Will dimmer switches dim LEDs?

February 20, 2007 6:06 AM

If I replace halogen bulbs with LED clusters, will my dimmer switches still work?

Our house has a collection of halogen light fittings which look very cool but which seem to use a lot of power as well as loosing a lot as heat.

I'd like to replace these bulbs with LED replacement sets that replace a single halogen with a cluster of white LEDs. They tend to run on standard 230v mains lighting circuits, so I assume they have some sort of circuitry inside to back it down to a handful of volts.

The question is, how will these interact with the dimmer switches already on the circuit? Will the LEDs dim like the halogens do, or will the just shut off, or is the transformer like circuitry likely to stabilize the voltage to 5v regardless of the incoming voltage?

posted by twine42 to home & garden (8 answers total)

They will barely dim through most of the range and then in a very narrow range will dim to nothing. LEDs are not very linear in response. I think you can buy specific dimmers made for dimming LEDs though.
posted by caddis at 6:57 AM on February 20, 2007

Couldn't they make LED clusters that reduce the number of LEDs illimuninated as the light is dimmed? If they don't, they should. I'd buy them to replace my halogens.
posted by alms at 7:15 AM on February 20, 2007

First, have you checked out the dimming compact fluorescent question?

LEDs are unlike compact fluorescent bulbs, in that they aren't based on fluorescence; however, they are like CF bulbs because they are both controlled by electronics, rather than being directly connected across the line voltage the way incandescent bulbs are.
Well, technically, I guess you could string a bunch of LEDs in series, and stick that right across the line (with a smoothing cap), but that would be very failure prone. Like old strings of Christmas lights - one burns out, and there goes the whole string.

Since LEDs suffer very few of the limitations of fluorescent lighting - they turn on instantly, there is no special start-up procedure, they don't mind being flicked on and off a lot - there are many fewer fundamental problems using a dimmer with them. However, I suspect that the integrated power supplies in most LED bulbs are designed so that they can't turn on and off 120 times per second (rectifier and bulk cap across the line?). On the other hand, I don't see any reason why such a limitation is necessary, so it will depend on the specific details of each bulbs design, and it is certainly worth experimenting.
I guess if the design allows the lights to flicker with the line voltage, you would have to add 10-50% more LEDs to get the same brightness, so there could be a legitimate cost issue - or just as likely a completely illegitimate cost issue that companies use to force consumers to pay more for "specialty" goods..
posted by Chuckles at 9:00 AM on February 20, 2007

A dimmer works by cutting the voltage, which in an incandescent bulb means less current and less energy, hence less light.

LEDs are either on or off, and the transition is measured in nanoseconds. You control perceived brightness in an LED by turning them on and off rapidly (anything above a hundred herz) and controlling the duty cycle. That gives you much better control over the brightness but it requires control electronics.

The replacement LED fixture contains a power supply converting line current into low voltage needed by the LEDs. If it's a switching supply, which it probably is, then reducing the voltage has essentially no effect on how much power it feeds the LEDs, down to a certain threshold. Then below that threshold it can't provide power to the LEDs at all.

It's very doubtful that it's designed to start playing games with duty cycle in response to an input voltage drop.

[For the pedantic, white LEDs don't transition in nanoseconds because the white light is being produced by a phosphor which is energized by the blue LED behind it. So the ramp time, on both ends, is slower.]
posted by Steven C. Den Beste at 9:36 AM on February 20, 2007

Well, technically, I guess you could string a bunch of LEDs in series, and stick that right across the line (with a smoothing cap), but that would be very failure prone. Like old strings of Christmas lights - one burns out, and there goes the whole string.

I don't think even that would work without limiting current, as LEDs have very low resistance. In addition, LEDs don't have a very high reverse breakdown voltage.

Back to the question, I don't think the dimmer will work at all. Dimmers typically operate using a triac to only let part of the sine wave through, basically. You could probably modify the dimmer and LED circuitry to do the job.

Having said all that, the lights should work fine at the maximum setting, so you can replace the bulbs with the LEDs if you don't mind not having dimmable lights.
posted by 6550 at 9:41 AM on February 20, 2007

It's very doubtful that it's designed to start playing games with duty cycle in response to an input voltage drop.

But, triac dimmers play those games for you. All you need to do is allow the power supply to turn on and off quickly, and it will all work out. However, this will lead to a dead time near the zero crossings in the AC line voltage, even when the dimmer is set to full brightness, hence you would need some number more LEDs than you would with a more straight forward design.
posted by Chuckles at 10:13 AM on February 20, 2007

Not an answer, but be careful: a friend replaced the 8 halogen bulbs in his hallway for LED equivalents. They suck - they are noticably less bright and the light is very directional, leaving pools of light on the floor. Imagine shining a Maglite from your ceiling. I'd strongly advise you to try one bulb first, before replacing the whole lot.
posted by blag at 2:35 PM on February 20, 2007

MeFi's own Mathowie says - LED lightbulbs: not ready for primetime
posted by gen at 3:50 PM on February 20, 2007