This is a recessed light I pulled out of my ceiling. There is a black cylindrical thing attached to the junction box and wired in series with the live wire to the bulb base. What is it?

The junction box and the black thing are both an integral part of the light fixture, ie they came with it.

The cylinder has "A10A9 Mexico" marked on it. This is in NJ, USA and the light fixture is probably around 1970s or 80s era.

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  • 1
    Do you know the type of light bulb it uses? Might narrow down the choices. Maybe a temperature limit switch if haligon
    – crip659
    Apr 17, 2022 at 22:27
  • It is designed for an incandescent reflector flood, like an R30. Replacement CFLs and LEDs can be used too. It would be a strange place for a temperature switch. The junction box is connected to the actual high hat by a 6-inch horizontal metal arm. You can see me holding that in my hand. The temperature would have to be carried to it via the wires. It might be a fuse but I've never heard of a fused fixture for incandescent lights.
    – jay613
    Apr 17, 2022 at 22:49

1 Answer 1


It is a thermal protector for a recessed ceiling light. It will cut out the light if the temperature of the outside of the fixture exceeds the rating of the protector. All UL approved fixtures will have them. If the protector fails, in many cases,the bulb will turn on and off signaling a bad protector.

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Picture from Lighting Supply.

  • 2
    @jay613 The heat could also be conducted through the metal tabs that secure the protector to the housing.
    – JACK
    Apr 17, 2022 at 23:32
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    @jay613 Critical to understand is that recessed lights come in two major flavours - IC (insulation contact) rated, and non-IC fixtures. What the question shows is a non-IC fixture - it's meant to be installed in an empty ceiling cavity. If insulation is packed around it then it will become too hot for the wires. That protector is there to cut power to the light if that happens. It's not there to measure the bulb or the cone, but the ambient temperature around the fixture, which must remain low and in free air so that the rest of the fixture can shed heat.
    – J...
    Apr 18, 2022 at 13:38
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    @jay613 The sensor has a heater built-in and is designed to sense the ability of that heated element to shed heat. See here for more detail. I quote : "The heater is so effective that when the RP is inserted in insulation without the lamp installed, the bimetallic protector will operate. In this manner, the RP functions as an insulation detector."
    – J...
    Apr 18, 2022 at 13:41
  • 2
    @J... brilliant ... thanks for the education!
    – jay613
    Apr 18, 2022 at 14:41
  • 1
    Ah. I'm used to the IC type, where it's a simple in-line switch, typically at the top of the can, that turns off when it overheats.
    – Ecnerwal
    Apr 18, 2022 at 15:42

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