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I have a Chinese scope with a two-pin power cord and a plastic body, and measure megaohms between its ground terminals and the actual earth, so I believe it has an internal isolation transformer (I suppose I could open it to confirm...) I've used it to measure small floating voltages before without problems (connecting the ground to +5V relative to earth, and the probe to a current sensing resistor also around +5 V), but now I want to measure power supply voltages where the ground of my scope will be connected to a rectified wave at maybe 200 VAC relative to Earth. Any problems? Does an isolation transformer make everything good?

I know that what you're supposed to do is use two separate probes and use the Math function to subtract them, but that doesn't work in practice because the common-mode voltages are so much higher than the differential voltage.

... Oh. Besides the, um, safety reasons. The whole scope will then be at 200 VAC relative to Earth ground, so if I touch one of the scope's BNC connectors and a real ground at the same time I'm in trouble. I guess that's a big problem, but is it the only problem? Floating the device under test with an isolation transformer would not have this problem? But that transformer would have to be a lot bigger to handle the high power. Does it have other problems due to parasitic capacitance, etc.?

Is there any trick to using the differential math function method by reducing the common-mode voltage with a capacitor or large resistor, but not connecting it directly enough to produce a safety hazard?

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    \$\begingroup\$ On these voltages you may need x100 probes that can take the voltage, you may be close to the limit with x10 probes. The trick of using the scope to take the difference only works if both inputs are within the scope's common mode and at low frequencies where the probes are matched. The better technique is to use a differential probe amp. Although these probably cost more than your scope. \$\endgroup\$ – Martin Mar 25 '11 at 16:06
  • \$\begingroup\$ @Martin: Yes, they cost more than the scope. I'm wondering if there's a trick to doing the differential math method. Like float the scope, connect the ground of the scope to the reference point of the supply through a large resistor or small capacitor to reduce the common-mode without providing a direct connection, and then connect only the two probes to the circuit and diff them. \$\endgroup\$ – endolith Mar 25 '11 at 17:30
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When we needed to do something like that where I worked many years ago, when testing power controllers that had a direct mains connection (no transformer) under load with a scope I specified a high-power isolation transformer in an "earth free" test area. The setup cost a lot of money, but was necessary for safety reasons, and worked very well.

People sometimes remove the scope ground connection in those circumstances, but it isn't a good idea.

In your case, I'd use an isolation transformer and an earth-free area.

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  • \$\begingroup\$ Mine doesn't even have a ground to be defeated. :/ \$\endgroup\$ – endolith Mar 25 '11 at 14:39
  • \$\begingroup\$ What make and model is it? \$\endgroup\$ – Leon Heller Mar 25 '11 at 14:43
  • \$\begingroup\$ JingCe JC2062M :D hzjingce.com/products.asp?id=2 \$\endgroup\$ – endolith Mar 25 '11 at 14:50
  • \$\begingroup\$ "In your case, I'd use an isolation transformer " on the scope or the DUT? On the scope wouldn't change anything, right? Why would I need an Earth-free area if the DUT is floating? \$\endgroup\$ – endolith Mar 25 '11 at 14:53
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    \$\begingroup\$ In the case I mentioned we used a large isolation transformer for both the scope and the DUT. If you simply use an isolation transformer on the scope in an earth-free area, you should be OK. \$\endgroup\$ – Leon Heller Mar 25 '11 at 15:13
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Floating the device under test (DUT) is the safe bet, you've mentioned the reasons.

Your scope appears to be a class II piece of equipment (no protective earth = PE connection). Usually, stuff like that is tested with a high voltage (some 1...2 kV) between mains and secondary, so this is what the scope's mains transformer should be able to take. (Just for clarity, in case it's not obvious: In your case, as seen from the sope's power supply, secondary means everything around the scope inputs and interfaces.) The high voltage is just used for production testing and is by no means meant to be a working voltage that you should continuously apply.

However, to be sure, there should be something in the manual about this issue. AFAIK, it is mandatory that scopes be grounded even if powered with a power supply that doesn't have a PE connection, or there must be a specification. (The Tek THS7020 battery scope is an example, but this one has isolated inputs that can even be floated with respect to each other).

Of all non-battery-powered bench type scopes with ordinary (non-insulated) inputs, this is actually the first one I've seen sold as a class II piece of equipment, i.e. as one that doesn't have a PE connector.

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    \$\begingroup\$ The manual's in Chinese... \$\endgroup\$ – endolith Mar 25 '11 at 15:33
  • \$\begingroup\$ High-voltage devices under test should be mains-isolated, granted. Is there any practical alternative to floating a scope, however, for situations were e.g. one wishes to measure the voltage drop on a 24-volt high-side driver for a device which is connected to a grounded PC via USB port? Connecting probes to +24 and the voltage of interest and using math mode would on most scopes only work if both channels were set to 5V/division, which would make it somewhat hard to see a small voltage drop. Would there be anything wrong with floating the scope in that cases? \$\endgroup\$ – supercat Mar 10 '15 at 15:27
  • \$\begingroup\$ Differential Probes are the solution. They are expensive, but they are also the safe way to go. Scopes are usually class I equipment and must be connected to protective earth. If you decide to not stick to that rule, many things that are usually safe suddenly become "hot". Benches tend to be messy, and it may be hard to see what's "hot" and what's safe. \$\endgroup\$ – zebonaut Mar 11 '15 at 7:40
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If the scope has 2 wire supply then it is not earthed and has to have an isolated supply inside.

What you describe is sometimes done but you should be extra careful and always verify the polarity of things (a screwdriver-like device with a small incandescent light inside — how do you call it?).

Also never move (or even touch) the scope probe while the circuit is "live". Always disconnect the power to the DUT if you want to change your connections.

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    \$\begingroup\$ And of course YOU personally should be the only one who makes and uses this setup. \$\endgroup\$ – jpc Mar 25 '11 at 16:06
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    \$\begingroup\$ Also, never touch the scope itself or its knobs while "live". They are likely plastic, but not necessarily reinforced-insulation type plastic... And some plastic knobs have small metal screws on the side... \$\endgroup\$ – zebonaut Mar 27 '11 at 9:54

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