12V transformer, & supplied at 240 volts by a single 1.5mm twin & earth cable with each transformer connected along it. The transformers are ‘lost’ in the ceiling space alongside the lamp fitment. But occasionally this isn’t possible. You can’t ‘lose’ transformers in ceilings where the ceiling itself forms the fire barrier between floors of the building or is of solid concrete. So you install a very shallow halogen fitment & mount a large transformer capable of supplying all the lights at 12V. And this is where it gets bloody complicated & expensive. It’s that Volts x Amps = Watts thing. Heavens, even you public schoolboys must have done that. To produce the Watts the lamp requires, if you reduce the Volts, you have to raise the Amps. And Amps is current. And the more current the thicker the wire because of Resistance. Remember? Resistance depending on current not voltage? So to run at 12V for only a few meters I’m up to 4mm cable, minimum. I’ve actually got a complicated table for calculating cable requirements/wattage/distance. Past 10 meters (& 10 meters isn’t far in wiring terms because you can’t always wire in straight lines) I can be up to 6mm. Or higher. Because at low voltages/high amperages the resistance of the cable causes it to heat & consume current. Adding to the Amps. There’s a point where you simply couldn’t go any further in terms of distance because you’d have to wire in cable as thick as your finger. Think the battery cable on your car which is a 12V circuit. It’s £5 meter & impossible to manipulate.
    And there’s also something going on at the transformer. All this cable resistance requires current. Which the transformer has to supply. You can get to the point where more of the current is disappearing heating the cables than powering the lights. Transformers stop being things the size of a Mars bar & become bloody great units costing £100+, yet only supplying a dozen lights.
    Now, I just looked up LED lighting & a typical light requires 5 Watts. (The voltage isn’t important at this stage because of that W=VxA thing. You can trade one for ‘tother) It’s a lot lower than the requirement for 12V halogens. But if you want to supply at 5V that current thing is going to bite very quickly. Typical halogen draws 35W so you’re only reducing the current by just over a third.
    You want to wire a whole house at 5V from a single point you’d better find someone to sell you room temperature superconductors.

  25. Incidentally & a WARNING.
    Just seen somebody turn a very expensive laptop to garbage because they didn’t understand the above. They wanted to get a better connection on a mobile internet dongle, plugs into the USB port. If you use one, you’ll know they get quite warm. The draw current. At 5V.
    Person thought getting the dongle out of the steel structure they were in would improve the signal. Which it would. So they stuck it on the end of two 5 meter USB extenders. Which are very thin LOW AMPERAGE WIRE. The wire heats, hot wire has more resistance so draws more current & round & round we go. Weak point in the system is the USB controller on the motherboard. Was on the motherboard rather. There ain’t one now.

  26. I’ve found LEDs to be fine. The light is a bit different again to CFLs and halogens, but no worse, and I’ve no problem with it. They don’t get hot, and I hate lights that get hot and cook everything around them. They last longer. And the prices are rapidly coming down.

  27. Back when I was a lad in my sparking days, I remember being told a very general rule of thumb, that the correct distro voltage is feet converted to volts. So, in vague approximate terms, you wouldn’t distribute 12V more than 12 feet; you’d go to to a higher voltage.

    NB this isnt an official calculation, just a ballpark. Years ago, I had to design the cabling for a playroom for the Sultan Of Brunei at a place he was having built in Southall. No mains in the room, so the kids couldn’t possibly fy themselves, so it was 12V to halogens all the way; and there were lots of them too. A fairy garden thing full of them, for instance. Can’t remember the specific cable sizes I ended up with, but they were huge.

    I retrofitted halogens in my sister’s kitchen to replace the halogens. Just used a couple of the old trannies; could have only used one but wanted two circuits in case of failure, and there’s never any harm in under-running a transformer (so long as it’s not copper/iron and going to result in overvoltage). Sis and hubby much prefer the light quality. Definitely the way of the future. But not powered from a central 12V source, not in any home of reasonable size anyway.

  28. @Simon Jester:

    I bought them just over a year ago (15 months maybe?) and almost exactly on their first anniversary the first one gave up. Each bulb was about a tenner each. Underwhelmed!

  29. There already exist 5A 240V sockets (they take old fashioned style round 2-pin plugs) designed to take floor standing lamps, so it wouldn’t be a major effort to define a new socket shape for 12V at 2A. I would suggest a rectangular opening, a bit like USB. These sockets would be on a different circuit to the washing machine etc.

