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Need help with oscilloscope connection

N

NeoPTLD

Platinum Member
I'd like to monitor the waveform of two locations marked by question marks in the diagram below.

Diagram

Neither legs can be grounded and they're floating off the ground by a considerably voltage. I could try to isolate the oscilloscope from AC with a transformer, but with the power supply operating at ~500V above ground at 30KHz, it will find a path to ground through capacitive coupling and I'm afraid it might destroy the scope.

I was thinking of using a resistive divider for the voltage across the lead, but for the current shunt, the voltage drop is only few tens of mV and the whole circuit is some 500V above ground @ 30KHz.

How do I go about hooking up a scope?
 
1) A battery powered scope is ideal for this type of work.

2) Back in the day we would just cut the ground lug off of the scope power cord. This is not ideal and do not touch or adjust the scope while the circuit is powered.

While its true that capacitive coupling will find a way to ground. It will weather a scope is connected or not. An isolating transformer before the device would remove ground refference and make everything much easier.
 
What is in play here is the voltage rating of the capacitor in the input amp of the scope that is used for AC coupling. It may be that all you have to do is select AC coupling and you're good to go. Check the operating manual of the scope. My old B-K scope would easily handle a 500V DC component.
 
I dont think most modern digital scopes will take more than 150V or so, unless you get special equiptment.

For this type of measurement, though, if you are interested only in the AC part of the wave you can just run it though a capacitor with a voltage rating of 500VDC or greater. Capacitors have infinite impedance for DC and you will be left with your AC coupled waveform.

Just check digikey and see what kind of stock they have. I'm sure you'll find something.
 
As far as the voltage issue goes, you could use some resistors to make a voltage divider. Then, you can scale the input back by a factor of 10 or so. For example, use two 45K and a 10K resistor and measure across the 10K (put the 10K in the middle of the two 45K ones). And then use an isolation transformer. The resistance should keep the scope from frying.
 
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