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Today while installing a 12v LED strip on my windows with the power adapter plugged but the LED turned off, I noticed the led that I pushed down against the wall got turned on partially. It wasn't right. My finger shouldn't have enough resistance to turn on those led with 12v. Right away, I tested the DC adapter output line with a 90-1000vac surveyor and found out that the 12vdc line from the DC adapter carries a high voltage AC. When testing with a meter, it measures about 12vdc, and unstable AC range from 38-0Vac. ( properly higher AC voltage, since my meter may not be fast enough.)

1, Is it safe to use this power adapter? ( It powers up the LED strip fine. I have also found a 5V usb charger that carries a HIGH voltage AC before, of which I discarded right away.)

2, What is the cause of this HIGH voltage AC? ( Is it because it's not isolated? or it's not filtered properly at the output end? I can't open the case, as it's sealed completely with no screws. )


marked as duplicate by Andy aka, Olin Lathrop power-supply Aug 9 '17 at 11:02

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  • \$\begingroup\$ You should always disconnect the supply during the installation - errors such as drilling in the wrong place can have serious consequences... \$\endgroup\$ – Solar Mike Aug 9 '17 at 8:24
  • \$\begingroup\$ "I can't open the case, as it's sealed completely with no screws." ahh, so naive... \$\endgroup\$ – dandavis Aug 9 '17 at 17:47

1 - I would say yes, it is probably safe, current kills, not voltage. These types of spurious high voltages are almost always detectable in ungrounded SMPS designs where capacitors are used which bridge between the HV and DC output stages. For quite a good explanation see the thread linked in the comments by Andy AKA.

As already mentioned, as good (and potentially life-saving, in other situations) advice - always perform any installations while disconnected from power. If required, offer the light up first and mark installation position / holes, etc.(while lit, so long as we are only talking safe voltages like here). Disconnect it, THEN install and test.

2 - See the link Andy AKA gave for more detail, but in short, you may be able to reduce / remove this with grounding resistance if available / where potential for touching of the electronics is possible. Alternatively, protect your strip with a case (again if possible).

You say the LED strip stopped working when installed? In such a case many strips are metallic backed. You may be finding that the strip is shorting to the window or by means of whatever you have used to install it (screws etc?).

By no means a comprehensive answer but hope this helps :)


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