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I just wired up the following schematic and both sets of LED recessed lights will flicker when dimmed below full power. It doesn't matter what the other dimmer is set to (full power, low power, off). The flicker is very quick, it is sporadic and doesn't follow a pattern. All lights on each line are included in the flicker. When set to full power on the dimmer slider, no flickering occurs. Both of the dimmers are the same and they are listed as compatible on each set of lights. All of the wiring that I ran was 12/2 based on this being on a 20 amp breaker.

All of the LED lights come with junction boxes and screw on pigtails down to the actual light. In the set of 4 LED's on the left of drawing, I have all 4 junction boxes wired up, but only the last light is connected by its pigtail as the wife wants to wait for the countertop to be installed before deciding how many lights she actually wants over it. So I may be removing a 1 or 2 of those lights from the line.

The middle switches in each 3 gang box are simple on/off switches to run either a ceiling fan or an exhaust fan.

I am curious of what steps I should take to try and diagnose the flicker. I just rechecked all wiring connections and nothing is loose.

My thoughts are that either the three missing lights on run one are causing the flicker. Or the USB outlets sharing the neutral line are causing the flicker. Or the dimmers, even after being listed as compatible actually aren't.

Is there anything in this schematic that jumps out to someone before I start buying different switches and outlets to troubleshoot with?

Wiring schematic

  • Can you plug in all the lights? That junction box for the lights must have a transformer in it, because the little pigtail with the color control switch is not going to be line voltage. It could act up if the transformer doesn't have a load on it. – JPhi1618 Jan 20 at 19:08
  • Related LEDs might look dim-lit even if off. – Mefitico Jan 21 at 16:01
  • i would scrap the analog dimmer switch and get dim-able smart bulbs. They can manage brightness without carefully controlling the power delivered to them and their peers. This lets them play well with other devices on the line, and allows per-bulb brightness tweaking that's simply not possible otherwise. – dandavis Jan 21 at 17:38
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The specifications of the LED lights you have say they will dim down to 10% without any flicker or buzzing. Some lights don't go down that far, and some will go even further.

Based on the manual for your dimmer switch, there is an adjustment to change the low end of the dimming range:

dimmer adjustment

I would assume the switch would be factory set to work with most lights, but maybe the quantity of lights or their exact dimming capability is a little outside the norm. Follow that adjustment procedure and see if it helps. If you feel like you have to adjust the minimum dimming to be quite high, you should also plug in all the lights and make sure that is not making the problem worse.

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    Given the current setup, this is the best answer. Unfortunately, there are limits to how well dimmable LEDs and LED-compatible dimmers will work, but the Lutron dimmers with the range setting has been, in my experience, the easiest/best way to match dimmer to LED. The gold standard is to use a separate low-voltage signal for dimming (i.e. 0-10V DC), but it's a pain to retro-fit that, since it requires a separate low-voltage line between the switch and LED driver. Other than accepting the limited minimum brightness or 0-10V system, it's just a matter of trial and error. :( – Peter Duniho Jan 20 at 22:00
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LEDs on AC Power Don't Like to be Dimmed. Period.

(LEDs on DC can actually be dimmed quite easily - just look at an older battery powered LED device (like handheld computer games from the 1980s) and you can see the LEDs dim when the battery is low.)

But most AC powered LED lights - whether complete fixtures or Edison bulb incandescent replacements - have a driver circuit that takes high voltage AC and turns it into low voltage DC. That process does not lend itself well to dimming.

A modern dimmer is not, contrary to popular belief (including what I thought when I was a kid) simply reducing the constant amount of current or voltage. Rather, it chops up the power into little pieces and only sends some of those pieces to the lights.

With incandescent lights, only the total power really matters much. It also helps that the incandescent lights use the power "as is", where an LED light has to convert from 120V AC down to very low voltage DC. So this works fine on incandescent lights.

However, CFL & LED lights are, essentially "on or off". So dimming becomes a lot more complicated. With LEDs in particular, it takes very little power to produce light, so you often end up in one of two situations:

  • Flickering - The LED gathers up enough "bits of power" until it has enough to produce light. Uses that power to produce light. Waits for some more power.
  • Dim - This is actually what you want with a dimmer. But with LEDs this can happen even with a very small trickle of current to power a timer/motion detector/etc., with the result that "off" is never really "off".

There are technical solutions, aka "dimmable compatible LEDs" together with "LED compatible dimmers". Essentially, the LED light has to figure out that the chopped up bits of power are not a deliberate attempt at flickering the lights but rather intended to dim the lights. But despite the best efforts, sometimes they just don't work well together. As you have found out the hard way.

