Unexpected power supply behavior

groundingmainspower

I am using this large 24V power brick (rated for 9.2A) to power a small circuit (an arduino controlling some RGB LED strip, drawing less than 0.5A).

If I physically pull the plug out of the wall, the circuit runs on the transformer's stored energy for about a second before shutting down, which makes sense to me.

If the power supply has been plugged in for a long time, and I switch it off at the wall outlet, then the circuit continues to run for a very long time– more than an hour– before shutting down. But if I physically pull the plug, it still shuts down in a second or so.

  • The switch on the wall socket is working correctly (tested with a desk lamp), and as I understand it, this switch interrupts the 240VAC live conductor.
  • The power supply connects to the live, ground and neutral conductors on the AC side, and on the DC side provides a +24V and an AC ground connection
  • Because I use an Apple laptop, I know the ground and neutral wires in my home are not at the same voltage

So, where is this circuit getting its power from when the live AC is disconnected?

Best Answer

Okay, given the comment that it's just a normal UK outlet, I'm going to take a wild-a** guess and suggest that your outlet may be wired incorrectly so that the wall outlet switch is breaking the neutral and thus the Y capacitor in the supply is continuing to conduct some current to through the ground connection and hot.

This is based on some assumptions about the UK residential electrics that may not be valid, and I'm sure a right-pondian correction will be supplied if necessary.

schematic

simulate this circuit – Schematic created using CircuitLab