The only reason I can think of for that to happen would be a short (a wrong connection), or maybe what you think is the chair’s ground isn’t really ground. The resistors in your voltage divider will limit current through the “hot side”, so any excess current has to be going through the ground.
Also, since the halogen lamp won’t be connected, can you measure three different voltages with a “light load” (like maybe a 1K resistor)?
Have you got the LED & dimming figured-out? I mean, have you got an acceptable LED lamp, power supply, and controller, and can you dim it under software control?
…It might be hard to find something as bright as a 150W Halogen that fits mechanically, it has to be dimmable, and I assume that for medical-safety reasons you can’t use a regular dimmable 120V or 220V LED flood light.
I connected the two wires from the secondary of the Ac transformer to the voltage divider. both times a fuse blew. But then, both wires has 10VAC, and not 18V and 0V. And the diode was (is) only in one of the AC connections of my circuit. (Perhaps use one in each to avoid positive side of the current on where it should be the negative pole of Arduino?)
LEd dimmer circuit figured it out and working perfectly, in a direct connection (avoiding my control circuit) with a switch, at max brightness for now. Pretty pretty bright. I turned on the lathe an … aluminum heat dissipation seat for the led, that fit the original lamp’s place, and it doesn’t get hotter than it was before.
I have measured before with a resistor between the AC wires, and that’s how I tested and decided the values of the resistors. I thought I’ve burned down the original control circuit when the fuse blew, but they’re working just fine.
I just have to be sure how to connect both AC secondary wires to my circuit, or if I use only one wire and the chair’s ground as a “general” ground…
Does this attached circuit makes sense? Would it work? Now with a second diode? As I don’t have a 0-18V AC on my secondary, but a 9V-9V AC.