will a dummy load cure all ills for LED dimming? - Page 1
Forum section: electronics / projects

author: electrolust
author_rank: Unknown
timestamp: January 10, 2019, 11:24:52 pm
content: For luminaires, ie complete light fixtures that include a driver, not retrofit bulbs, my understanding is that the phase-cut dimming control (w/ coincident power, ie typical "legacy" existing installation) input for the driver is generally desired to be ELV / reverse phase per the luminaire manufacturer. This saves cost such that the driver doesn't have to present a minimum load to a triac dimmer, or worry about inrush current having bad effects on the dimmer, and other such problems. Apparently it's generally cheaper to replace the dimmer with an ELV dimmer than to have a triac-compatible driver anyway.But in my situation, I cannot replace the dimmers. Which are very high quality (Lutron RadioRA), but are for MLV.It's also my understanding that the dimming performance of the fixture will generally be derived from the driver+LED combination, which the manufacturer has made sure are compatible. They expect some arbitrary ELV dimmer control to be in front of it. That ELV dimmer isn't the critical factor in dimming performance of the luminaire.I've found a scant few references that problems can be patched up with a dummy load.My dimmers already have a neutral wire, so minimum load is not a problem at least as far as powering the features of the dimmer itself. I don't know for sure for my specific dimmers, but I imagine the neutral also provides stable zero crossing detection for the dimmer.Will a dummy (resistive) load help otherwise? I think obviously (?) it will cure all the problems of a capacitive load negatively affecting the dimmer. But, will the driver behave correctly?Whether MLV or ELV, the driver still expects a phase-cut voltage-modulated signal, so does forward (MLV) or reverse (ELV) phase really matterto the driver?Does a typical driver actuallydependon the phase characteristic per se?My thoughts are that this will work well. The reason it isn't widely discussed as a proper fix is because it eliminates the efficiency of LED and in most cases, it's easy enough to replace the dimmer.There are triac-controllable 3rd party drivers, but good ones are in the $150 range and I have to void warranties by replacing the manufacturer driver, and experiment with compatibility. A UL listed dummy load is $10.All newer easily sourced modern styled luminaires are LED. And of course there are many styles that simply can only really be done with LED. Otherwise I'd easily address this by installing incandescent.

author: Zero999
author_rank: Super Contributor
timestamp: January 11, 2019, 10:46:30 am
content: LEDs and phase controlled dimmers don't mix well. There are some LED drivers which are supposed to be TRIAC dimmer compatible, but they often have issues. I replaced some incandescent lamps with dimable LEDs at my parent's house about five years ago. They were fairly cheap ones off ebay and probably don't comply with the safety regulations, since the level of insulation between the metal collar and LEDs is insufficient, but I made the judgement that it's very low risk, since they're out of reach, the luminaire is earthed and one shouldn't replace the bulb, without disconnecting the power first.There are two dimmer switches in the room: one with three lamps and another with six. The one with three lamps doesn't always dim properly, but reliably works at full brightness. It will often flicker, at a low frequency or just not work when dimmed, presumably when he mains voltage is on the low side. The dimmer with six lamps dims quite well, although there is a tiny amount of 100Hz flicker, especially when set to the minimum brightness.A dummy load on the three lamp circuit would probably solve the problem. I imagine replacing one of the lamps with an incandescent would work, but I haven't tried it and I don't see the point, as the dimmer function for the three lamp circuit isn't used anyway.In your case, you could try adding a small incandescent lamp to the circuit, but it might not make any difference.

author: ajb
author_rank: Super Contributor
timestamp: January 11, 2019, 05:35:22 pm
content: What sort of LED fixtures/bulbs are you using?I have a handful of Cree dimmable LED bulbs around the house on standard lutron dimmer switches (1-4 bulbs per dimmer) and never had any trouble. The LEDs don't dim all the way down to zero, but that's expected.

author: electrolust
author_rank: Unknown
timestamp: January 11, 2019, 08:53:46 pm
content: I'm shopping complete luminaire assemblies. The LEDs and the drivers used aren't specified. The only spec is the control input, eg ELV, 0-10V. I absolutely need 5% dimming, and 1% is preferred. Also I cannot have "dead zone" issues, eg dimmer doesn't start to do anything until below 75%. Basically it has to behave qualitatively like incandescent/halogen.Check this out:https://www.edn.com/design/led/4368149/Efficient-method-for-interfacing-TRIAC-dimmers-and-LEDs-4368149QuoteThis suggests to me that a dummy load is in fact a good solution to triac dimming performance, just inefficient. Anyone have comments on that?The article goes on to describe efficient approaches, which are obviously what are used in triac-controllable drivers. Of course, to varying degrees of success depending on the design and the load characteristics.

author: electrolust
author_rank: Unknown
timestamp: January 11, 2019, 09:07:25 pm
content: see attached.the dummy load ensures the triac doesn't misfire, and also keeps the current within the design range.

author: james_s
author_rank: Super Contributor
timestamp: January 11, 2019, 09:14:08 pm
content: I've found through experimentation that a dummy load of a small incandescent lamp like a 5W nightlight bulb improves things considerably. I have not tried a resistor but I bet it would work, you'd just want to experiment to find the highest value resistor that does the job so that you are burning up as little power as possible.

author: electrolust
author_rank: Unknown
timestamp: June 12, 2019, 09:34:25 pm
content: Didn't work in my case.I bought a 10k resistor packaged specifically for this use. I won't embarrass myself by posting the price. That's 1.4W. I actually bought 3. Even 3 of them in parallel (3333r, 4.3W) didn't do the trick. I suppose I could try going up to 10W or more? I fear I start to have heat dissipation problems though. The little 10k resistor just packs into the junction box with the light's wiring.I suppose that for a simple dimmer switch, where all you are powering via trickle current is a light, or perhaps a memory of the dim setting, it would probably work.But I am powering a RadioRA switch, as I noted in the opening post. It's a complete radio and MCU in there. I guess I should measure the current draw to better understand my situation. Anyway it certainly requires more power than a simple dimmer (but less than will visibly light an incandescent bulb).What did end up working is a Lutron "synthetic load", LUT-LBX-WH. It requires its own junction box and it has some serious heat dissipation. I measured the resistance across it and it was mega-ohms, so obviously it's not a simple resistive load. But it dims down to 5% (maybe more -- my switch only goes to 5%) and turns off completely. The LED flashes once (3-500ms?) at the lowest few settings before settling down to the dim/off setting. At the higher dim settings there's no flash, it just smoothly changes to the next setting. The dim settings appear to be fairly linear in nature just as if it were an incandescent bulb.When I'm able to get a second one of these I'll open it up and see what it's made of. Any guesses?

author: joeyjoejoe
author_rank: Frequent Contributor
timestamp: November 30, 2020, 11:23:34 pm
content: I'm reviving this, as there is so few discussions on dimming out there and it's hard to search for things without just finding home articles about where to find a dimmer that doesn't need a neutral wire.I'm in a similar hell with flickering lights, minimum brightnesses, etc. It's all way over my head.But, I wanted to chim in as well - there is the LUT-MLC which is a capacitor to generate some load. It's overpriced for what it is (a capacitor), but it does have UL approval which was more appearling to me then just buying a cap from digikey and getting weird looks. It did help resolve my issues initially, although now they may be back to haunt me. I have 2 of them, so I'm going to remove one to see what happens.Not sure if you ever managed to resolve your issues.I would love for big clive to tear down these various types of dimmers and show us what it looks like on a scope and such. MLV, ELV, etc etc....