Modification of Smart Theatre Light Dimmer

[Don Furr]

Is there a modification I can make to this Smart Dimmer so when dimmed, the power goes completely out? This dimmer work well except with LED's.
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[Mark Gulbrandsen]

Not all dimmers will work with all LED's. BUt some LED lamps will work just fine. Occasionally you may have to hang a 100 incandescent watt lamp off the output in parallel with the in house load in order to trick the dimmer to go all the way down. Kelmar dimmers have a similar issue. But with theirs works perfect with Cree lamps.

[Marcel Birgelen]

When we last updated our screening room, we got rid of our old spot lights and replaced them with LEDs. Those where of the "dimmable" type, but despite that, they had issues. One of them was that our old dimmer would still leak like 5V even on "0", sufficient for the LEDs to still glow considerably. The other was that, while the LEDs dimmed based on voltage, the light output was nowhere near linear to the voltage applied. So, eventually we replaced the dimmer with one we control via an analog output on our PLC. We, or rather I, needed to write a routine with a curve in it, to compensate for the non-linearity. The dimming sequence is actually the only time our old-school MODBUS is filled with commands to the bandwidth maximum with updates towards the analog output module, in order for the dimming to look smooth. Anything less, and the dimming sequence looks like it stutters.

[Don Furr]

Thanks Mark and Marcel. I just finally found some LED bulbs at Home Depot that in fact dim all the way to off!! Problem solved. Thanks for your help.

[Marcel Birgelen]

Great find. You may want to write down the make and type number. Although those LED bulbs easily outlast their incandescent counterparts, I've found most LED advertising regarding their lifetime to be highly unreliable. I'm sure they've come up with some bullshit statistics and math to justify it, but we've had several of those LED bulbs fail within a year.

[Mark Gulbrandsen]

Excellent Don... Could you list the make and part number of the bulbs for future refrence for everyone here?

Thanks!

[Steve Guttag]

Finding a working lamp is no guarantee on future lamps of the same make/model. We had a customer do likewise (find a working make/model lamp) only to find on their next purchase that the lamps were completely incompatible. So, make sure you get enough lamps including some replacements from the same batch!

Another thing that can be done is to put a 1K/25W (or higher wattage) resistor across the output to always force enough current to flow to keep the triac seeing a load. Some dimmers may need a lower resistance/higher wattage resistor to accomplish this (hence a reason putting one incandescent lamp on the load often works...it is like the resistor only it makes light and burns out. Kelmar even makes a fancy load thing that switches in different resistances based on where you are in the dimming cycle.

LEDs do not dim like incandescents. For one, their color temperature does not change towards yellow/orange as they dim (unless the manufacturer deliberately uses amber LEDs that are the last to go off as it dims). Conventional dimmable LED lamps are not designed to dim to zero...that is merely a happy circumstance for some. Most are designed to dim to a point (10% of full brightness) and that is it because residential dimmers have both the adjustment and an On/Off switch. The range you have for the dimming is not going to follow a typical dimming curve it will be VERY compressed at the low-end so you may only have 30% of your dimmer's range that is usable.

We are moving towards a designed solution by EPRAD where they make their own dimmer module and lamp (low-voltage DC) where one can use the standard 120V wiring/fixtures but their output module and lamp (they make or are making an output module that can be put between any standard "chopper" dimmer and their lamps so get a true dim to zero solution. They also have their own dimmers that take their lamps directly, for new builds.

[Marcel Birgelen]

Finding a working lamp is no guarantee on future lamps of the same make/model. We had a customer do likewise (find a working make/model lamp) only to find on their next purchase that the lamps were completely incompatible. So, make sure you get enough lamps including some replacements from the same batch!

Yeah, it's quite a bummer with many of those brands who just put a name badge on some Chinese import. We've been using Philips branded bulbs and although I suspect them also to be re-badged OEMs, their behaviour has been consistent across batches... (keeping fingers crossed).

We are moving towards a designed solution by EPRAD where they make their own dimmer module and lamp (low-voltage DC) where one can use the standard 120V wiring/fixtures but their output module and lamp (they make or are making an output module that can be put between any standard "chopper" dimmer and their lamps so get a true dim to zero solution. They also have their own dimmers that take their lamps directly, for new builds.

