Earth and mains decoupled power supply

Question

I just had an application where ground capacitance and stray capacitance of all power supply options (Switch mode wall warts, grounded switch mode supplies) was to large and spoiled measurements by injecting relatively large mains frequency dependent voltages.

I actually helped myself out by building an optical power insulator (a LED driving a small solar panel).

I wonder why I found no low capacitance (stray and to earth) power supplies.

All supplies I checked out either showed symmetric mains capacitance (providing about half the mains voltage at the output GND) or ground connection (output GND tied to mains GND).

Is there any special type of supply that connect mains or earth in the smallest possible way? How would it be named?

Answer 1

Is there any special type of supply that connect mains or earth in the smallest possible way? How would it be named?

You can use a piezo electric transformer like this one by TI (formerly a Nat semi design): -

Disregard the lower half of the circuit because that is for a high isolation amplifier.

This overcomes the problem of capacitive coupling between primary and secondary producing mains voltages at your output terminals although you still have to be aware that any floating node will capacitively couple to your AC supply voltages. However, I expect this to be an order of magnitude (or more) down on conventional SMPS units.

Here's the link to the TI data sheet/article how to build one.

Another way is to use RFID technology with tuned primary and secondary coils spaced a centimetre apart to substantially reduce coupled capacitance to the power source.