  30. Blue Eyes-

    MTBF. Even with a long one, you’ll get some failures well before it. The MTBF of human beings is about 70, but some humans conk out as children.

  31. BIS, that’s interesting (#26), thank you. I knew about needing thicker wires at lower voltages, but I didn’t realise how much, or how much it depended on distance.

    But these new LED bulbs seem to be 12V and 7W, so 0.6A (at least the one I’ve just ordered off Amazon to try is that). Your halogen circuits seem to be 35W and 12V, so almost 3A. If the LED bulbs are only a fifth of the ampage, does that mean the cables can be 5 times as long?

    Would that be enough to run through the existing lighting crcuit of a reasonably sized house, or is it still not there yet?

  32. Converting incoming 230/240V AC to low voltage DC requires two things: a step down transformer and a rectifier.

    Stepping down and rectifying 240 V AC to 5V DC would be big, expensive and complex particularly if handling three phase supply and lose energy in the heat and noise created, not least in the transformer/rectifier unit, but as is explained in others comments, in the thick wiring needed to relay the supply around a home.

    Also by the way, if the circuit is to be 12 V, a transformer will still be needed for those appliances which use other voltages.

    The base of low energy bulbs contain both step down transformer and rectifier. Because the load on each bulb is low, a small transformer and single phase solid-state rectifier can be used, with little energy lost by way of heat or noise.

    In terms of energy efficiency it is the energy used expressed as watts which matters. Whether voltage is higher or lower, DC or AC, 15 watts is 15watts.

    The total energy bill for a light bulb is not just its rated output, but must include how much energy is lost in the system from the meter to the bulb.

    Given the additional wiring costs and energy loss in DC circuits, they are neither practical nor cost effective – there is no such thing as a free lunch.

  33. Ian B of course, but I’ve never had that happen with a CF bulb (I have only ever had two failures of CF bulbs in many years: one because my upstairs neighbour flooded me and one because it was about ten years old). Combined with the really stark cold light I am not a fan.

  34. BIS>

    Re your warning – that USB controller was not compliant with the standard if it did that. It should a) detect over-length cables and b) limit total current. The length detection should have made it refuse to negotiate a connection in the first place.

  35. Richard
    The LEDs themselves can’t be 12V. The diode usually works around the 4V range so they must have an integrated transformer of some sort. Don’t try & use a halogen type transformer to reduce 240>12V. It’s a different transformer.
    “Your halogen circuits seem to be 35W and 12V, so almost 3A.”
    NO!
    Go back & read the comment. The halogen fitting is 3A. (Don’t forget for more than one it’s 3A+3A…..) What I have to calculate is the current load of the cable as well.
    ” If the LED bulbs are only a fifth of the ampage, does that mean the cables can be 5 times as long?”
    No, because each time you lengthen the cable, the current consumption of that length is added to the load on the existing length. I actually use a table for available wire cross sections but I’d imagine the relationship’s more logarithmic rather than linear.
    Essentially, this is why civilised countries use house current in the range 220/240V. (Yanks also have 110V which may say something). When you get down to low voltages, wiring sizes can get very tricky & sensitive to length. It’s why trucks run on 24V. (Avionics 50V? somebody?) They’re bigger.
    Seriously. Don’t bugger about with low voltage circuits without understanding what you’re doing. You don’t get a shock but you could easily set fire to the house.

  36. they’ll be installed each with it’s own 240>12V transformer, & supplied at 240 volts by a single 1.5mm twin & earth cable with each transformer connected along it. The transformers are ‘lost’ in the ceiling space alongside the lamp fitment.

    Ah yeah. I’ve used them in my place in Thailand, about 3/4 the size of a Mars bar, and cheap enough. I have found they tend to fail as regularly as the bulbs though, but that might be the shoddy kit available in Thailand.

  37. BE@37
    When I moved into this apartment 18 months ago I put in 11 CF bulbs. The last of the 11 failed last week. Why the failure rate? My guess is our Dago power company can’t hack keeping the line voltage up. Voltage drops, the electronics pull more amps & bugger the bulbs. There’s an implication in this for the UK’s mad race for wind, because we get a lot of wind in our supply mix. Brown outs aren’t just incandescent bulbs going a bit orange. Whole lot of electronics is very supply voltage sensitive & inclined to go very expensively wrong. (See below)

    dave @ 39
    Yes, I know. That’s the theory. Doesn’t mean it actually applies though or bumble bees couldn’t fly. In this case it didn’t. I’ve blown a USB hub for the same reason, running a USB video device, but my laptops plug to independently powered hubs before much else, so it didn’t kill £500 worth of Tosh.

