LED Light bulb in lamp can't be turned off. What's the deal?
December 27, 2022 5:13 PM Subscribe
I have a table lamp that uses "A" type lamps (standard-size Edison-base light bulbs). The lamp has worked fine for years using incandescent bulbs. Now I've put an LED equivalent bulb in it instead. When the lamp is on, the bulb lights up like it should. But when the lamp is (supposedly) switched off, the bulb remains lit - but at a very low brightness. And it remains at that low level, never going dark. What's going on here, and is there a way I can make this lamp work normally with LED bulbs?
The lamp has what seems to be a dimming switch. It's a rotating knob. From the off position, with a short clockwise twist there is a click. From there the knob smoothly rotates clockwise until it stops at a maximum point. The incandescent bulbs in the fixture have only ever been on or off, though; there has been variation in light level, no matter how much the knob is rotated (past that on/off "click" point).
The LED bulbs we've tried using are a dimmable type, and 60W equivalent (the previous incandescent was 75W). But like the incandescents, these bulbs don't vary in brightness regardless of how much the knob is rotated past the on/off click point. When the knob is rotated until it clicks in what should be the "off" position, as I said, the bulb remains lit, at a very low level.
I guess I can just get incandescent bulbs again (though those are getting harder to come by, and also not energy efficient). Or maybe non-dimmable LED bulbs? Either way, what is going on here?
The lamp has what seems to be a dimming switch. It's a rotating knob. From the off position, with a short clockwise twist there is a click. From there the knob smoothly rotates clockwise until it stops at a maximum point. The incandescent bulbs in the fixture have only ever been on or off, though; there has been variation in light level, no matter how much the knob is rotated (past that on/off "click" point).
The LED bulbs we've tried using are a dimmable type, and 60W equivalent (the previous incandescent was 75W). But like the incandescents, these bulbs don't vary in brightness regardless of how much the knob is rotated past the on/off click point. When the knob is rotated until it clicks in what should be the "off" position, as I said, the bulb remains lit, at a very low level.
I guess I can just get incandescent bulbs again (though those are getting harder to come by, and also not energy efficient). Or maybe non-dimmable LED bulbs? Either way, what is going on here?
The incandescent bulb dimmer in the lamp is likely not compatible with LED bulbs, causing your weird symptoms. Dimmable LEDs need a special LED dimmer to function correctly.
posted by monotreme at 5:31 PM on December 27, 2022 [7 favorites]
posted by monotreme at 5:31 PM on December 27, 2022 [7 favorites]
The issue isn't (directly) the dimmer or phosphor persistence. This Stack Exchange answer explains why it happens.
posted by wierdo at 7:04 PM on December 27, 2022 [3 favorites]
posted by wierdo at 7:04 PM on December 27, 2022 [3 favorites]
Like weidos' link says the LEDs are staying dimly lit because the switches in the lamp don't turn 100% off by design. You can use halogen bulbs instead of incandescents which while not nearly as efficient as LEDs are 50-100% times better than incandescents.
posted by Mitheral at 8:17 PM on December 27, 2022 [2 favorites]
posted by Mitheral at 8:17 PM on December 27, 2022 [2 favorites]
If you still want to use the lamp, get a secondary switch that it plugs into at the wall. You can turn it on and off completely there regardless of its design.
posted by BlackLeotardFront at 9:32 PM on December 27, 2022 [5 favorites]
posted by BlackLeotardFront at 9:32 PM on December 27, 2022 [5 favorites]
This a detailed explanation of problems you can have with a non-led dimmer. An incandescent bulb sounds like the easy answer. Or a switch like BlackLeotardFront suggests.
posted by H21 at 9:37 PM on December 27, 2022 [1 favorite]
posted by H21 at 9:37 PM on December 27, 2022 [1 favorite]
Best answer: Take a look at the circuit diagram for an incandescent dimmer on this page.
Capacitor C1 will conduct a tiny AC current from the mains through the bulb regardless of the switch position or the rotating knob which controls the variable resistor. That tiny current is not enough to cause an incandescent bulb to emit perceptible amounts of visible light, but it is enough current at a high enough voltage to make your LED bulb glow.
You could stop it only by unplugging it or using a secondary switch such as BlackLeotardFront suggests.
posted by jamjam at 10:17 PM on December 27, 2022 [3 favorites]
Capacitor C1 will conduct a tiny AC current from the mains through the bulb regardless of the switch position or the rotating knob which controls the variable resistor. That tiny current is not enough to cause an incandescent bulb to emit perceptible amounts of visible light, but it is enough current at a high enough voltage to make your LED bulb glow.
You could stop it only by unplugging it or using a secondary switch such as BlackLeotardFront suggests.
posted by jamjam at 10:17 PM on December 27, 2022 [3 favorites]
Incandescent bulbs need to use at least 10% or so of their rated power to get hot enough to glow. If you have a dimmer circuit that is supplying 9% of the power needed by the bulb, internally the filament will be quite hot, but not glowing hot.
With an LED bulb, as long as there is enough voltage to turn on the LED, it will generate light.
posted by coberh at 11:09 PM on December 27, 2022 [1 favorite]
With an LED bulb, as long as there is enough voltage to turn on the LED, it will generate light.
posted by coberh at 11:09 PM on December 27, 2022 [1 favorite]
"there has been variation in light level" Is there a typo in here? Because the rest of the sentence seems to be saying that the switch doesn't dim the incandescent light when rotated. (and therefore probably isn't a dimming switch)
posted by achrise at 7:01 AM on December 28, 2022
posted by achrise at 7:01 AM on December 28, 2022
Response by poster: achrise - ""there has been variation in light level" Is there a typo in here? Because the rest of the sentence seems to be saying that the switch doesn't dim the incandescent light when rotated. (and therefore probably isn't a dimming switch)" Yes, I meant to say "NO" variation in light level. I tried to edit the question to fix this, but apparently there's no way to do that!
Thank you to all for the good responses. As usual I was able to resolve a vexing issue here.
posted by Mechitar at 9:06 AM on December 28, 2022
Thank you to all for the good responses. As usual I was able to resolve a vexing issue here.
posted by Mechitar at 9:06 AM on December 28, 2022
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I admit this doesn't sound like your symptoms, but before you go further, can you confirm it goes dark when you unplug the lamp?
After that I think you're looking at a faulty switch in your lamp that leaks current when the switch is off, which might be enough to be visible in an LED bulb and not quite enough to show with an incandescent.
posted by How much is that froggie in the window at 5:23 PM on December 27, 2022 [2 favorites]