Electronics > Beginners
Floating the DUT
ArcticPhoenix0:
Hello all. Please forgive the ignorance you're about to read; there's probably a lot of it.
I'm very new to Oscilloscopes and EE in general and it's safe to say that I know just enough to be exceptionally dangerous to everybody and everything involved.
With that said, I think I understand why it's necessary to float the DUT to prevent ground loops and making my probes into fireworks. I'm also pretty sure I understand why the scope should never float but always be grounded.
It seems to me that cheater plugs would be a cheaper, albeit probably not safer alternative to the newbie probing around an mains AC+Grounded DUT. Of course an isolated 1:1 transformer is the safest way to to do this but is it at least feasible? I'd rather just plug the thing into a 3/2 plug instead of carry around a transformer provided it's as safe.
Secondly, would it not be a good idea to simply omit use of the grounding clip in mains AC+grounded devices? Seems like the best way to avoid ground loops altogether to this very inexperienced and ignorant hobbyist EE.
Anyway, I'm not interested in blowing up anything in my $500 scope; however pretty such an event may be.
I plan on sticking with the battery powered stuff or at least the non-grounded stuff until I understand this better.
Thanks in advance.
james_s:
No, it's not safe, end of story. You MUST either use an isolation transformer or a HV differential probe when probing the primary side of line powered equipment.
How do you expect to get a measurement without grounding the probe at the DUT? You have to have a complete circuit, without the probe grounded all you will measure is noise.
BravoV:
--- Quote from: ArcticPhoenix0 on January 28, 2020, 03:58:51 am ---Anyway, I'm not interested in blowing up anything in my $500 scope; however pretty such an event may be.
--- End quote ---
Forget about the scope, ask your self, while in safe low voltage probings, how many times in your life you made mistakes in those probings ? Say 99.9% you've done it right, cool, impressive number. :clap:
Now, when it come lethal HV or mains, a single mistake at 0.1%, or just once, once only, is enough to toast your limb or even lost your life, not worth it.
As you can afford the scope at $500, HV differential probe is within your reach.
An example of the HV diff. probe in action, the scope was measuring the mains line at 220 VAC that was powering it self.
EEEnthusiast:
I would recommend to use a transformer along with the probe. That should provide enough isolation from the mains. You could use a regular step down transformer with 20:1 turns ratio.
Brumby:
--- Quote from: ArcticPhoenix0 on January 28, 2020, 03:58:51 am ---I'm very new to Oscilloscopes and EE in general and it's safe to say that I know just enough to be exceptionally dangerous to everybody and everything involved.
--- End quote ---
It is good you acknowledge your limitations and even better that you present yourself here with such an introduction.
In regards to working with mains - whether it's 120v, 240v or whatever - always consider it lethal and always consider it capable of delivering chassis melting power. This is basically because that is exactly what it can do.
I have been an electronics hobbyist for over 50 years and I have yet to probe a mains circuit with a scope. I prefer a properly rated multimeter, some observations and exercising the grey matter. So far, I have been able to avoid such a risk, despite being sorely tempted on one occasion. Such is my recommendation: stay away from mains wherever possible. (It's a lot harder to zap equipment or yourself if you don't come into contact.)
Having said that, I will concede there are times when you need to know what's going on. In those instances, I would strongly recommend the differential probe solution. These are designed for the express purpose of dealing with this type of situation, providing the safest option to both person and property. Yes, they can be pricey - but they are still cheaper than a funeral.
IMO, an isolation transformer should not be considered a safety measure. Its purpose is to ensure operation of a device is unaffected by the circuit in which it is placed (well, as much as possible). A side effect is that you have less constraints when attaching a scope - but, at the same time, you must be aware how you connect the scope to the DUT. Attaching the "ground" connection of the scope to a point on the DUT which is at a significant voltage could render the chassis energised with the same potential (but opposite sign). This creates a far more dangerous scenario.
Of course, it is possible to deal with the dangers directly - but taking the steps to avoid them will yield greater safety, especially when you have those "oops" moments.
Take care.
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