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I have a 12V led strip connected to a (cheap) 12V 5A switching mode supply.

The supply is connected to a remote control switch.

The supply does not have earth connection (only neutral and live)

The supply have an European plug ( Live and neutral can be switched by turning the plug 180°)

Since the remote control switch only cuts one connection (I think) if I connect the plug in a certain way the live or the neutral wire still reaches the power supply even if the "return path" is blocked.

In one of the 2 cases I noticed that if I touch the led strip some of the leds near the point that I touch turn on very dimly. I have no body->earth connection except for the body capacitance.

The questions:

  • Is this caused by me working as an antenna or by me working as a capacitor for some low current AC signal on the output of the power supply?
  • Is this safe?
  • Is the power supply working properly?
  • Is the remote control switch working properly?
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    \$\begingroup\$ Capacitor not antenna. \$\endgroup\$
    – Andy aka
    Jan 10, 2018 at 14:09
  • \$\begingroup\$ Leakage current though the Y-capacitor in the power supply, the LED, though you and finally to ground. \$\endgroup\$
    – winny
    Jan 10, 2018 at 15:18

2 Answers 2

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Is this caused by me working as an antenna or by me working as a capacitor for some low current AC signal on the output of the power supply?

Forget Antenna, indeed it is mostly capacitive coupling and leakage of your body to all the mains wiring and mains connected devices in your house.

Is this safe?

Yes, assuming a properly designed power supply, the current will be so small that it cannot harm a human. Even if you'd touch the LED strip and touch a grounded conductor with your other hand.

Is the power supply working properly?

Yes, there will always be some capacitive coupling to the mains lines through a power supply. It cannot be avoided. As long as it is not too much (meaning a harmful (to humans) current can flow) this is OK.

Is the remote control switch working properly?

Yes, switching off only one line is OK as there should be enough isolation in the powers supply to safely touch the supply's output even if it only is connected to the live wire.

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Since the wavelength of line frequency is far greater than body size there are no antenna effects. But stray E-field can be coupled thru your hand to cable and feet thru earth which can act as neutral return. So when you switch Neutral instead of line, you are bridging the neutral switch with about 1nF with one hand near wire and shoes to grass.

1nF is approx. 3 MOhm at 50 Hz and I estimate that 100uA is dim yet visible on efficient LEDs. There may be be little or no sensation felt with this effect. UL/CSA and IEC consider 500uA as safety threshold for leakage testing.

The electric field would be similar to touching an ungrounded 10M scope probe and viewing >50Vpp of E-field.

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