Also I am getting continuity between the scope ground and the metal chassis of the ATX PS. Isnt this dangerous?

That is essentially what this ENTIRE thread has been about. I think you finally understand!

--ETA: No, it isn't dangerous, since it is ground, Earth Ground, 0V, so if you are standing outside and wet, you won't get a shock from touching the enclosure.

Yes, I pulled 10k out of the air as a "Resistor that will limit current enough to not damage anything, yet show a value on his meter if there is a path to ground"

However, 5V to 12V should work fine for 7V if you use just the red wires, and call 5V "circuit ground" and 12V "+7V", That said, I'd still suggest an isolated supply. There is a reason they aren't as cheap as computer supplies, well, several reasons, but isolation is one of them.

 

A few years ago I came across a site with some very great articles, for people of all levels of education and expertise. These articles, specifically, would probably be of great use to you.

On proper earthing: http://sound.westhost.com/earthing.htm
On electrocution (and how to avoid it): http://sound.westhost.com/articles/electrocution.htm

 

I don't think that using 5 and 12V to create 7V is such a great idea. If the 5V isn't loaded - so that there still is positive current coming out of it, the 5V rail will rise if you pass any larger current into it. This might activate the overvoltage protection and switch the supply off.

 

If you want to learn and read about grounding. I would recommend these two books
Electromagnetic Compatibility Engineering by Henry W. Ott. And Grounds for Grounding: A Circuit to System Handbook by Elya B. Joffe and Kai-Sang Lock. Perhaps the library can get the books for you

IEEE-STD-1100 provides the following definition of the term “Ground”:
A conducting connection, whether intentional or accidental, by which an electric circuit or equipment is connected to the earth, or to some conducting body of relatively large extent that serves in place of the earth.
Note: Grounds are used for establishing and maintaining the potential of the earth (or
of the conducting body), or approximately that potential, on conductors connected to it and for conducting ground currents to and from earth (or the conducting body).

 

If you want to learn and read about grounding. I would recommend these two books
Electromagnetic Compatibility Engineering by Henry W. Ott. And Grounds for Grounding: A Circuit to System Handbook by Elya B. Joffe and Kai-Sang Lock. Perhaps the library can get the books for you......

Grounds for Grounding: A Circuit To System handbook: This looks perfect. Thanks man.

 

Grounds for Grounding: A Circuit To System handbook: This looks perfect. Thanks man.

The best book of the two is in my opinion. By far is the "Electromagnetic Compatibility Engineering by Henry W. Ott" This book is on google books. It is not complete by you may check it out
http://books.google.com/books?id=2-4WJKxzzigC&printsec=frontcover#v=onepage&q&f=false
Edit found the other also http://books.google.com/books?id=1WfPWbSN7pUC

 

It is best to always make measurements with the black scope clip from Earth ground.

This is why the alligator clip on a probe snaps on and off, leave it off when working with AC and all measurements are from ground.

Sorry if my request sounds ridiculous: could you rephrase both sentences? (non-native having problems to grasp the idea exactly).

And eventually, could you provide additional explanation ?

 

Sorry if my request sounds ridiculous: could you rephrase both sentences? (non-native having problems to grasp the idea exactly).

And eventually, could you provide additional explanation ?

The black alligator clip/probe shield is ground. It connects to the probe with a U shaped clip and is quite short to reduce noise.

If working on an AC device, such as a TV, without an isolation transformer, there usually isn't a nearby convenient place to clip the black lead, so removing it will give you the measurement, since both are from ground (The TV, and the Scope). If the TV is not connected to ground, then you need to clip the alligator lead to it.

 

There's a guy selling some 1kVA 1:1 toroidal isolation transformers for $150 each, you'd need to provide the enclosure, but that's a great deal. They weigh 26lbs, so probably add $50 for shipping as well.

Keep an eye on eBay for name brand types being sold by somebody getting divorced, and the medical grade ones typically add $200 to the price just to get that stamp of approval.

So, expect to spend $200-$300 on one, not much you'll be working on will be drawing over 10 amps, so a 750VA or even 500VA would do for things like LCD televisions, allowing you to get into the sub $200 range.

I was looking around and found this Hammond 169G 500 VA 1:1 isolation transformer for $119 from Mouser.