Using PC Power supply for 7.5 volt power adapter

[DeLuSioNal29]

Hey all,

I wanted to wire up my PC speakers directly to my PC's power supply.  The power adapter for my PC speakers say "7.5v DC" on it.  Do you think I can get away with wiring it up as 7v directly from the power supply?  My thought is that I want them to turn on when my power supply comes on.



DeLuSioNaL

[ed12]

try it on your 5volt rail..not the 12v..
or open it up and see what the main filter cap is rated for,and if there is a regulator
in there..7.5 is a odd voltage,but as a rule i would see it if it is going into a 5v reg.

ed

[DaOld Man]

12 volt + and 5 volt + (red and yellow wires) will give you 7 volts.
I don't like doing this, but I have heard of others doing it.
How much current do the speakers need?
Switching to experimenters mode: Measure voltage from yellow to red, I think red  yellow will be negative, but make sure, my head's a little fuzzy right now. Once you have polarity hammered, hook it up and try it.

[lilshawn]

+5 red to NEG and +12 to POS gives 7 volts.

the current carrying capability is that of the smallest current channel you are using IE: if you wanted 10 volts and you used the red +5 volt and the white -5 volt line (to get 10 volts) the +5 volt line is capable of delivering 60 amps, but the -5 volt line is only capable of delivering 175 milliamps, your 10 volt "line" you created is only capable of driving 175 milliamps.

handy dandy chart...print that sucker out:

[ed12]

which both ideas are perfect
expcet 1 little thing
u CANNOT USE THAT SAME POWER SUPPLY TO POWER THE TOWER
gee i wonder why u say.. hmm 12v 5 volt..your carzy ed..welp think again
that pesky little thing called common ground...
biacth as it is...

ed

[dkersten]

I was just thinking the same as ed, if you do this be careful of your grounds or you will just end up shorting out one of your rails.  Soon as you plug in your audio jack to the PC you will complete the ground loop and either fry a rail, fry the amp inputs, fry your audio chip, or at the very best have a ground loop in your audio that will cause a hum.  If you still want to try it, then before you plug in and after you hook up your power from the PC, meter between the lower part of the 3.5mm audio plug and the jack in the computer.  If you are seeing something like 3 or 5 volts DC, I wouldn't try to plug it in, you should see zero if the ground for the audio is isolated from the ground of the PSU.

[MonMotha]

To reiterate, while it's possible to get 7V from a PC supply, the reference ("ground") for that 7V is not the same as the reference used by all the guts inside the PC (the black wire labeled "ground").  If you tried to connect them, say, through the audio cable, you'd be shorting the 5V rail out.  Given that the 5V rail on a lot of PCs can supply a few dozen amps, you definitely don't want to do that.  Melty wires could well result.

The easiest thing to do here is to just get a wall wart and find a way to switch it.  Some PC speakers actually need a floating supply.  They actually establish an internal reference separate from either of the power input lines and tie the audio ground to it.

If your speakers don't do that, you might find that the speakers work OK on 5V and just won't get as loud.

[lilshawn]

---steaming pile of meadow muffin---, you are right. 

the ground on the power supply and the ground used on the shield of the audio cable are tied together. BZZZT fried cables.

you could plug into the PC through a ground loop isolator though... this separates the ground (and audio) and eliminates the DC path...

http://www.radioshack.com/product/index.jsp?productId=2062214

[DeLuSioNal29]

Perhaps I should explain what I'm planning to do:

I'm currently using a recycled PC power supply to power a 60 in 1 board using the molex connector.  I have a Jamma harness connected from tbe 60 in 1 to the controls (power wires not connected to Jamma connector since the molex is giving it power).  I have an LCD.  I have a cheap pair of PC speakers with a power adapter (7.5v).  These are all plugged into a power strip.  (3 plugs).

