safe way to use grounded scope with non isolated equipment ?

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I want to start experimenting with SMPS.

The scope will still be directly plugged into the mains outlet socket and connected to Protective Earth via it's own earth wire.

I plan to use an isolating transformer to power up the mains side of the mains equipment.

Does this arrangement remove the risk of killing myself and stop blowing up my equipment?

Is there anything else I should be aware of?
 
No. The risk you have eliminated - well, reduced some anyway - is that of the direct connection to the mains. And you can now connect the scope ground to some point that would otherwise be hot. But if the SMPS rectifies the mains and makes 340vDC, then you still have 340v, even if it is no longer referenced to earth. a 340v battery in your hand is dangerous.

A tube amplifier uses a power transformer, so all its circuits (other than mains wiring) are isolated. But those 500v circuits can still kill you perfectly dead. Just so the SMPS.

Here in 120v land, one side of the mains is neutral, which is bonded to earth. In my typical SMPS, that is rectified into +170v and -170v with respect to earth. The primary side runs on that. In many if not most of them, the control circuits for the switching run on about 12v. But the "ground" reference for that 12v is the -170v rail. The controller IC sees +12v on its power pin, but from my perspective, that pin is at -158v. I cannot connect my scope ground to that -170v to probe the control circuits without the iso. The iso allows me to do just that. After the iso, my -170v rail is no longer -170v with respect to my earth. But it is still 340v from the other end of the filter caps. I cannot forget this, and should not feel like the SMPS is risk free.

And never lose sight of the fact your scope ground - and with it the frame of your scope - is connected to that point in the circuit. So even with the iso, if you connect the scope ground to that -170v, you cannot also connect some other earthed test equipment ground to some other circuit point. It would short those two points together through the ground leads.


SO yes, you need the iso for your SMPS work, but as to danger, it makes the SMPS no less dangerous than a tube amp.
 
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Andrew, what you propose sounds OK but be aware that HF leakage currents can be more than you might expect. It would be OK once connected but you could still feel quite a jolt from earth to the running but isolated PSU if you touched the two points. It depends what you intend doing. You could also run into trouble if you connect more than one grounded piece of test equipment at a time to different parts of the circuit.

(I used to work all day with SMPS and we had each bench powered from its own isolation tranny. All the test equipment and soldering irons were fed off isolating trannys too. There was no protective safety earth anywhere near, all earth leads in the equipment were isolated, the benches were wooden, the flooring rubber matting. This allowed us to be able to directly connect say a scope probe "ground" lead to the reservoir cap on the primary side of a non isolated PSU with no danger of shock or zapping anything and to be able solder and work "live" using a "mains soldering station on the running PSU)
 
You might want to consider some proper true differential HV(as in 600V) hf probes to be able to measure gate drive signals in high side drivers fex. Better CMMR than using the scopes A-B function, and safer for the scope too.

Using a isolation transformer om the supply is wise, as the scope is still earth connected. using it on the scope, while having a direct feed to the supply is lethal, but you knew that already probably.
 
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The problem with isolating the scope is that the scope (and all test gear) and you in touching it, are actually in contact with the non isolated mains. The gear you are working on would assume what we somewhat incorrectly term "half mains potential". If you were working in a plastic bubble it may not be so bad but you are not. You can touch things, your feet are on the floor etc etc. Thats no good imo. Isolate the gear you are working on and its isolated full stop. A small isolation tranny also provides a lot of safety in the event of a short on the secondary side. Its a bit like a bulb tester... it can't deliver that much current (obviously depending what size the tranny is of course). I'm trying to recall the size we used to use... something like 250va comes to mind and that powered large screen CRT tv's etc.
 
I agree with Mooly. Tektronix have a very good app note about why using an ordinary scope isolated from mains, but the supply connected to the mains , is a very stupid idea unless you know EXACTLY what you are doing.

Fundamentals of Floating Measurements and Isolated Input Oscilloscopes | Tektronix

It is tempting, as the scope demands a far lesser tranny than most supplies (if you need a full power test of course). I am very very happy for my full isolated 750VA Variac that I picked up on a bargain 10 years ago. My wife learned quickly that it is not an item to throw away, if the variac goes , I go with it :)

But still, as mentioned capacitive coupling or even stray magnetic fields due to Hf switching can build up potentials at floating parts causing nasty surprises when touched but I can not imaginge energy stored being of sufficient size to pose a health hazard (or I might be wrong, please correct me then)

Be careful. Be very careful.
 
Andrew,
As soon as you place your scope earth on your isolated equipment under test it becomes non-isolated and lethal to you if you make only a single mistake.

If you isolate the scope - same thing but the scope is lethal - although when I have had to use this method for special cases I had to break out the glove and mat safety gear and signage etc.

Best solution is a non-isolated scope on an isolated DUT using differential probes. Easy enough to make or use A-B if you have it. Also recommend you use the lamp tester with a big red push button that must be held on to supply power - the dead-man switch. For the rare occasions in DIY when I need full mains I have a special shorting socket to replace the lamp (I used to have a 200W incandescent but you cant buy them anymore, here at least).
 
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