If both the LED fixtures and the dimmers claim compatibility, I would start by trying a different brand of dimmer to see if that helps.

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    "A dimmer is not" Well, LED dimmers are not. Incandescent dimmers can very well be. That's one of the reasons you can't put a LED behind a normal dimmer. – Mast Jan 21 at 7:22
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    +1 LEDs proper (the actual diode that emits the light) have no problem being dimmed, I don't think they mind at all! The light output will be some monotonic and roughly linear function of current. (fig 2a) However, any intervening circuitry may behave quite badly when providing it anything except a fairly constant DC supply, so maybe you should begin with "Commercial LED lights" rather than just "LEDs"? – uhoh Jan 21 at 7:38
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    @Mast If dimmers worked like that (resistively), a dimmer at 50% would produce as much heat as the actual lightbulb did. Maybe some smart person can invent an inductive or capacitive dimmer which doesn't waste power. I've never heard of one. – user253751 Jan 21 at 10:04
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    @user253751 Not resistively, but with a transformer. – Mast Jan 21 at 10:20
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    While variable autotransformers transformers (variacs) do exist and I believe they have been used for dimming stage lighting in the past, I have never heard of them being used for domestic dimming, they are too bulky and expensive. Much cheaper to simply cut up the waveform with a Triac. – Peter Green Jan 22 at 2:34
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Check that your LED bulbs are compatible with dimmers, and check that your dimmers are compatible with LEDs. If their respective packages don't say, then they probably aren't compatible and you will have problems.

Even if everything claims to be compatible, you still might have issues. In that case, the best you can do is experiment with different brands and models of each until you get a combination that works.

Both AC dimmers and AC LED bulbs employ some electrical trickery to perform their intended function.

AC dimmers make the light appear dimmer by turning the voltage on and off very quickly. This on-off effect happens at twice the line frequency (e.g. 120 Hz) which is usually too fast to see.

LED bulbs typically need less voltage than is provided by the AC line, as well as constant forward polarity (in contrast to AC's alternating polarity). Packed inside the bulb is a small circuit to make these adjustments.

In isolation, both of these things work pretty well, but when combined, things can start behaving weirdly. The dimmer's rapid switching disrupts power to the LED bulb's voltage regulation, and the LED bulb's voltage regulation prevents the dimmer's rapid switching from having a useful effect.

There are "dimmable" LED bulbs that are designed to cope with dimmers, and "LED compatible" dimmers that are designed to cope with LED bulbs, but neither are guaranteed to work well in all cases.

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I'll tell you a secret: they flicker all the time

You just don't notice it, because the higher the power, the longer they stay ON and quicker switch on/off. As you dim down them, their flickering becomes more even and noticeable.

What about randomness ? Well, it flickers randomly to minimize harmonics disturbance. Wait, what's that ? It's topic for another question on stack exchange...

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Electronic devices like LED compatible dimmers that chop up the AC power into tiny chunks tend to generate electronic noise that consists of harmonics of the line frequency but at very high frequencies. These signals will bounce around between other devices nearby through the wiring. This can cause interactions between those devices which would be more likely to be noticed in LEDs operating at low output levels.

Some electrical devices are marked "Surge Protected" such as "Surge Protected" Receptacles, which contain special circuitry to absorb the electronic noise that may be present.
The protection works best when it is very close to the source of the electronic noise or close to the susceptible load.

If, for example, you insert a "5280" surge protected receptacle between the dimmers and the LED lights you should see a noticeable reduction in the flickering. There are many other devices marked "Surge Protected" that would also work if they fit betteer into your specific application.

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  • Hello, and welcome to Home Improvement. Thanks for the answer; keep 'em coming. And, you should probably take our tour so you'll know how best to contribute here. – Daniel Griscom Jan 22 at 10:11
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LED flickers, because it is a light emitting diode (a solid state lamp). The Lamp will only light when its power cycle is "on". A dimmer in essence, SLOWS DOWN THE CYCLE TIME, therefore slowing down the time frame in which a cycle is repeated. So, the lamp appears to flicker, because it is turning ON and OFF!

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  • Hello, and welcome to Home Improvement. In general, that's wrong: dimmers leave the power at 60Hz, but reduce the drive time on each phase. And, you should probably take our tour so you'll know how best to contribute here. – Daniel Griscom Jan 22 at 20:31

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