Choosing the right power supply!

I am trying to choose the right power supply for my project such that it will fit nicely and I won't die using it. Here are the requirements:

1) It needs to provide 12A or more at 5V.

2) I'd have a nice latching switch that I'd like to use to switch the power supply on.

3) The smaller / cooler / safer the better.

I have 2 brand new ATX power supplies which meet the power requirements, but perhaps not the size or switching one. I know there is a green wire in there that you can use to switch on an ATX supply if the normal switch on the back is 'on', but if power is only coming into my supply through the socket on the ATX supply, I don't konw where to get the power for the switch to ground the green wire!

I've seen some 'open' power supplies on sites that look like the inside of the ATX supply, but I don't want to electrocute myself if those aren't safe. It will eventually be enclosed and bolted down in an acrylic box.

Can I have any advice you might have?

Open frame supplies are perfectly safe as long as you don't go probing around (with fingers or anything else) while they are plugged in!

:)

Seriously - I've used open frame supplies for powering motherboards in "embedded" situations with no problems; if you can find one that meets your requirements, use it. Wire it up to line voltage as the manufacturer datasheet recommends (if it requires a grounded plug - USE IT). Then put it in your acrylic enclosure (be sure to add ventilation and fans if needed).

One other thing: A nice open frame power supply can be had from a TIVO - they aren't badly sized, and they provide enough power to run the motherboard and hard drive of the TIVO, so for embedded PC systems (Mini-ITX and the like), they could be perfect. I tend to find old TIVOs for salvage at thrift stores like Goodwill (plus, you generally will find an 80 gig or so hard drive inside as well!)...

:)

Ok, that's a good tip. A couple more concerns:

How do you switch these power supplies on?

I have a latching switch I would like to use: press in once it stays on, press in again, supply goes off.

Also, will the datasheet tell me if I need a fan? Do any of them include a fan? I just need this to be low profile vertically, it can be huge horizontally (up to 12"x12")

Also I'm a bit wary of wiring up mains - are there any precautions I need to take (other than obviously making sure I'm never touching ANYTHING when it's plugged in, not jamming my finger in the caps after it's been on)

I was thinking of trying to mount one of these:

http://parts.digikey.com/1/parts/100072-conn-ac-rcpt-8mm-snap-solder-716w-x2-01.html

in the panel. Then I suppose I somehow wire those to my power supply? Maybe I want a 12 amp relay that my switch drives which connects AC to the power supply?

Just want to be as safe as possible... What wires to use, what insulation to use, what is a grounded plug? (Three prong?)

With these power supplies you connect the green wire on the ATX plug (PS_ON) to a ground pin (one of the black ones - the closest one is fine). It will then turn on whenever there is power to the power supply.

I presume you are ok getting 5v from it. Just use a red and a black wire...

Mowcius

Yes, I know that much, but I am wanting to switch the power supply on with this external latching switch.

So, I would actually like to use the green wire. IE, I would like to do something like always keep green high, and when I switch the switch, green goes low and the power goes on.

But I don’t have the faintest clue how to wire that up:

AC Power comes in from the wall. I hook it up to this power supply I’ve bought. Now I want to be able to switch on the power supply with an external switch. The switch can only handle 5A, so I can’t put it between wall AC and the power supply.

If I use an ATX or similar, I can turn it by bringing that green wire low, but how on earth can I have my switch do that? How can my switch even have a voltage or current, if I haven’t switch the supply on yet!

Ahhhhh - does the green wire on ATX carry 5V when the switch on the supply is on? Could I just run that green wire to my latching switch, and the other side of my latching switch to a black ground wire on it?

How much current would go through that switch when I opened it? Aren't I shorting green to ground through a switch that can only handle 5A?

From what you are saying it sounds like you just want to have a switch to switch it on. Well put the switch between the green wire and ground...

I presume that is not quite what you mean though

If you want a standby line then there is 5V_SB which can normally carry 1-1.5A while the power supply is not 'on'. Bear in mind that if you don't turn the supply off at the wall (or at the switch on the back of the supply if it has one), then it will still be using electricity...

Mowcius

Also I'm a bit wary of wiring up mains - are there any precautions I need to take (other than obviously making sure I'm never touching ANYTHING when it's plugged in, not jamming my finger in the caps after it's been on)

From the plug to the power supply, use proper gauge wire (probably something like 12-14 AWG), and for extra safety, put in a fuse inline on the hot side (most power supplies have such a fuse already - check and see if it does, and if not, add it - use a standard fuse - not slo-blo - of an amperage equal to or a little less than the max current rating for the input side of the power supply).

I was thinking of trying to mount one of these:

http://parts.digikey.com/1/parts/100072-conn-ac-rcpt-8mm-snap-solder-716w-x2-01....

Well - you would likely want the opposite version - based on the image at that link, that seems like a "female" recepticle, you would want the male (like that on a PC power supply).

in the panel. Then I suppose I somehow wire those to my power supply? Maybe I want a 12 amp relay that my switch drives which connects AC to the power supply?

You are supposed to use a DPST switch, with hot and neutral connected to each half of the switch, so that both are switched at the same time (do not put the switch on only one or the other side; that can create a dangerous situation in certain failure modes); look at how old AT-case PC power supplies switches were wired.

Just want to be as safe as possible... What wires to use, what insulation to use, what is a grounded plug? (Three prong?)

Three prong, yes - is a grounded plug. If you are confused by any of this, or have other questions, it would be best for you to do a lot more research, and to also see if you can find somebody local to you to show you how things connect up. Mains power isn't something to be casual about; it isn't difficult to work with if you know what you are doing, but if you don't, or if you are careless, it can bite you, and hard. You can get everything from a hard tingle to death if you're not careful and understand what you are doing. There is a lot of information out on the internet on how to do all of the above. The basics, though, are (bad ASCII art ahead!):

                           DPST
                          Switch

    ______                   /
   |      \                 / /                    +--------------+
==|      |-- Hot --------+/ /  +-------[FUSE]-----|              |
   | Plug |-- Neutral -----+/   +------------------| Power Supply |
==|      |-- Ground ------------------------------|              |
   |______/                                        +--------------+

Hopefully this will illustrate what I was saying, but it would be best to study this and other information online (as well as at your local "big box" home improvement center on electrical wiring), before trying anything (for one, you need to learn how to identify the hot side and the neutral side of the plug).

Good luck, and if you are at all jittery about something, stop, and take a look at what you are doing before you plug anything in.

:)

[edit]NOTE - In my illustration above, the PLUG is supposed to be a 3-pronged plug, not 2 as was illustrated for brevity...[/edit]

Two points:-

1) You are thinking that because you have a power supply that will delver 12A at 5V, then you need a mains switch that will be able to handle 12A at mains voltage. It won't draw 12A from the mains it will draw much less. The power output is 12 * 5 = 60 Watts For a 240V mains input 60 Watts is derived from 60 / 240 = 0.25A Double this for margin and efficiency loss and you are looking for a switch that will handle half an amp. If you are running at a lower voltage mains then it is an amp.

2) A 12A power supply is very unforgiving as regards short circuits, things start to melt very quickly and fires can start. Until you are more experienced then please sick to 500mA supplies.

A 12A power supply is very unforgiving as regards short circuits, things start to melt very quickly and fires can start.

Which is why bench-top power supplies exist (though you can cause fires with them, too, if you don't have your overload setpoint set low enough!).

:)

There's nothing quite like seeing what you thought was a solid wire go up in a flash of smoke and light (and generally a loud bang) because of short circuit with a large amperage supply. It really makes you stop and think about what electricity is capable of.

;D