How to get +7 volts from computer power supply?

I have used modded computer power supplies many times to power my creations, but in the past I have always relied on just using either the +5V red wires, the orange +3.3V or the +12V yellow wires.  However, most sites that offer advice on diy lab power supplies always say to hook the 12V to the 5V to get the 7V.  My question I guess is do I hook the +12V to the +5V to be the new +7V positive, and then rely on one of the black ground wires to be the negative? Or, is it the +12v  wire hooked up to my 7V devices positive lead and the +5V wire hooked to the devices negative lead?  Any help would be great, kind of stumped.  Multimeter was lent to a friend, and I'm not really in the mood to fry electronics to test it.  Thanks and sorry for the oddly worded question, having trouble following myself at this point.

Comments

[steveastrouk]

People do some dumb things to get what they think they need. The 12V would be linked to the 7V line, the 5 V to the "ground" line. That means that the 5V line is SINKing current, whcih it isn't designed to do.

This is a really dumb idea.

You'd be much safer putting a regulator off the 12V line and using real ground as ground. What are you powering, and how much current does it need ?

[frollard]

It never occurred to me that doing this was bad -- I guess running series to boost power can be appropriate (so long as there is no centre tap to ground) -- but subtracting voltage really must be damaging since it runs one charge against another to reach the goal... Noted!

[iminthebathroom]

Fair enough, several instructables are vague on this point, they just say that you can output various voltages by using certain wires, one of them being 12V and the 5V lead. It is powering an array of 4 LED's, each LED is protected by its own resistor. The led/resistor combination's are linked to a photo-transistor creating a dark sensing circuit. Further redundancy is provided in a 12v resistor. I have ran the circuit for a week straight so far on the 12V provided by the computer power supply and all seems well. Here is where the 7V comes in. When fitting the led arrays on to the base, I found I was able to power the arrays with a 7v battery I had laying around for testing purposes. Since the other instructables stated you could output 7V i thought why not go with the lower current. Thinking that perhaps this would put less strain on the circuit. Please forgive my ignorance to this as it is information gleaned from various sites and tutorials. No training what so ever in electronics, as of yet. The LED arrays I have built are not on board, they are not on a micro controller, they are soldered together rather red neck style. I'll post a couple pictures to laugh at, however they do work.

[iminthebathroom]

Frollard, hopefully you can see the reply to steveastrouk, if not leave a comment and I will post the whole response. Just wanted to say thank you, you had answered a question for regarding omni directional mic's. It worked just as you said it would, awesome!

[steveastrouk]

I wouldn't worry. What resistor have you used ? and what voltage are the LEDs supposed to run on ?

Stick to 12V.

Steve

[iminthebathroom]

I believe the LED's were 3.2v paired with 1/4w 470ohm resistors rated for 12v , these were from the supplier specifically for these green LED's. An additional 22,000 ohms ± 5% resistor is on the positive power lead running into the main circuit. Sorry about the iffy-ness of the components exact power as I bought these several years ago and the documentation to begin with was lacking. The resistors feeding the LED's are yellow, purple, black, black, brown. I believe 5 digit color coding is military oddly enough. The larger resistors are red, red, orange, yellow. Once again feeling like I'm standing on the shoulders of giants trying to pull this off. And yes, I believe 12V will be the chosen voltage. I have uploaded several more pictures, this time with higher resolution.

[steveastrouk]

5 digit colouring is perfectly normal.

How does this 22K fit in ?

Steve

[iminthebathroom]

Was a recommendation from the couple at www.evilmadscientists.com. I am uploading a diagram of the circuit. Sorry it is not an actual schematic, learning to read/draw with the proper icons will be next on my list.

[steveastrouk]

Very neat drawing.

I have my doubts about the wiring of the transistors, as shown. I think you have the emitter and collector.

Steve

[iminthebathroom]

Think your right, thanks again, just wanted to show how the 22k tied in, Not really sure if its needed or not, as the other resistors are rated to 12v, but... the circuit is working as desired and somewhat moot as they 60 arrays are all ready built.

[lemonie]

Why 7V...?
I'm sure there's a better way (steve's comment noted & agreed)

L

[iminthebathroom]

Oh for sure, My misunderstanding that if a circuit could be powered by less voltage, as in the 7V that it would be easier on the circuit. I have ran it just fine on 12V and that is what I'll be doing. On several Wiki-hows and instructables that convert a old ATX power supply into a lab type power supply they always state a variety of potential voltage outputs, one of them being 7V - which happens to be the battery voltage I was using to test the circuits after moving them about. I do appreciate all the input, one learns often by failure. I must be learning a lot!

[lemonie]

Well I guess you've got a trade-off with the circuit being more efficient against the PSU being less?

L