Transformers - are they safe?

[meow2222]

Whole cities were on dc for half a century or so, dc switching isnt a problem, as long as you dont use ac only switches.

NT

[meow2222]

I tihnk this is a misunderstanding. If we were to run all our mains appliances of LV then we'd need thick cu, but not for the tiddly powers warts are now used for. 12v 2A would run all the wart adaptors I have here easily. Multicore burger alarm cable can handle that no trouble. Bear in mind some V drop can be dealt with by upping the psu v a bit, plus we put the psu in the centre of the house to keep wiring shorter. Or in a big house have 2+ psus.

of course, because today those come at no extra cost with appliances. If 12/24v domestic distribution ever took off that might change and it become self-paying, but not today.

Au contraire, it looks so much better to get rid of all those warts and just use tiny 5mm plugs instead. If the word gets around the present messy wart proliferation will become socially undesirable. It means less ext leads too.

Today we have lots of mains sockets, some of which will power warts. If you take the need for sockets to power warts out of the equation, you need less sockets. Still need them in all the same locations, just not as many.

But that isnt really the point, this system would be used to add sockets not use less. Keep the mains ones as now and add tiny 12v connectors which take up almost no space, and you get socket clusters that will support more appliances with less bulk, less extension leads, and more tidily. Thats what wil appeal about it.

Thats simple, but I doubt thick cable would be used. If it were, the socket has flying leads on with vampire connections on the end. The 6 way 12v socket is round, maybe 1.75" across, with one central fixing screw.

For installation a holesaw takes out a 1.25" shallow hole in the wall, and the cable simply passes through the space without and need for loop or ends. The socket fixes into the wall with 1 screw after the vampire taps have been connected to the bigger cable. There are many other possibles of course. But that is quick, simple, cheap, and would give 4 or 6 LV outlets in one pop.

A better option might be to distrib both 6v and 12v, using 2 different socket sizes, or same size but different centre pin etc. It means more cores with lower i rating, and eliminates the need for pretty much all regulators.

If it ever becomes popular we would probably see double mains outlets with a row of LV connectors down one edge, and an internal partition wall inside to keep LV and HT separate. They can then take no space at all. Or maybe LV skts in the top corners or something.

What do you say that reason is?

NT

[Dave Plowman (News)]

Ah. They wouldn't here and by quite some margin. At an average you're talking only 6 or so. More than that around this computer. 20 amps might be more like it. Probably more with some occasionally used devices.

Then regulating it at each and every appliance? Starts getting expensive that - as to maintain efficiency you'd be into using switch mode regulators - analogue types are horribly inefficient.

I suppose you could disguise it as a plantpot?

I'd want a backup type anyway if running everything off one.

[Jonathan Schneider]

That's before considering EMC problems which would be a nightmare to deal with.

Jon

[Andy Wade]

Some are, but many are in the 6-12 VA range.

[Dave Plowman (News)]

Think the largest non SMPS one I have is a 12 volt 1 amp which charges my jump start pack.

[djc]

Better to design a universal wall wart with sockets in it. Then one wall wart could supply all the power for multiple appliances from one socket.

Stage two: build wall wart type supply into a standard size double mains socket box. So you can have one standard mains plug and LV sockets at the same outlet, no need to rewire.

[meow2222]

I guess what I'm looking at is something thats good for mass use. For the minority of people with much greater demands, you/they will need to either use a few warts for the bigger loads or use an uprated LV system. I run most PC related LV loads off the built in PC PSU, its a lot neater than using separate warts.

no, keep it simple and low cost. 12v appliances can handle 13v fine.

I guess. If you want :)

Yes, fair point. Thinking of which it would be relatively easy to add battery backup for the few that wanted that.

I can even see a maybe way to reduce the cost of this using distributed supply.

NT

[meow2222]

But that only gets you LV at one location.

