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I need an isolated power supply for powering a device that’s very sensitive to current leak (see below). I bought one such isolated power supply from Aliexpress. What surprised me is that there seem to be two capacitors bridging the isolating transformer.

Top view Bottom view Detail

I wonder if these two components somewhat undermine the “isolation properties” and if I should perhaps remove them? What are they there for anyway?

BTW the black rectangular component adjacent to the top side capacitor on the first photo is an optocoupler so that should be ok.

FYI the sensitive device is an Atlas Scientific pH probe measuring my pool pH connected to a Raspberry Pi. Apparently the current leak between the mains switching 5V powersupply and the pool water is enough to completely throw off the measurements. When the RPi is powered from a battery power bank it gives accurate results.

Can / should I remove the two blue things?

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    \$\begingroup\$ First you might try to earth the negative output of the power supply so to divert leakage current from the pool. Removing the capacitors could not be a good idea, they are supposed to locally close high frequency noise current. \$\endgroup\$ – carloc Feb 16 at 10:01
  • \$\begingroup\$ See my answer here: electronics.stackexchange.com/questions/478607/… And here: electronics.stackexchange.com/questions/388155/… \$\endgroup\$ – Andy aka Feb 16 at 10:08
  • \$\begingroup\$ @Andyaka Thanks, your links seem to confirm that there may be a current leak through these capacitors. What if I remove them? Will that harm the RPi? \$\endgroup\$ – MLu Feb 16 at 10:58
  • \$\begingroup\$ It could harm the RPi. \$\endgroup\$ – Andy aka Feb 16 at 11:06
  • \$\begingroup\$ @carloc: Voluntary earthing a SELV circuit is in most cases not permitted because it violates regulations. \$\endgroup\$ – Ariser - reinstate Monica Feb 16 at 11:38

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