Computer power supply outputs question

[Mokele]

I figured this was more appropriate than the Computer science forum, since this is more of an electronics question.

 

I've noticed that many computer power supplies have a label on them, listing things like "+12V, 15A" and "-12V, 0.3A" and suchlike. I know the V and A mean volts and amps, but I'm a bit unclear of the meaning of the + and - signs with respect to voltages. Isn't voltage always a difference in potential, thus doesn't get a sign like that?

 

Basically, I want to hook something up to the power supply with a 2 Ohm resistance, and I want to know what wires to connect it to.

 

Mokele, probably well out of his depth on this project.

[YT2095]

+12 0 -12

 

all is taken in relation to the 0 volts or Ground (Vref).

 

if you take 2 car batteries and link them in series to make 24v total, where they connect in the middle you decide to call 0v and all voltage reference is taken from this point.

so from Vref to the + side will be 12volts, and from the Vref to Other side will be -12v respectively

 

edit; btw, you never said what voltage you want to put the 2ohm load across?

all is taken from Black (vref 0v).

Yellow is the +12v line and Red is the +5v line anyway

be carefull though, as even at the 5 volt supply your resistor will draw 2.5 amps and needs to be 12.5 Watt rated to sink that current.

these values increase further still on the 12v line.

[Lance]

You can think of a negative voltage as less than ground (0). In the computer PSU you could replace the black wire with the negative voltage output in a circuit to create a higher voltage difference. Such as the +12V and -12V to achieve a difference of 24V.

 

Normally this would be quite useful but in most computer PSU's the negative line can't supply anywhere near as much current as the positive. On mine the the +12V line can supply 18A and the negative 0.8A.

 

Also, line voltage fluctuates between +120V and -120V or higher depending on where you live.

[Mokele]

Ahh, thanks very much YT and Lance, that explains it! Now let's hope I really do understand and don't burn my apartment down.

 

be carefull though, as even at the 5 volt supply your resistor will draw 2.5 amps and needs to be 12.5 Watt rated to sink that current.

these values increase further still on the 12v line.

 

Well, I'm plugging it in to a peltier system whose sole purpose is a heat pump (to keep the roots of a fussy plant cool). Currently, the system has 5 volts running across it, and it's not doing jack crap (the heat sinks on the hot side are barely warm, and the pot of the plant is definitely not cool enough).

 

I'm hoping to experiment with this at a more leisurely pace over the winter, when the plant in question is in dormancy (and therefore won't be killed by my inevitable screw-ups).

 

Mokele

[Lance]

Are you sure your peltier is rated for 5V? I would suspect that as the junction heats up the resistance will rise.

[Mokele]

Well, considering it's on 5V now, I'm pretty sure. The stats I got from the dealer said my model was rated up to 15V, iirc. Special "hi-temp" thingy.

 

Mokele

[Lance]

Well' date=' considering it's on 5V now, I'm pretty sure. The stats I got from the dealer said my model was rated up to 15V, iirc. Special "hi-temp" thingy.

 

Mokele[/quote']

 

Ah, I see. I suppose 'rated' isn't a good word in this instance. I think you would have *much* better results closer to the voltage it was designed for. Although, please don't do this simply because I said it would work, you should be able to run it a bit over it's rated voltage assuming you have a large enough heat sink and fan. The only real danger is the junction burning up. Obviously if you drive it at 1/3 of it's intended voltage then it's not going to draw anywhere near as much current as the data sheet states and thus you won't get much of a temperature difference.

[Mokele]

::nods:: Thanks very much! The only reasons it's on a mere 5V is that a) I was afraid it's get too cold and freeze the plant and b) I couldn't find a power supply that would "fit". The two peltiers together are 3 Ohms, and I looked all over for something that would give both a high voltage *and* could take the resultant amps. (I posted about that before, and IIRC, you also responded to that one). The best I could find was an old laptop power supply at 5V. Or at least, the best which didn't cost far more than I was willing to spend. It was only recently I hit on the idea of using a scronged computer power supply instead.

 

Of course, if there's some cheap and obvious alternative that I'm just not aware of, please clue me in, since I know that this electronics project is about 4 levels above my experience/knowledge in the subject, and I'm sorta (ok, *really*) winging it.

 

Mokele