Quick question about wiring 12v & 5v from molex.

[quarterback]

I just want to make sure I don't fry my LEDs.

Okay, everybody knows you can get 1vDC and 5vDC out of a PC power supply via the mokex power connector



Some web pages show one of the black wires as being the (-) for the 12v and the other being the (-) for the 5v, but others simply refer to both black wires as grounds.

Question: Can I use just one of the black (-) wires for both 12v and 5v simultaneously?   In other words, can I use a single molex plug and connect one thing to pins 1 (5v) and 2 (ground) to my 5v leds and  then connect 4 (12v) and 2 (the same ground) for my 12v leds and be okay even though I'm sharing the exact same black wire?

I know I've read that there's some kind of 'trick' to getting 7volts out of a molex plug by connecting some combination of wires together.  I do not want to do that.  I just want to be able to get both 12v and 5v while only using a single one of the black/ground/(-) wires.

I believe I can do this without issue, but I just want to make sure.
Thanks

[Derrick Renaud]

Both blacks are the same ground.  They are only separated so current spikes on the 12V line are not reflected on the 5V line.  Causing problems with the control circuitry on the 5V line.

You can use a single ground, if you are not using a circuit that pulses/spikes a high current on one line that could interfere with the control circuitry on the other line.

D.

[pmc]

I usually use the black wire next to the yellow for the 12V circuit, and I use the other black with the red for a 5V circuit. I always figured that it's right next juice wire so it goes together with it -- and I've often seen it listed as "12V gound" and "5V ground" indicating that they should be matched like that.

That said, ground is ground so I don't see why you cannot take artistic license! There are 7 ground wires on an ATX power supply harness and they are often all labled "common". They all go to the same place.

-pmc

[Derrick Renaud]

I usually use the black wire next to the yellow for the 12V circuit, and I use the other black with the red for a 5V circuit. I always figured that it's right next juice wire so it goes together with it -- and I've often seen it listed as "12V gound" and "5V ground" indicating that they should be matched like that.

That said, ground is ground so I don't see why you cannot take artistic license! There are 7 ground wires on an ATX power supply harness and they are often all labled "common". They all go to the same place.

-pmc

Yes the grounds all go to the same spot in the power supply.  There are 2 separate grounds on the HDD end so the power spikes from the one supply do not affect the other supply, as I stated in my last post.

I'm not about to go into all the technical electronics about it.  But it boils down to you should never daisy chain the ground path, unless you are using minimal current with nothing pulsing the current.

I would be curious to see the ground path layout in the LCD TopGun recoil circuit.  It probably has a bad ground layout, leading to the disconnecting of the device that people have mentioned.

D.

[bleemus]

Yes, you can do that.  I'm not sure if there is anything else with it that we need to know, but it seems to work just fine for me.

I have two 12V LED bulbs run from two bayonet bases, and then two Perfect 360s run off of the 5V.  I took apart a female molex fan extension cord so I wouldn't have to chop up the power supply.  I just used wire nuts for the connections, but may switch to quick disconnects when I actually mount all this junk into a cabinent.