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I have 3 12v LED under cabinet lights that I plan to install. The final install goal is to have the line power (120v) controlled by a pre-installed light switch (1-way) which currently operates 4 pot lights above my kitchen sink. This means one switch will control 3 LED lights and 4 pot lights. I haven't gotten that far though, has I have hit a bump in the road. I removed the AC adapter module board from the wall adapter, so that I can discretely install it in the junction box with the switch and soldered two leads onto the board for attaching to the switch. I then terminated the soldered leads to the switch L to the hot wire and N to the switched wire, before I had the chance to test the operation my LED light turned on right away, but my 4 pot lights were off, I flipped the switch and the LED shut off and the pot lights turned on. I then reversed my two soldered leads, but it didn't make a difference. I'm not sure if this is common for LED lights, or if it is maybe some sort of electrical component on the module board that somehow only conducts to the load when the line has no power, but that doesn't make sense because its meant to plug into a wall socket, which is always live.

Why is my LED on when switched off but off when switched on?

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What you have done is connected the LED lights in parallel to the switch. This way, when the switch is on, the LEDs are shorted out and power flows to the pot lights. When the switch is off, you end up effectively with the LEDs and the pot lights in series. Because the LEDs are so low power, the power flows through the pot lights without giving them enough power to visibly light.

Check your switch box to see if there is a neutral wire there (probably white if you’re in the US). If so, you can connect the LED driver between that and one side of the switch. If you don’t have a neutral wire there, you’ll need to run a cable from a pot light to the LED driver. Connect it in parallel with the pot light.

One more comment. I’m assuming that the LED driver board is not designed to be in a junction box. Doing what you’re doing is probably against the electrical code (at least in the US) and could be dangerous, especially if the board shorts out to the box. If this causes a fire, your insurance may have grounds to not pay, or at least to drop you afterwards.

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  • Your absolutely right. What I was attempting was just to preview the light, but it now works properly once installed correctly. I needed the 3rd wire like you are saying, but only have 2 in the switch box. I now have my AC adapter installed at the pot light and one lead is on the Line side Neutral and the other is on the switch leg, now it turns the light on and off with the pot lights. I wanted to preview at an easier access point which was the switch but it could never work from there and I just wasn't thinking straight.
    – Tom Paquette
    Apr 18, 2019 at 16:33
  • I had also hoped to have the LED's built in switch to run under a 3-way switch circuit. The LED's switch is motion activated, is low voltage and would have required some serious modifications, including getting a third wire to my switch box and I was just going for above and beyond expectations so that the lights could be motion-switched on one end of the counter and switched regularly from the other end. Now the regular single pole switch controls both, and the motion switch is next to useless/pointless. Thank you a ton for your response and sorry for asking before really thinking.
    – Tom Paquette
    Apr 18, 2019 at 16:35

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