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Lets say you have a flyback with a 20V dc input and 2 secondary side outputs, 5V and 12V. The 20V power in is provided by a benchtop power supply. What is the safe way to measure the input and output sides of the converter?

Can you connect to the secondary side rails using a single ended probe only or would this cause ground loops through the bench equipment? Can you connect primary using a single ended probe and secondary using a differential probe using the same oscilloscope?

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    \$\begingroup\$ Is there any reason you can't connect the 5 V and 12 V grounds to the 20 V DC ground? You lose isolation while this is done but you can monitor both sides. Once you connect up your probes you are doing that anyway but risk a high current fault taking a shortcut home via you test equipment. Bonding them directly should protect against that. \$\endgroup\$
    – Transistor
    Feb 23, 2021 at 20:09

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If you have access to a decent 4-channel Oscilloscope you can use a single-ended probe on each of +Vin, -Vin, +Vout, -Vout. (You can leave the gnd clips for each probe unconnected.)

Use your oscilloscope math function to subtract (+Vin - -Vin) to measure the input voltage and do likewise to measure the output voltage (+Vout - -Vout). In this way you are measuring all signals with respect to a somewhat arbitrary GND reference, but the point is it is the same reference point and therefore is canceled out by subtraction.

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