| Electronics > Beginners |
| General questions about Isolation i.e. how NOT to blow up your 'scope follow-up? |
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| DDunfield:
The desire to "float" a scope or DUT arises from the desire to measure something that is not ground referenced, and wishing to avoid a path to ground through the scope. This subject comes up quite often, do some searching of the forums and you will find lots of opinions. In doing so you will find the words "safe" and "safer" mentioned a lot, as in "floating the DUT is safer than floating the scope". I prefer the term "less hazardous". Below 40V or so isolation is generally OK - as coming into contact with <=40v is usually not very hazardous. At higher voltages things get dicey. BAD ways to measure non ground referenced HV signals: BAD#1: Floating the scope. Isolating the scope and attaching it's ground to HV means that every exposed metal bit on the scope (BNC connectors, Calibration ground etc.) is now sitting at HV, and as you are used to doing things like connecting/disconnecting BNCs with your hands, there is a very real possibility that you will have a "brain lapse" and grab onto HV. Also, although I would hope a TE manufacturer would assume the worst and provide really good insulation, you scope isn't spec'd for this purpose so this may not be the case. BAD#2: Floating the DUT Isolating the DUT and then grounding some HV signal (via your scope ground lead) means that all the normally ground bits of the DUT are now sitting at HV. Since the DUT was likely not designed with testing in mind and the manufacturer never intended it to be "grounded" this way, there are likely a LOT of them, and you may not immediately discern what is and isn't ground, and again "brain lapse" will factor in .. there is a very real possibility that you will grab onto HV. And don't even think it has been acceptably insulated for this use case. ++ You will see a lot of forum comments saying that floating the DUT is "less hazardous" then floating the scope. This is because traditional scopes a large "grounded" metal boxes which you really don't want sitting on your bench at HV ... modern scopes are more insulated, so I'd say it's become more 50-50 ... what this means is that you remove "less" and end up with just "hazardous" no matter which end you float - AVOID DOING EITHER! GOOD ways to measure non ground referenced HV signals: GOOD#1 - use the "math" function of your scope to measure the difference between two channels. - Cheapest way (free) - Takes two channels for one measurement. - Limited by the HV ratings of your scope/probes. GOOD#2 - Use a HV differential probe - Good HV-differential probes are expensive (and you WANT good). - Only uses one channel, allows all the normal measurements/math of your scope. - Can get HV-diff probes for really high voltages ($$$) GOOD#3 - Use a properly insulated battery powered scopemeter - Pricey new, but can be easier to find older good ones used. - Must be designed for HV .. just because it's a scopemeter doesn't mean it's "less hazardous" to use on HV. I personally have opted for #3 - I have two scopemeter I use for non-ground referenced measurements: UT-81B .. ~$150-200 new, 8Mhz, single channel, connects like a multimeter (test leads although there is a BNC adapter). - Rated like a multimeter 1000vdc, 750vac. Fluke97 .. ~$200-$400 used. 50Mhz, dual channel, uses BNCs - Make sure you get original 10x probes with it - they are insulated and rated for 600v - Scope itself is rated up to 3KV with the proper x10 probes (as high as 25Okv with correct 1000:1 probe - I don't have one!) Dave |
| rstofer:
--- Quote from: FakeDave on April 28, 2019, 05:41:07 am ---My next several-hundred dollar purchase will be a Rigol 1054Z with all those additional pricey software keys now included. It's even gone down in price since I started watching them. In that case at least, it pays to be 4 or 5 years late to the party. --- End quote --- I have the 1054Z and the first thing I did after unboxing was to unlock the advanced features. Today, I would be looking at the Siglent SDS1104X-E and unlocking it to become the SDS1204X-E or, perhaps, just buying the 1204. That extra 100 MHz of bandwidth would be nice and the UI is said to be much more responsive. Sometimes the party moves... |
| FakeDave:
--- Quote from: rstofer on April 28, 2019, 01:28:15 pm --- --- Quote from: FakeDave on April 28, 2019, 05:41:07 am ---My next several-hundred dollar purchase will be a Rigol 1054Z with all those additional pricey software keys now included. It's even gone down in price since I started watching them. In that case at least, it pays to be 4 or 5 years late to the party. --- End quote --- I have the 1054Z and the first thing I did after unboxing was to unlock the advanced features. Today, I would be looking at the Siglent SDS1104X-E and unlocking it to become the SDS1204X-E or, perhaps, just buying the 1204. That extra 100 MHz of bandwidth would be nice and the UI is said to be much more responsive. Sometimes the party moves... --- End quote --- I can see why, that's a nice looking scope. |
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