Dimming LEDs with an "old-school" dimmer is somewhat of a hack. Low-voltage LED drivers are the way to go, but it requires a new approach for your lightning. LED dimming is done by Pulse-Width modulation, so a dimmable LED needs to adjust the "pulse width" depending on the input voltage. Some LEDs do this better than others. In my experience, for LED solutions, it's indeed best to use low-voltage LED drivers that do the modulation for you. During our last overhaul, we installed a bunch of LED strips too, one is in front of the screen and replaces a few spots that were there with a more continuous strip of light. We also have installed a bunch of "mood lightning" strips in the sides of the ceiling. Those LED strips are driven by a bunch of Meanwell LED drivers that have a 0-10V analog input. The dimming seems to be pretty linear, so you can use almost the entire range from 0-10V. Everything below 0.5V will effectively be off. Also, the driver seems to do a bit of interpolation between dimming updates. If you go from e.g. 10V to 5V, you'll see that it will dim the LEDs on a curve within about half a second. This allows us to reduce the amount of updates we need to send to the analog output module over MODBUS and still have a smooth dimming experience.

It's true that most LEDs don't simulate the color temperature change of an incandescent dimming sequence. I guess a full-color RGB LED strip could get relatively close, if you put in enough efforts, but it never really annoyed me. As long as the dimming itself is smooth and doesn't look like an animation stuck at 5 frames per second, I think it looks great.

[Don Furr]

This most likely won't help anyone but the bulb I was looking for to completely dim was a low wattage ambience bulb. They are 18,000 hour bulbs and only 3 watts. They were perfect for my application. This was found at Home Depot.
Brand name....Energetic LED
Part # YGA03A44-3W-Blue-1

[Frank Cox]

I bought two dozen incandescent flood lamps of the type that I use in my auditorium lights a couple of years ago. I figure that since I have to change one of those bulbs less than once per year that supply will last as long as I need it to even if the incandescent floods disappear completely.

Out of idle curiosity, though, is it possible to convert my dimmer switch to use led's? It's one of the white boxes with three pushbuttons on it from Kelmar.

[Gordon McLeod]

The issue realy is not the dimmer but the lamp ballast
The dimmer if under sufficient load (typicaly 60watt) will provide a variable output as normal the issue is that LED ballasts need power at the beginning of the cycle to start working which is not the case with a Thyristor dimmer so they tend to flash at the bottom end

[Carsten Kurz]

Out of idle curiosity, though, is it possible to convert my dimmer switch to use led's? It's one of the white boxes with three pushbuttons on it from Kelmar.

You can only try. We went through two dozen LED bulbs until we found a type that would finally work with our specific dimmer. Since then, we are quite satisfied, the cost for buying them was very low, the saving is dramatic, and we did lose only one of 42 bulbs since 2016. Which means something by itself, because changing them is quite a bit of work.

But I think we were quite lucky to find some that worked at all. With an electronic dimmer like the Kelmar, your chances are probably higher, but I don't know how easy it is for you to get a range of bulbs for testing. Maybe ask Kelmar - I am sure they have a list of working bulbs if you tell them your type of dimmer.

- Carsten

[Gordon McLeod]

Kelmar does ell a add on box that they claim is compatable with every brand of LED

[Mark Gulbrandsen]

Kelmar does ell a add on box that they claim is compatable with every brand of LED

Well, some LED brands don't need the add on box. If you call there and ask Tom about it, he can tell you which ones do work and which ones don't and will require the box. Cree works fine with a straight Kelmar dimmer. I think the box only contains a load of some sort to fool the dimmer.

[Dave Macaulay]

I believe all new Kelmar dimmers have an internal load resistor to help LEDs dim fairly well. Older ones don't. Not sure what the LED output accessory does but it seemed to work with LEDs where we've installed them.
Incandescent lamps have a complicated resistance vs voltage curve, the cold resistance is considerably lower than the full voltage resistance. Adding a 100W incandescent bulb to the load side of a typical dimmer will settle down the flickering and flashing at low dim levels for LED lamps. I haven't found any triac dimmer setup that dims any LED lamp to full off smoothly or dim up without flashing on fairly bright instead of dimming up smoothly from off. The LED module just eliminates the flickering and such that some LED lamps do on a triac dimmer,
You can get network controlled dimmers that can dim LEDS pretty well if configured properly. I haven't found any with dry contact control. You can configure the dimming curve and lamp characteristics and get close but they will still not go down smoothly to full off: LEDs on a triac dimmer go down to pretty dim then just go off... but not all your lamps will go off at exactly the same time as they all dim down. Same going up - some will pop on (how bright depends on the lamp) before the rest. Even a variac dimmer will look the same.
You need either DMX or 0-10 VDC controlled LED fixtures to dim properly from full off to full bright like incandescent bulbs do. Even then the color doers not shift at dim levels to amber (like incandescent) as already mentioned. Many DMX fixtures can change the light color though...