I want to power up the arcade by simply reaching in the back and flicking the switch of the power supply.  Then all 3 devices will power up.  I planned to accomplish this by:
1)  Cutting off the power adapter from the speakers and connecting the power cord to a molex connector's +12v and +5v (creating 7v).
2)  Splicing the power cord from the monitor (a typical PC power cord) to the power supply plug's terminal inside the power supply. (110v).
3)  Leaving the 60 in 1 board plugged into a molex connector and clean up the extra wires/connectors not needed.

I think it should work, since I'm not using a PC with ground.  I do have a headphone jack on the 60 in 1 board.  Would I fry it if I disconnected it for any reason while the power was on (causing the rail to fry).  My main concern (originally when posting this) was that the 7v created from the molex would not be enough to power up the 7.5v needed for the speaker's draw.  But now I'm concerned about shorting it out completely.

DeLuSioNaL29

[dkersten]

Just save yourself the headache of trying to do all that and get one of these:
http://www.amazon.com/gp/product/B0050HH70E/ref=oh_aui_detailpage_o03_s00?ie=UTF8&psc=1

There are about a million versions of them out on the interweb, many cheaper than this one.  You also need to get a fuse for them based on 110 or 220v. 
Then take your power strip, cut the plug off, and wire it to that socket. 

Now you can use that C14 power cord, have your single switch, and have a power strip with however many outlets you need, and you can use the 7.5v power supply.  And you don't have to mount your PSU so it sticks out the back of your cab..

[DeLuSioNal29]

Great idea for a full sized cab, I love it and will probably use it on my next full sized build, thanks!  But this hack will be for a Vigolix clone for my 6 year old.  So there is no back to the mini arcade. 

D

[dkersten]

OK, if you are set on using the psu for the master power, you could also use a simple bosch style 12 volt relay to jump your 110v over to that wall wart.  The typical 12v 30a relay is good for 3 amps at 120 volts on the switch side (12v trigger). 

(see below for diagram)
Pin 85 to ground on the PSU (black)
Pin 86 to the yellow wire on the PSU (12v)
Pin 87 to the black wire on the 110v (where your monitor power is tied in)
Pin 30 to the wall wart (small spade on the plug if polarized, otherwise just one of the two spades, doesn't matter which)

Then the other spade of your wall wart to the common 110v (white)

When you turn on the PSU, the relay will close 87 to 30 and turn on the wall wart and you get 7.5 volts for your audio..

[lilshawn]

this is a good option. that way it turns off and on with your other power.

another option is to wire a powerbar inside the cabinet and plug everything into that. that way, when you hit the main power switch...the bar gets power and your power supply for the game, speaker adapter, and monitor all get power at once.

ADDENDUM:

the reason powering it with the power supply like i mentioned won't work in this case is because the shield of the audio output jack of the computer (or xx-in-1 board) is tied to ground internally on the board. that means all the black wires in the powersupply and all the exposed metal on the chassis are tied to the black "ground" connection. (this includes the USB and the shield connections on all the IO ports and monitor connectors...even the metal case itself.)

when you power it with 7 volts from the power supply...the audio input plug on the speakers is also integrally tied to ground. (to keep noise from entering the audio amp and causing buzzing.)

when we power it with the (-) lead attached to the 5 volt line to get 7 volts it is effectively attached to the shield of the connector.

if you leave it unplugged, nothing would happen the speakers would operate normally (plugging an mp3 player would have it working fine)....but as soon as you plugged it in to the ground sheilded audo connector on the board, the power supply would shut down (hopefully) because the 5 volt on the shield now is being attached to ground and shorted out. worst case scenario is the power supply continues to power the 5 volt rail and melt all the wires into a fiery mess.

CONCLUSION:

powering items straight out like fans or LED's or motors using a mixed supply like this is fine. but not for anything that is attached BACK to the computer through another connection (IE: audio,usb,serial,video,etc.) or anything that will be attached or touch to the metal chassis of the computer. (because the screw holes on the board may be tied to "ground")

[DaOld Man]

DOH!
I didn't think about the ground either. Don't use your pc power for this project!
EDIT: Don't use your PC power +5 and +12 to get 7 volts.