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Deck 01-04-2012 10:40 PM

Any electricians? Question about a DC switch. Switch is on ground wire(s).
Hi I have an electrical question I can't figure out. I have a PSU (Power Supply Unit) in my computer. The unit converts AC power to DC power which powers everything. There are cables called molex cables which have 4 pins, one 12v, one 5v, and two ground.
I need to turn one of my hard drives on and off (EDIT: I don't mean off while it is being powered/in use. it's more complicated than that. read my replies) and I'm going to buy a switch for that purpose. I found a switch but it is not wired as I would've expected. The actual switch part is on the ground, not the +12/5, and the grounds are combined. I contacted the company and they say they wire the switches that way because it's easier than having to wire a DPST switch. Here's a picture of what it would look like if I used their switch on my device (hard drive):

So my question is do you think that's safe? If it is why do power supply units give two ground wires if they're both the same ground circuit? Maybe I don't understand ground. What I was expecting is a DPST that breaks both the +12 and +5, leaving the grounds untouched and not combined. Does the way the switch is wired mean my device would always be receiving power but it would be ungrounded unless the switch was closed?
Furthermore, if DC power always flows from negative to positive, does that mean the ground electrons are flowing back into the 12 and the 5? It just doesn't look right to combine the grounds like that.

I realize this may be a lot to take in and I have no idea what I'm talking about. Sorry about that.

Julius793 01-05-2012 02:24 AM

I would think it can be an issue because you will have to different voltages flowing on the same neutral but then again all I do is residential. However an electronic engineer would really be able to tell you.

dmxtothemax 01-05-2012 05:18 AM

It was done that way simly for economy,
One switch instead of two switch .
Is it safe ?
this is too general a question !
Under most circumstances yes !
But under some other circumstances,
its not the best way to do it !

So what specifically are your concerns ?

In a circuit it doesnt matter much where you break it !
Once it is broken it stops !

Where switchs are put is dictated by other concerns,
like maximum safety under all circumstances.

zappa 01-05-2012 05:47 AM

What you are showing will work just worries. There are different reasons for running multiple ground wires but it this case it is for current handling and manufacturing reasons.

AllanJ 01-05-2012 07:53 AM

Computers and automobiles are not subject to the same rules as household electrical wiring. In the case of automotive wiring it is quite common to switch the ground (or neutral) and there are even situations where the same wire acts a hot for one circuit and, at different times, a ground for a different circuit using a double throw switch.

For the switched ground cable you have diagrammed I trust that there are no devices (loads) that commingle the 5 volt and 12 volt power, for example to obtain 7 volts. In such a case turning off the switch would not kill the portion of the circuit that used both the +5 and +12 lines.

joed 01-05-2012 08:41 AM

Those are not really ground wires. They are negative wires. The ground is connected to the case and the ground pin of the 120v plug.
In a DC circuit it doesn't make any difference switching the negative. It doesn't create a safety hazard of shock to ground like your 120 volt ac house wiring does. The only logical reason for two negative wires is for increased current capacity. If you open the PSU you will find all the negatives connected to the same place.

The reason the manufacture does it that way is to cost. It is much cheaper to use a single pole switch on the negative than a double pole switch on the +12 and +5.

zappa 01-05-2012 09:03 AM

I got to thinking about this on the way to work and I wanted to add another reason for separate gr.... negative (thanks joed) wires. While they connect to the same point electrically, there may be some filtering. One of your hot wires is dedicated to the spindle motor and the other powers the electronics. The thought is to keep motor noise from entering the electronics through the negative wire. Maybe some computer folks could confirm this and give more advice for your circuit but I'm still not seeing any major concerns.

ddawg16 01-05-2012 10:08 AM

I wouldn't switch the common if the 5 and 12 vdc lines are still hooked up.....

If it was only one voltage...then, yea, you can break the common.

But with the 5 & 12 v lines hooked up, you still have 7 volts of potential between the those 2 power inputs....depending on your drive, this may or may not hurt it....I personally wouldn't take a chance.

BTW...DPST switches are a dime a dozen....

mpoulton 01-05-2012 01:51 PM

Computer power supplies generally don't support the use of the 5V and 12V lines together as a 7V source, because that results in the 5V line sinking current instead of sourcing it, which the rectifier and regulator components often don't allow. Some PSU's will work this way, but many won't, so computer hardware doesn't attempt to do it. Also, there just isn't really a need for a floating 7V source for anything. Thus, switching the negative connections is just as good as switching the positives. The only catch could be if both the drive and the PSU have the negative busses bonded to the chassis. If they do, then the switch will do nothing because the current will return through the metal cases. Most computer hardware maintains DC isolation (but not AC) between the negative bus and the chassis, so this is unlikely to be a problem. And yes, both negative lines connect to exactly the same place in both the PSU and the drive. There is no distinction between them; they are in parallel only for increased current handling.

The only problem you could encounter might be that the switch doesn't turn the drive off if the chassis is a functional current return path. There's no way I can think of that switching the negative lines could damage your hardware.

gregzoll 01-05-2012 05:02 PM

Personally I would not do it. One reason is that the operating system at one point is going to forget that drive exists, due to it will corrupt the MBR either on that drive, or the file table on the master, the other is that there really is no need to do that.

If you are only wanting to use that drive, set it to spin down in power settings, or if not wanting someone to access for who knows what, encrypt and use a password. Now of course, you could use hot-swap trays if you have a 5 1/4" face, which is the old way of doing things, or get a USB drive enclosure, or install a e-sata port to hook the drive up to, using a e-sata enclosure.

Deck 01-05-2012 05:16 PM

I really appreciate all that you guys do here. This forum has been a great source of information for me. I didn't expect so many replies. It really means a lot!

Deck 01-05-2012 05:19 PM


Originally Posted by gregzoll (Post 813621)
Personally I would not do it. One reason is that the operating system at one point is going to forget that drive exists, due to it will corrupt the MBR either on that drive, or the file table on the master, the other is that there really is no need to do that.

The BIOS locks the drive so I need to power it up after boot. I do understand what you're saying, but once I turn the drive on I don't plan to turn it off. I use AHCI and it will detect the drive after boot. You make a very good point though.

gregzoll 01-05-2012 05:32 PM

It is your risk, I personally would not do it, since there are other ways to do this. And I personally do not see a reason to put in a switch inside, unless you are thinking of other methods. Best way if you are really wanting to save money, etc, use a SSD.

All you are doing is trying to reinvent the wheel. Here is a option for SATA drives
Hot swap tray

pw5599 01-06-2012 01:58 PM

It's not a good idea to switch power on & off to a drive while the computer is running so I'd advise against it, you may harm the drive. So why are you wanting to do this?

davido30093 01-06-2012 02:38 PM

I agree with the others that this is NOT the best way to do this. You are likely to damage the drive by hitting the switch at the wrong time. While many DC circuits including most automotive equipment and most communications equipment turn off and on by interupting/connecting the negative side of the power supply, if you do this to your hard drive at the wrong time, damage is very likely to occur.

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