NT

[meow2222]

Trouble with this is no small appliances will run off it, and mass prduced low cost PSUs arent available. Might have been good if the existing kit didnt exist yet, but compatibility rules.

NT

[Mike Barnes]

I'll vote for that.

[Mike Barnes]

Is that always safe? In most cases I'd guess that the appliance would also be connected to the PC through a signal cable or similar, and I wonder if there's potential :-) for a positive/negative earth sort of problem IYSWIM. Similar considerations would apply if connecting two related appliances to the same wall wart.

[Dave Plowman (News)]

A wall wart, as has been stated, is restricted in maximum output by the physical need to plug it in to a double socket and retain access to the second one, and or switch - and even then some don't allow this. In practice it seems to be a limit of about 12 watts for a transformer type supplying DC and perhaps 20 for an SMPS. So although you might get away with a couple of low powered devices the average punter wouldn't know how to calculate what the total load is.

The thing that many seem to forget about is that sharing a PS isn't always that straightforward. There could well be rubbish on the supply rails of one device which would get transferred to the other connected in parallel unless special filtering is provided - and this can be difficult, bulky and costly. Which of course separate PS already do. So any savings aren't quite as obvious as might first be thought.

[Dave Plowman (News)]

Well, I've got several 'peripherals' that don't allow powering from the PC PS. A cordless phone. External speakers. Printer. Scanner. And a few more. These may or may not have built in PS or use wall warts. Whatever, they still require a mains distribution board on the back of the workstation, etc.

The actual voltage drop will depend on the gauge of the cable and the runs involved as well as the device current. And the last thing you really need is a high impedance power supply - it can cause all sorts of problems. Including EFI.

;-) Glad you appreciate the reference.

I'd have a dual PS as well - common in my field with large low voltage supplies.

My main concern is the way appliances just appear to grow from nowhere. A few years ago I had nothing that used a wall wart. First thing I got IIRC was a modem. Now I've got dozens. So if you installed a central system today with 'plenty' of outlets chances are it would be totally inadequate in a few years time. Exactly the same happens with mains outlets. Just look at most new houses built about 20 years ago.

[meow2222]

You'd have to spend a whole lifetime searching for a positive earth PC appliance. Perhaps someone somewhere has made a germanium amp and for reasons known only to ET decided to make it positive ground.

NT

[meow2222]

Some things always will. I dont see that as a problem.

I'm also wondering quite how so many of your periphs dont allow LV power, I've not seen any PC LSes that wont run off the PC quite happily for example, and nearly all scanners use 12v which is one of the PC voltages etc.

No, you're replacing a wallwart which is a high impedance supply, so there isnt a problem. The impedance of a run of copper, while it exists, is generally much lower than that of a typical wart. And of course the Z of a PC PSU is even less.

Quite so. But this is quite workroundable if you're willing to go with distributed supplies. Imagine we put this setup in 15 years ago with a

3A 12v psu, and now have 3x as many warts. Just hook the PC PSU to the 12v line and yo've got another 10A or so to play with, plus the distributed nature reduces system V drops..Anyway, cable is not expensive and running the thing in 2.5 T&E would be cheap enough. And good for distribbing 2 voltages as well.

NT

[meow2222]

Good point. But again easily solvable. Mark your psu with max current in amps, mark your loads with max load current, and if they pay no attention and overload it the psu goes into current limiting.

But we're talking about appliances that are designed to run on crap psu rails with no filtering. These appliances normally all have reservoir caps built in, so cant put much crap on the rails, and are fairly crap resistant too.

NT

[Anita Palley]

Thank you Chris, Gorty needs to untwist his knickers.

[Dave Plowman (News)]

Sounds ok in principle, but you'd be surprised how many b****er up invertors in caravans etc because they can't understand wattage ratings.

The actual mains transformer is a major part of the filter.

[meow2222]

Yes, which is why I dont suggest using wattage ratings markings with this system.

yes. There still isnt a problem though :)

NT