  38. @BiS: “Seriously. Don’t bugger about with low voltage circuits without understanding what you’re doing”.

    I’d end that there actually. 12VDC is safe-ish, but once you get up to around 24VDC (truck/boat voltage) you can shock yourself quite badly. Even a 9V battery can start a fire if you’re not careful.

  39. Tim @41
    Thanx for the compliment.
    Out of interest, unlike Ian I’m not a qualified sparks – though I have wired everything from substation switchgear through racing cars, boats, houses, circuit boards…. So I couldn’t legally wire my own house in the UK. I just used advise our qualified electrician when he got fucked. (whistles small tune) Regulations innit.

  40. Also, a bit tangential, but this thread has got me wondering as to when Hollywood are going to stop depicting “gritty” visions of the future by having a switch-start fluorescent cycling on and off. IIRC copper iron ballasts aren’t even legal any more.

  41. ¡ Bloody hell ! A voltage chauvinist @ IanB

    That’s gotta be a first!

    Know what you mean @49 though. They had them on the Alien starship, didn’t they?

  42. BIS>

    Theory or not, it’s about who bears responsibility. Either the laptop manufacturer, or, just conceivably, the device manufacturer, is responsible for providing faulty equipment which caused damage.

  43. @IanB: yeah, I always thought it was funny that a 430V 600A 3-phase motor controller was considered “low voltage” and as a result could be worked on while it was running.

    I once told off a senior sparky ’cause he was working on one while standing on a rickety chair so that he could reach over the top of the bus-bars. I didn’t want to end up with the job of cleaning his remains off the opposite wall.

  44. BIS-

    No, I draw porn these days. Haven’t engineered in some years, hence I’m a bit rusty. It sometimes amazes (and depresses) me how much I’ve forgotten.

  45. Dave @ 52
    “Theory or not, it’s about who bears responsibility.”
    No
    It’s who accepts responsibility.
    My case, probably due to a £15 video converter sourced from e-bay. The video converter works fine. So what’s my beef?
    My associate? Jeez! You imagine the wriggle room the vendor/manufacturer would find around a fault on a year+ old laptop? They’d need your level of expertise just to have the discussion.
    Hence the caution. Better not to have the problem in the first place.
    Tim
    There you go, then.

  46. I am pretty sure the resistance losses for 12V DC would be too high, so it would have to be 12V AC, but a diode bridge and some capacitors is a hell of a lot smaller and cheaper than a full blown transformer.

    This sentence is all you have to read to know the original author is being silly. As has been pointed out, the resistance losses don’t change whether it’s DC or AC. Same voltage, same current, same volt drop. At least at this scale (where you can pretty much ignore induction/capacitance issues).

    But also, you don’t drive LEDs with a diode bridge rectifier, not if you want them to last.

    And no, I’m afraid cabling a house with 5/12VDC is not practical.

  47. We’ve had a Sanyo system U/S for nigh on 2 years now. ExPat Brit installer sourced it bent & f/u’d the install so the warranty wasn’t honored. 6000€ worth.
    So far we’ve had:
    The Spanish aircon co charged 40€ to tell us they could sell us a new system.
    A Brit chancer.
    The Sanyo Spain recommended service tech who
    1) replaced a perfectly good mainboard with a totally inappropriate one for a different model. 2)Then put the original back & promptly blew it (they cost 500€) due to incorrect wiring
    3)tried to replace it with another totally inappropriate board
    4) supplied fitted the correct board but didn’t get the system to work
    5) submitted an estimate for about 1500€ to get it running. Includes cost of board they fucked.
    6) Are already into us for 180€ paid during the 1/2 hour period @ 2) the thing was making cold air.

    Lessons I’ve learned.
    My cuz (whose apt it is) was a twat for paying the geezer who was doing the building work’s mate, at a distance of 1800km cash, to install aircon.
    Anyone to do with aircon down here does not need the word air in the company name to provide an accurate description of their services.
    All Spaniards are incompetent cunts but their aircon techs excel themselves.
    I know bugger all about aircon but more than Spanish service techs.

