tbdannyPower is volts x amps, is it not? The desktop power supply I've got is 500W, and the mains AC voltage here in Australia is 240v. So, on the input side, from the power socket; 500/240 = 2.08A draw On the output side: 500/12 = 41.66A available. Is this correct? Will my approach of upgrading the wiring also work? Depending on how bright the lighting is, I may drop the middle strings.
The wiring is correct - you definitely want more than #28 wire for this! I'd run seperate bus runs to each section and fuse them off at something around the expected draw. If it's 5 amps for 15 meters, that's roughly 1 amp for every 3 meters. So just add up the length of all the strings you have connected to a given set of feeders and divide by 3 to figure how many amps you need to that group.
The breakout box is a good idea, that's what I was going to do - so if the power supply went out, all I have to do is plug in another one. Beats cracking them open and making the mods - plus no worries about the high power caps inside which can give you quite a jolt even if the supply has been unplugged for hours. You're a little optimistic on the draw from the wall, figure on basic computer power supplies running at about 80% efficiency, on a good day. So if you need 40 amps at 12V, 480 watts out, it will be more realistic to say it draws 600 watts from the wall, at 240VAC that's 2.5 amps.
Based on what I've seen others do with similar widths, I figure I will need 3 strands on most of my layout, but I plan to make one of them an RGB strand so I can do daybreak/sunset/night effects as well as have it on full white with the other two for broad daylight.
--Randy
Modeling the Reading Railroad in the 1950's
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INPUT SIDE:
First off power conversion isn't 100% efficient. So you lose some of that conversion of 240VAC->12VDC to heat. So figure 85% efficiency conservatively (depending on how well the power supply is built and load). So you're actually drawing closer to 2.08A / .85 = 2.44 Amps (If it's a true 240 outlet and the power supply can handle 240V....most in the states are rated for 120/220 operation)
Now on the OUTPUT side:
Just because a power supply is rated at 500 Watts doesn't mean it can deliver all 500 watts on a single 12V rail. That's the combined power of all the rails. (3.3, 5, +12, -12 etc...) And a 12V rail pigtail (the cord that comes out of the box is a pig tail because it's attached on) might be actually only rated to handle as low at 3 amps on a single rail.
The real telling clue is to look at the label slapped on the side of 99% of quality power supplies which tells you the amp rating of each rail.
Didn't know it was so complicated did ya?
The good news is even a mediocre quality 500Watt power supply should be enough lighting and power for accessories for a good size layout.As a rule of thumb White LEDs are about 6x more efficient then a standard incandecent. So let's say your power supply label says the combined 12V rails can put out 300 Watts. Well that's only about 5 60 Watt incandecent bulbs...but with leds, that's about 5x6 or 30 incandecent bulbs equivalent lighting for the same power. And as a perk they last about 20,000 hours. Now you see why the government, power companies, and manufacturers push these things so hard.
Here's a good tutorial...
http://makezine.com/projects/computer-power-supply-to-bench-power-supply-adapter/The only thing I don't like about the above tutorial is that the 12V and 5V pins might be on multiple rails. So I like to wire together the 12V wires together, and the 5V supply wires together from the accessory wires as well.
Don - Specializing in layout DC->DCC conversions
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rrinkerI've tried a whole bunch and adding that load resistors have been pretty much unecessary. Maybe mine were better quality power supplies. Back in the 286 days, AST computers came with a load resistors in the case that you had to hook up unless you added a hard drive. Talk about low quality power supply...well, the whole things were low quality but that's beside the point. And especially if there is any sort of load on the power supply - the load resistor definitely is not needed.
LION not so sure. Modern computer power supplies are switching supplies, which do require a load or they will not work. Put the load back and they should work.
OLDER supplies may have been regular transformer supplies that may not have required a load, or may have had a built in load on the assumption that a simple thing like a computer (sans all of those drives) did not draw wll that much power anyway.
Computer supplies are the same for USA and for Astralia-Europa,etc. since they have a little switch to select between one and the other, and the newer ones have not even that, they will take whatever you put into is and upt out the right stuff.
