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?
Best Answer
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.