    QED
    We need competence. And probably new int/ext units.
    It might be more cost efficient to fly someone down who knew what the hell they were doing.
    For;
    Payment
    A safe refuge from the English winter. Today sunny 64deg.
    All the alcohol that can be drunk
    An endless selection of the most beautiful woman from the four corners. Affection very much negotiable in a buyers market.
    A million half € apt to do it all in.

  48. @johnb

    >Converting incoming 230/240V AC to low voltage DC requires two things: a step down transformer and a rectifier.

    Stepping down and rectifying 240 V AC to 5V DC would be big, expensive and complex particularly if handling three phase supply and lose energy in the heat and noise created, not least in the transformer/rectifier unit, but as is explained in others comments, in the thick wiring needed to relay the supply around a home.

    My phone charger seems to manage it in a space of about 5 cubic centimetres :-)

    One further point – 12v wiring needs to electrician, of course.

    One could get a properly insulated house, and use incandescent bulbs and no central heating.

  49. Don’t knock the great Brit chancer, Tim.

    A Brit chancer will:
    1) Turn up on time
    2) Fiddle about in inexplicable machinery hoping to find the loose wire/ hidden restart button that fires it up. Charge you 150€ for 1/2 hour. But you do have a working appliance. Not bad value & less than the manufacturer’s tech. to do the same thing.
    or
    3) Plead the need for a 3/8 Gribley wrench & never be seen again. No charge.

    The great Spanish chancer will;
    1)Turn up anywhere between 2 days & a year late. With swarthy accomplice with no apparent role.
    2) Fiddle about in inexplicable machinery hoping to find the loose wire/ hidden restart button that fires it up. Break several parts. Not succeed.
    3)Scrawl invoice for 150€ for service.
    4)Take householder for court for non-payment of above.
    5)Win case.

  50. Tim Newman

    …the electricity in Phuket is pretty sketchy.

    Surely you meant “…the electricity in Phuket is Phukt”.

    The angels weep when an opportunity for a good gag is missed.

  51. BIS #57>

    Fair points. That’s why people keep giving me money, after all.

    What’s the laptop worth? I’ll get you a warranty repair on a no-win no-fee basis if the money’s good enough for me to browbeat support monkeys. Sometimes I suspect that’s what I actually specialise in anyway.

    “A Brit chancer.”

    Oh, not me.

  52. Thanx Dave but it’s 1800km from UK. It still works but with no USB it doesn’t network. If it had the old PMCIA slot I’d stick an adapter card in for him but there isn’t a similar thing for the new card slots is there?

  53. Hadn’t noticed this one till now but as we’ve had a few misconceptions above, another little warning;

    “SadButMadLad // Nov 30, 2012 at 2:32 pm

    There already exist 5A 240V sockets (they take old fashioned style round 2-pin plugs) designed to take floor standing lamps,……” etc etc

    Stop & think!
    The 5A is at 240V. That’s almost 1.25kW. Small fan heater. At 12V, not even 2 halogen downlighters. Same applies to switches. Just because you can run 240V through those little plugs doesn’t mean they’ll take the same wattage at 12V. They’ll melt or catch fire.

    Worth remembering. You can plug an arc welder into a 13A domestic socket. After the welder’s transformer’s dropped the voltage, you can burn holes in 1/4 inch steel plate with the amperage. 2000 degrees

  54. “The angels weep when an opportunity for a good gag is missed.”

    And at 56 we have a man who draws porn & is amazed (& depressed) at how much he’s forgotten.
    Drip ¦
    Drip ¦
    Drip ¦

  55. Another thing to consider is switching DC. With AC, the voltage drops to zero every cycle so arcs stop automatically. Not so with DC; an arc will continue until the switch melts back enough so it can no longer jump the gap. Not often a problem with 12 VDC, but a big problem at higher voltages.

  56. ZT, I work a lot on 24V systems, many of which are fairly high integrity because they have safety and agency constraints. I tend to use a series combination of relay contacts, and mosfet, with the fet breaking the current first, followed by the relay. That makes it less likely that any single extraneous outside influence could cause extraneous activation of the circuit.
    Also, a low impedance current monitor is quite easy to build in to that, so that a current overload can be detected, and used to switch off the mosfet to isolate the problem until it is fixed.
    Also ground faults are routinely monitored and detected. You can make things quite robust, without breaking the bank.