ROAR
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Here there be cats. LIONS with CAMERAS
The fact that it is a switching power supply does not mean it requires a load to work. It's the design of said switching power supply. Most of the ones I've tried in realtively modern times (beyond Pentium III days) have turned on just fine under no load. The 12V side is usually a bit low, 11.5V or so, until a load is applied, and the 5V lines run about 5.03V, which drops under load (but that part is likely due to the nature of the supply which typically produces just 12V and the rest is all DC-DC converters making the lower voltages). Despite the voltages not being dead on (and even WITH load - they seldom are, just read any of the decent sites that review power supplies and actually chart the outputs under various loads, like HardOCP), they turned on just fine then connecting the proper lines, with nothing else attached to any of the connectors. Non-computer ones, if you look up the specs of ones solder by Mouser or Digi-Key, you will see some specify a minimum load and some say they do not need a load - all switching power supplies.
Old transformer supplies mostly went away by the PC-AT era. Those AST computers I was talking about that actually had a load resistor attached to the top of the power supply, or in the hard drive cage area, were definitely switching power supplies. The last computer I had that used a transformer and not a switching supply, hmm, I was thinking my TRS-80 Model 4P but I'm not so sure, I think it would have been a lot heavier with a big solid transformer in it. My DCC power supply is a simple transformer. My first computer has a BIG transformer supply with a physically huge main capacitor in it, and a row of TO-3 case voltage regulators arranged on a thick aluminum plate for a heat sink. It's meant to allow the computer to be expanded, the basic system plus the expansion board I have draws maybe 2-3 amps total.
As for design - most 500 watt and under supplies are signle rail, only a few oddballs did multiple 12V rails in such a low power supply. ANd these days, the bigger ones are going back to single rail, because it's just easier for people to figure out, especially when hooking up multiple video cards. There are plenty of supplies that have a combined output plenty high enough to support dual video cards, except that the arranagment of the rails is such that you can't evenly divide things up, especially if you have multiple hard drives. It's almost insane some of the ones you can get now, 1200 watts total, with more than 90 amps on one 12V rail. The label is the place to start to determine the layout of the outputs, although there have been plenty of cases where the label says one thing and an analysis of the internals says another/ It's easy to tell a single 12V rail design if you open it up - if all the +12V wires end up bundled together in one giant solder point, it's a single rail supply.
No, do not connect the rails in parallel. If there is no documentation on which connections go to which rails, you might be able to glean this insformation from the label. It should at lease indicate total capacity for each rail. If it's a modular power supply, the 12V connection on the 20+4 motherboard connector will almost always be a different rail fromt he PCIE6/8 connections.
Randy,
Below is the label of the power supply. It is literally all the information I've been able to find on the PSU:
These are the connectors coming off it:
My main concern is that the connectors won't be able to take the 12A current I'll be pulling down them.
EDIT:
I have actually got a second PSU, which is used (as opposed to the unused one in the photo above). This is a Tauro XTK-TB-0600B, which has a single 12V rail capable of putting out up to 36A.
I'm guessing that this 36A output can't be put over just the one connector, otherwise it'd melt the plugs. Given that this is the one rail, would connecting multiple plugs from it help to distribute the load?
On the dual rail, you'll probably have to crack it open and trace the wires to see what connects to what rail. You cna also google the exact model number to see if any PC site has done a review which might have the details.
With one rail, you can parallel however many connectors you want, or at least wires - the 20 pin has multiple wires for 12V so you can parallel all those for higher current. You can either prallel everything, then divide this up by fuses to individual runs, or parallel 2-4 wires for each 5-6 amp run - again with a fuse or breaker on each one. Either way is electrically equivalent.
Well, I tried googling in a more general sense, and found this: http://www.playtool.com/pages/psumultirail/multirails.html
Seems that one rail goes to a 4-pin connector, which can take 8A across each pin - 16A in total. I'll use this to drive the 6A circuit and one of the 4A, then the other 12A from the other rail.
This is why I wire mine into a breakout box. Inside the breakout box I take the PC board connector and 2 PCIe connectors and wire the +12V together inside the box and crimp then together with a spade and then solder that to be double sure. I then use a single ground of 12 gauge which are spaded to each terminal.