I want to use my oscilloscope to check the cleanliness of the output a high-voltage power supply (electrophoresis type, up to 3000 volts DC).

I built a 100:1 voltage divider (out of 2 resistors - roughly 700k and 7k ohms), hooked up the scope to the "1x" output of that (to protect the scope from HV damage), then turned on the power supply (set to only about 200 volts, just in case...).

The power supply immediately complained of "leakage current" and turned off the output. (This is a safety feature of the power supply - I wish I knew how to defeat it, but I don't have schematics.)

So - I bought a used isolation transformer on eBay, checked that it worked (it does seem to), plugged the power supply into the transformer, and tried again.

Same thing - leakage current.

Then I put a 1 megohm resistor in series with the 120 VAC supply, and measured the AC current going thru that into the ground lead of the oscilloscope.

Current when plugged into normal AC outlet: 118 microamps (about as expected)
Current when plugged into isolation transformer: 29 microamps (I expected zero)

What is going on? How can any AC current pass thru the scope ground lead if the current is coming from an isolation transformer? What is the source of the leakage?

And - how can I do this measurement?

  • \$\begingroup\$ Maybe the capacity of the probe opens a path for the ac current? \$\endgroup\$
    – Arsenal
    Feb 6, 2015 at 19:29
  • \$\begingroup\$ What happens if you use 3 resistors, 700k, 7k, 700k? The reading won't be correct but the leakage will be smaller. \$\endgroup\$
    – tomnexus
    Feb 6, 2015 at 19:33
  • \$\begingroup\$ Power the scope with batteries, a small computer UPS or similar? Also, be aware that some resistors have a maximum voltage, perhaps 100 or 200 V, after which their value deviates from specified. So be careful with the 700k, maybe use 30 x 22k in series... \$\endgroup\$
    – tomnexus
    Feb 6, 2015 at 19:36
  • \$\begingroup\$ The scope is grounded. If you connect the ground terminal of the scope probe to the circuit you're providing a path to ground. \$\endgroup\$
    – Hot Licks
    Feb 6, 2015 at 19:37
  • \$\begingroup\$ @HotLicks no, he has an isolation transformer. But it only takes 1 nF of inter-winding capacitance in the transformer to cause 30 uA of leakage. Try earthing the transformer core? Reversing the mains polarity? Add an aluminium foil shield (but not a shorted turn) \$\endgroup\$
    – tomnexus
    Feb 6, 2015 at 19:40

2 Answers 2


There are a couple of things going on here.

First of all, many isolation transformers do NOT isolate the ground terminal on the load side; they simply connect it through to the ground on the supply side. I've never understood this reasoning, but there it is. This means that the input ground of your scope is still connected to the building ground even when using the transformer, which sort of misses the point.

The second thing is that unless the isolation transformer is built with an internal electrostatic shield, there is capacitive coupling between the primary and secondary windings, which can easily pass the 29 µA of current that you're seeing. This is why if you measure the voltage between either terminal of the secondary and the building ground, you'll often see a reading of about half the line voltage.

  • \$\begingroup\$ Thanks. I'll try disconnecting the ground on the isolation transformer & see if that helps. The 29 uA doesn't worry me in itself, the problem is just that it seems to be enough to trip the leakage safety detector in the power supply, which turns it off when I don't want it to... \$\endgroup\$ Feb 6, 2015 at 20:55
  • 1
    \$\begingroup\$ Much to my surprise, disconnecting the ground wire on the isolation transformer made all the difference - now no complaint about leakage! Unfortunately I can't upvote the answer because I don't have +15 rep, but THANK YOU! \$\endgroup\$ Feb 6, 2015 at 21:14

I have a couple of power supplies intended for electrophoresis use and they have a ground fault indicator. In the case of my power supplies, this fault indicator trips when there is either a missing AC ground OR if the power supply negative terminal is connected to earth ground.

So check your supply for both conditions.

1) ensure that the input power cord for the power supply has a good ground.

2) Don't have anything connected to the output of the power supply and turn it on. It should power up without any problems.

3) Now connect your load and see what happens.

In the case of my power supply, I simply can't use it if the power supply negative pin has to be connected to something that is earthed. That hasn't been a problem for me so far.

  • \$\begingroup\$ The power supply works OK (if there is a load < about 500 kohms). It's only when I try to connect it to the oscilloscope (via the voltage divider) that I get the leakage complaint. \$\endgroup\$ Feb 6, 2015 at 20:53

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