  57. Maybe I should mention that the reason I am beginning to like the concept of at least some of the local domestic supply being DC, is that it lends itself to a degree of consumer self-sufficiency if the AC grid goes down.
    At present, you can put panels on your roof, but if the grid fails, you will have no electricity, even in blazing sunshine. I think it’s a regulation, so that your panels can’t feed 240VAC into a broken transmission path and surprise their repair man who thought the down line was safe. Fair enough.
    But if you could diode feed your local backup straight into a 24V spur, that would give you some independance of supply, and here is no way any of that voltage would be able to propagate upstream. It could mean that you get to keep some stuff running, instead of losing everything.

  58. BIS>

    Re your warning – that USB controller was not compliant with the standard if it did that. It should a) detect over-length cables and b) limit total current. The length detection should have made it refuse to negotiate a connection in the first place.</blockquote

    This is one of the problems with USB that Firewire got right from the beginning. For the length and current negotiation to work, USB requires both the device and the host to be compliant. Cheap USB devices just ignore that and draw the full current from the beginning. Usually they just put a load across the power and ground lines with an impedance designed to take 500 mA from the 5 V supply.

    If you want confirmation, notice that the USB spec uses the data lines to negotiate current and detect length. These devices don't even connect the data lines.

    In short, it's the device's fault first and the protocol designers a distant second.

  59. But if you could diode feed your local backup straight into a 24V spur, that would give you some independance of supply, and here is no way any of that voltage would be able to propagate upstream. It could mean that you get to keep some stuff running, instead of losing everything.

    You can do that now. My wireless access point, ADSL modem and network gateway (an Alix SBC) all run off a bank of marine cycle batteries that get charged while the mains is on. If we have a power cut, all the laptops, tablets and phones still have internet access. Simple enough to add solar panels, you just have to have a separate bank from the ones you use for the mains.

  60. BTW, this blog must surely rank Global No.1, by a fucking mile, as having the most technically well-informed commenters. For a non-specialist blog, it doesn’t half attract the geeks.

  61. I’ve often thought that too Tim. Across a wide range of domains and disciplines too. My guess would be we’re all attracted by sarcasm and poking fun at people who don’t know what they’re talking about :)

  62. Matthew L, you could probably use the same bank of panels if you set it up so you could manually disconnect some of the panels from the grid connected inverter and connect them into a regulator that charges the batteries from the panels when the mains charger goes out (only some panels because the typical 2-3kW panel install would require a very hefty regulator to handle). You might be able to get away with a ‘break before make’ changeover switch so you can leave it all hardwired (and if you wanted to get really fancy that could be an automatic changeover switch sensing the mains voltage), but depends on the regs and the leeway the inspectors get I suppose. Also, the changeover switches I’m familiar with are for AC, I presume you can get them for switching DC but as someone pointed out above DC is a different kettle of fish.

    Given that solar regulators typically handle up to 150VDC input (usually each 3-4 panels in series) and that at full output you’re getting up to 200W per panel, that’s potentially 150VDC at 5A. Which is serious stuff, so if anyone acts on this get someone who knows what they’re doing to design it!

  63. BIS>

    “If it had the old PMCIA slot I’d stick an adapter card in for him but there isn’t a similar thing for the new card slots is there?”

    Which new card slots? If you’ve got a CF or ExpressCard slot you should be able to find a wireless network card for it. I wouldn’t be surprised to find that someone even makes a card with USB ports on it, but you’ll have a hard time finding one on google amongst all the card readers that plug into USB ports.

  64. USB 2.0 is included in ExpressCard already, but it’s just routed through the same connector so that will (probably) be fried as well. If you get a USB 2.0 ExpressCard, it’s almost guaranteed to be a simple mechanical adapter. USB 3.0, on the other hand, is going to be a proper host controller (and an upgrade to boot).

  65. @MattL
    I’m pretty sure the poor victim can still access memory cards, although he’s been giving me the stuff he needs printed on optical disks or e-mailing it. That the slot we’re talking about? Worth getting one of those adapters anyway. They’re cheap enough.
    Talking about cheap, so were the USB extenders so likely why no protection.

