why scopes/other equipment are not floating? - Page 1
Forum section: electronics / beginners

author: exe
author_rank: Unknown
timestamp: March 21, 2016, 02:37:17 pm
content: Hi guy,One question puzzles me a lot: why scopes and other gear are grounded in general? Wouldn't it be more useful to have floating tools?Even for repair tools like soldering iron I would prefer it to be floating (okay, with 1MEG resistor to ground to prevent ESD stuff).I heard so many ``funny'' stories about soldering equipment with grounded irons...So, what am I missing?

author: suicidaleggroll
author_rank: Super Contributor
timestamp: March 21, 2016, 03:05:46 pm
content: With scopes it's to protect the user. Most scopes are multi-channel, and all of the channels share a ground. This means that if you attached the ground clip on channel 1 to a live source, the jack for all other channels, and the ground clip for all other channels, would become live.

author: vasi_ro
author_rank: Unknown
timestamp: March 21, 2016, 04:37:14 pm
content: In general for user protection. As a rule you need to float the DUT not the test gear.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: March 21, 2016, 08:06:51 pm
content: It is for both safety and convenience, with a touch of arrogance thrown in for good measure. Posts above have identified the safety reason. It is mostly valid, although it can create safety issues in some situations. Those situations are probably less common so overall safety is enhanced.The convenience factor comes from the ability to rely on a ground return in most situations so a single probe connection is sufficient. It is very convenient when bouncing around between multiple measurement points with multiple channels.The arrogance? We any time you add a ground connection you are opening up another path for ground currents. In any low noise/high sensitivity system this may have a significant effect on operation. High power/high current systems can also have exciting results from additional grounds. But the makers and users of oscilloscopes are sure that their ground is more important than all other possible attachments and therefore tie it in their instrument.

author: Neilm
author_rank: Super Contributor
timestamp: March 21, 2016, 08:56:16 pm
content: There are scopes out there with isolated inputs. Trouble is each channel needs to be separately isolated so they are very expensive. Safety regulations state that any exposed metal should be safe to touch, so you can't use regular BNC connectors. Scope manufacturers want to be able to sell scopes in US / EU so will meet these standards.

author: exe
author_rank: Unknown
timestamp: March 21, 2016, 08:59:25 pm
content: Quote from: CatalinaWOW on March 21, 2016, 08:06:51 pmBut this path can be very long and it forms a ground loop, doesn't it? I mean, it can be very noisy and parasitic inductance is also no good...Another issue bothers me is that I often need to measure differential voltage. So, as I understand, I need to use two channels and a decent scope. Or expensive differential/isolated probes the probably would cost me more than the scope itself...

author: Zero999
author_rank: Super Contributor
timestamp: March 21, 2016, 09:05:07 pm
content: I agree with all of the above.Another one is that most switched mode power supplies have Y capacitors, that leak a significant amount of mains current to the secondary side, which could create ESD problems, when testing sensitive equipment and interfere with some measurements.It's perfectly safe to float a scope, by using an isolation transformer or a battery powered unit, but only on circuits running off 60VDC or less and is adequately isolated from the mains.

author: danadak
author_rank: Super Contributor
timestamp: March 21, 2016, 09:41:43 pm
content: https://xellers.wordpress.com/electronics/1ghz-active-differential-probe/Google differential oscilloscope probe, more hits.Regards, Dana.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: March 22, 2016, 04:03:03 am
content: Quote from: exe on March 21, 2016, 08:59:25 pmI almost completely agree with your comments. Much of the art of using a scope is understanding how these parasitic or sneak circuits form and how to control their influence on your measurement. Unless you got a tremendous deal on your scope you can get some forms of isolation for significantly less than the cost of your scope. Of course you can also spend a great deal more, and in some circumstances it may be worth it.

author: BravoV
author_rank: Super Contributor
timestamp: March 22, 2016, 04:20:17 am
content: Quote from: exe on March 21, 2016, 08:59:25 pmWhen it comes to a decent differential probing, its always expensive.Heck, this differential measurement is even separated into two main groups, cmiiw, 1st, the low voltage but high speed one which usually cost arm & leg, even used, and 2nd, for high voltage like mains related which is usually at much lower frequency, but with all those safety built in which is made it also expensive.My suggestion is to wait patiently for used handheld scope, as its worth it if you do lot of these differential probing.An example how a "good" handheld scope does, its like two galvanically separated scopes (one channel of course) that built in into a single package, good enough imho.

author: jfgilbert
author_rank: Newbie
timestamp: March 22, 2016, 05:46:20 am
content: That's a peeve of mine. I agree with the OP and the comments, as they tend to focus on scopes. To some extent I understand that scopes can be grounded, for security and, I suspect, as a leftover from the days when they had tubes and high voltages floating (pun intended) inside even when not connected to anything. What really annoy me, however, are the function generators. These are practically always low voltage, low power devices and their outputs have no need to be connected to ground, or when multichannel, to each others. It seems that a typical use for a signal generator would be to inject a signal to a DUT and check what it looks like at different points in the circuit with a scope, and it would be useful to show the input signal next to measured one. That's often not possible, and I can't imagine why. Our power supplies are typically floating, our DMMs are floating, there should be only one grounded instrument and the scope is a good candidate. Everything else should float, that would actually be safer.I know there are signal generators with differential outputs but they fall in two camps: The very cheap ones, powered by battery or wall wart, are obviously not grounded but they are slow and very basic. The fast ones with good features that also offer floating outputs tend to be quite expensive, and that spec is often not even mentioned. They do tell you, however, never to modify the plug or use it if the ground wire is not solidly connected to earth - as their lawyers dictated - never mind that it makes it much less useful.

author: exe
author_rank: Unknown
timestamp: March 22, 2016, 12:20:30 pm
content: Thank you, guys, now I know moreQuote from: Hero999 on March 21, 2016, 09:05:07 pmThanks, that's what I probably wanted to know. But what is "adequate isolation"? Is a single power transformer an adequate isolation? (Once I saw them in chain of two somewhere in the Internet).PS I didn't even think that all channels have the same ground on the scope. I'm used to using toyish single-channel scopes powered from batteries. So, probably, your advice saved me either a scope I'm going to buy, or a circuit.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: March 22, 2016, 04:51:16 pm
content: Safety is a discipline. Perfectly safe is both an oxymoron and non-existent. Safe enough is an opinion.In almost all cases IMHO it is safe enough to be isolated with a single transformer if you understand the issues and are careful.If you do not understand the potential dangers and how they arise, no amount of safety equipment and isolation is adequate.