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Old 06-28-2010, 07:39 PM   #1
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Default 'Grounded' leads on a DC plug?

I've seen leads of internal power connectors labeled as 'ground'. Is that the right phrasology? To me, 'ground' always suggested AC current, whereas I thought a computer's internal power was all DC.

One of the reasons I'm asking is because I need to know how to hook up 12 volt molex connections in my computer to get 24 volts to power a pump...
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Old 06-28-2010, 07:47 PM   #2
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Default Re: 'Grounded' leads on a DC plug?

Ground has nothing to do with AC / DC specifically at all - it's a generic term and usually simply means a potential of 0V. Current flows from a high to a low potential, so this essentially acts as the return path for the current for any positive voltage.
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Old 06-28-2010, 08:51 PM   #3
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Default Re: 'Grounded' leads on a DC plug?

Quote:
Originally Posted by gulfcoastfella View Post
I've seen leads of internal power connectors labeled as 'ground'. Is that the right phrasology? To me, 'ground' always suggested AC current, whereas I thought a computer's internal power was all DC.

One of the reasons I'm asking is because I need to know how to hook up 12 volt molex connections in my computer to get 24 volts to power a pump...
EDIT: Oops, read below
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Old 06-29-2010, 03:44 AM   #4
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Default Re: 'Grounded' leads on a DC plug?

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A way of thinking about it in terms of water flow is when you hook up two pipes of the same size, the pipe diameter doesn't change, but the force of water flowing through does. Amperage is this force.
No no no!!! What you've described there is voltage, not amperage.

Taking your pipe analogy, the pressure of the water flowing through the pipe would be the voltage and amount of water flowing through the pipe (diameter of the pipe for instance) would be the amperage.

If you've got 2 12V sources, you can wire them together in series or in parallel. If you combine them in parallel then it's the current that's combined between the two and the voltage stays the same. If you wire them in series then the voltage will combine and the current will stay the same.

Regarding the pump, is it definitely designed to be powered from a PC? All the ones I've seen for PCs are 12V (for obvious reasons!) If it's not then I wouldn't bother using it...
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Old 06-29-2010, 03:56 AM   #5
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Default Re: 'Grounded' leads on a DC plug?

Oops, definitely talking out of my ass. Would it even be safe or possible to wire molex connectors in series to obtain 24V? I'm pretty sure if the rail voltage ratings are there for a reason
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Old 06-29-2010, 04:30 AM   #6
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Default Re: 'Grounded' leads on a DC plug?

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Oops, definitely talking out of my ass. Would it even be safe or possible to wire molex connectors in series to obtain 24V? I'm pretty sure if the rail voltage ratings are there for a reason
The only way the OP can get 24V from the 12V supply is to build a small switching inverter circuit that steps up the 12V to 24V. It basically means switching the 12V on/off at high Fq, feeding it to a small transformer of the appropriate winding ratio then rectifying and filtering the output.
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Old 06-29-2010, 01:12 PM   #7
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Default Re: 'Grounded' leads on a DC plug?

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Originally Posted by Remeniz View Post
The only way the OP can get 24V from the 12V supply is to build a small switching inverter circuit that steps up the 12V to 24V. It basically means switching the 12V on/off at high Fq, feeding it to a small transformer of the appropriate winding ratio then rectifying and filtering the output.
Oooohhh. Yeah, I won't be going there... Luckily, all of the pump's operating modes run at 12 volts; the 24 volts is used to boost the highest mode to an even higher level, which I probably won't need.

I'm not sure that two molex connectors can be wired in any way to produce 24 volts, anyway. I've been going over it in my head and can't figure out a way to do it. I won't worry about it though; at this point I'm just considering the pump as a 12 volt unit.

Thanks for the input, guys.
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Old 06-29-2010, 01:37 PM   #8
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Default Re: 'Grounded' leads on a DC plug?

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Originally Posted by gulfcoastfella View Post
I'm not sure that two molex connectors can be wired in any way to produce 24 volts, anyway. I've been going over it in my head and can't figure out a way to do it. I won't worry about it though; at this point I'm just considering the pump as a 12 volt unit.

Thanks for the input, guys.
It wouldn't work because the ground and the 12V is common throughout the supply at all the terminals. Even if you use a multi rail PSU the 12V may be supplied from different rails but the grounds are still common throughout the PSU and thats the problem; you need to use completely independent supplies. Think about doing the same thing with batteries: it possible to put two 12V batteries in series to get 24V output because the negative and positive terminals on each battery are separate from the other battery. Now imagine, for some strange reason, that the negative terminal on both batteries is joined by a wire and the positive terminals are not common but you still try to put them in series to double the output... heat, cable melting maybe. Do this with an ATX PSU and you may well kill it.
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Old 06-29-2010, 07:59 PM   #9
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Default Re: 'Grounded' leads on a DC plug?

I knew I had that answer in my head; I just couldn't work it out because it makes so little sense to wire batteries that way. Thanks for clarifying, though!
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Old 06-30-2010, 07:04 PM   #10
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Default Re: 'Grounded' leads on a DC plug?

Remeniz has got there before me and he's spot on, you can't grab 24V from 2 molex connectors.

Just for theory's sake, you can get 24V from an ATX power supply beacuse on the main 20 pin connector pin 12 houses -12V. Connect that across from a 12V line and you'd have a potential of 24V.

However - don't do that! There's a good chance you'll start frying things...
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