    TimN @75
    “BTW, this blog must surely rank Global No.1, by a fucking mile, as having the most technically well-informed commenters. For a non-specialist blog, it doesn’t half attract the geeks.”
    I’ve noticed that. I’ve got stuff I’ve filed from here that’s helped me with all sorts of things. I’ve a theory why as well. Tim may write on economics but he’s got a proper day job too. The scandium thing. So do a lot of the commentators here. They do shit. Make things happen. Have real life experience. You do that you pick up all sorts of things over the years, very often working backwards. Starting with the practical application & working backward to getting a grasp of the theory.
    Coincidentally, you did a post at the Kitty Kounters about numeracy prompted me to churn out a load of garbage in the small hours. Way I see the difference between the real world & academia. You only have to look at some of the comments at CiF & some that creep in over here to see how detached a lot of people are from how the world really works & how little they’re interested in understanding it.

  66. Matt>

    Correct me if I’m wrong, but don’t the USB 3 cards draw power from a USB 2 port? If that’s buggered, the card will also need an external powered hub to draw power from. Which, ironically, is what should have been used in the first place to stop the magic smoke escaping.

  67. Dave: I hadn’t thought of that… unfortunately there’s probably no way to find out. Even without the USB 2 power you could probably connect it to a powered hub, but you wouldn’t be able to run any devices off it directly. Try before you buy.

  68. I think the other commentators have dealt with the 12V electricity distribution question well. It would require thick wires to be efficient and wouldn’t be very practical.

    The power from USB ports thing is a bit simpler than it’s made out to be though. It’s true that USB has a badly designed system for reporting power consumption back to the host controller. But, that’s no excuse for the power supplies on motherboards and hubs to burn out. Those power supplies have max input voltage, max current and max power dissipation specs that are known to their designers regardless of the system they’re connected to. USB may do stupid things, that shouldn’t matter to the power regulator chip much. It’s quite possible to design regulator chips with overvoltage, overcurrent and overtemperature protection on them to protect against those limitations. Regulator chips with all those three features on have existed since the 80s. There can be no excuse from the chip and equipment manufacturers for getting it wrong today.

  69. Tim Newman // Dec 1, 2012 at 1:42 am

    BTW, this blog must surely rank Global No.1, by a fucking mile, as having the most technically well-informed commenters. For a non-specialist blog, it doesn’t half attract the geeks.

    ——————-

    Imagine what we could get away with if we joined forces, and set our caps at a spot of mischief….

  70. Paradoxically and contrary to my comments, I have a plan to rewire most of the lighting in my house with LEDs driven from a solar charged 24VDC battery bank. But, specialised circumstances… it’s small (100 sq m), I can centralise the battery bank to keep the runs short, and there are various reasons why rewiring in 240VAC is not worth it – chief being that in extra low voltage I can do most of the labour. I doubt it would ever become a standard thing to do in most houses.

  71. Many of the contributions here are hurting this old electrical engineer’s eyes. Space here is too short to cover all of the errors, but for starters: these LED lamps do NOT have transformers in them. Instead, they have small switching power supplies, which are NOT transformers. The main ingredient in a “switcher” (as they are called) is an inductor. By running the switcher at a fairly high frequency, 50kHz – 200 kHz, the size of the inductor becomes quite small, maybe 1/4″ on a side and 1/8″ tall. Using a control integrated circuit and this small inductor, you can easily “step down” the rectified line voltage to the voltage (about 3.5-4.0V) and current that the LED needs.

    Each switcher is “non-isolated,” meaning that if you touch any of its components, you run the risk of electrical shock. For this reason, the switchers are sealed up where we humans can’t get at them without tearing the lamp apart. No isolation transformer is needed, keeping costs very low.

    12VDC throughout a house is a really dumb idea if you intend to run the entire house off of it. The short story: “voltage regulation.” Would you be happy if the voltage at the socket dropped to 10.1VDC while you had a bunch of LED lights on in the house? How about 8.7VDC? It’s very much like a house with clogged water pipes, and someone flushes the toilet while you are taking a shower: Yow!!!! It’s called “pressure drop in plumbing, and “voltage drop” in electricity. A system with lots of voltage drop has “poor regulation.” Regulation is horrid for low voltages, far less so for high voltage (110VAC). This is exactly what got Edison in trouble, requiring power stations be every 1/4 mile, unlike Tesla’s AC, which let you place the power stations hundreds of miles away from homes and industries. Same thing when you use 110VAC throughout the house. You can run 12VDC if you aren’t burning a lot of watts. But burn a lot of watts, and you’ll get real poor voltage regulation at your 12VDC sockets.

  72. “Each switcher is “non-isolated,” meaning that if you touch any of its components, you run the risk of electrical shock. ”

    How true, how very true, and the shock can be substantial. Be sure your finger is curled away from the live bit …

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