I'm setting up a project lamp for my workbench using one of these


I checked for continuity while doing a test setup, the live and neutral connectors are isolated from each other, and the backplane.

However, when I powered on the light and started probing around to test for shorts I found that my meter is reading about 44V AC between the backplane of the LED COB and ground.

The LED COB is attached to a heatsink with a thermal pad between them, then on the rear is a 12v fan.

The light and fan work as expected when powered, and if I connect a ground wire to the heatsink the breaker does not get tripped, so it should still be isolated correctly.

But why does my meter read it at 44V AC? Is there some sort of EMF that these lights emit?

Picture of the light and heatsink

  • \$\begingroup\$ Digital multimeter with 10 MΩ input impedance? \$\endgroup\$ – Transistor Sep 3 '18 at 22:17
  • \$\begingroup\$ Digital muiltimeter, manual lists the AC voltage impedance as 4.5M Ω \$\endgroup\$ – MKUltra Sep 3 '18 at 22:22
  • \$\begingroup\$ Do you mean wrt. ground? \$\endgroup\$ – Sunnyskyguy EE75 Sep 3 '18 at 22:36
  • 2
    \$\begingroup\$ I'm not entirely sure what you mean by "wrt.", If I measure voltage with my meter, one probe on the heatsink, and one probe on the ground wire, it measures abour 44V AC. \$\endgroup\$ – MKUltra Sep 3 '18 at 22:39
  • \$\begingroup\$ @MKUltra if the heat sink is isolated, and you have AC in proximity, this could be caused by induction. You can confirm this by grounding the heatsink and starting the LED, then confirming voltage to ground is now 0. Next, meter current to ground with the heatsink grounded in this way. If current is negligible, the substantial voltage you were seeing was simply because of isolation. As a final test, I would meter power consumption by the LED and just confirm that it does not change when the heatsink is connected to ground. \$\endgroup\$ – K H Sep 3 '18 at 23:45

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