Forum Suggestions

I second the suggestions. Particularly for a dimming forum, as that's what my current question is...

I currently have a house lighting system run on a rack of older Sensor SR12 dimmers, and part of that system includes a couple of circuits of 60w light bulbs in various colors for cove lighting. They're just currently standard A-style edison twist medium base light bulbs but they're exceedingly hard to access and take many hours to simply replace the bulbs. I've found a series of LED bulbs that can replace these - much lower wattage of course, but they put out enough light to serve for our purposes, if only I can get them to be dimmable. I have only a basic understanding of the dimming technology but as I understand it the LED's will respond properly to a waveform that would reduce in amplitude, but not to the traditional chopped-up on/off/on/off style that traditional dimmers use - does that make sense? The current situation with the LED lights seems to be that they require a ghost load to respond at all, but when they do respond, I get a relatively even progression from about 1-15% on the dimmer at which point the LED is running at about 90%of output capacity. Increasing the remainder 85% brings it to full output.

 Anyway, what I'm wondering is if there's a standard dimmer module ETC makes that I can simply swap out on the applicable circuits. I'd be replacing the entire load with these units, and there'd be nothing non-LED running on the circuits, so my hope is that one of the various non-standard dimmer modules would work to run these lamps. As I said, I've only got a basic understanding of the technology, so I don't know how complex an issue this is. This would be in a typical installation for a US 120v 60hz system with 20a dimmers, though I think the total load on each dimmer is currently more like 15a, and would significantly reduce with the use of LEDs. I think the actual power consumption on the LED replacements is in the 11w range.
 

Blackfaer,

You bring up an interesting but not unusual application and also a hole in the design and control of LEDs.  What you are seeing is the combination of several factors acting in concert.

First, it is not too surprising that you need an additional load to get the LEDs to dim.  This is because SCRs require a minimum load to remain in conduction and most likely the LEDs do not create that for you.  I can tell you that a Sensor dimmer will work fine in the 50W or greater region as long as the load is linear, or resistive, which the LED power supply may or may not be.

Second, and depending again on the power supply design of the LED lamp, it is completely possible that you will not achieve linear dimming throughout the range even with a ghost load.  Quite often the power supply is a basic half or full wave rectified with a capacitor to keep the power running to the LEDs without a drop out.  Since the supplies are designed to operate at a wide range of voltages they are not going to respond well to dimming.

Lastly, you should monitor these LEDs for the first few weeks of dimming as they may have a tendency to fail early.  Since phase control dimmers chop the waveform as you stated, some basic LED power supplies end up reacting poorly and sending half wave spikes to the LED elements.  These spikes are what you could say "amplified," and operate the LED elements above their rating and shorten their life.  Once one element dies (and they die as a short) if other LEDs are then wired in series, the voltage to the rest of the surviving elements increases and thus reduces their life.  As you can see this then propogates and exponentially then burns out the rest of the elements in the lamp very soon.

 It is important to know from the lamp manufacturer if their products are designed to work on phase-controlled dimming.  Most are not and unfortunately the ones that are tend to be expensive and require other control systems tied in.  ETC does not make a dimmer module to fit into your rack to solve this.  Yes, SineWave dimming will accomplish this without a problem but it is quite expensive for the energy and long life savings you'll experience.

Since your primary issue is lamp life as it goes to maintenance and access, I would suggest your get slightly higher wattage A lamps and trim the maximum scale voltage down in the rack.  If you drop the voltage by 5% you will probably double the lamp life but you also lose light output which is why I suggest a large wattage lamp.  You'll have the same energy usage but a lot more service light.

Does that help?

David

Thanks very much David. That's a great help.

Not the answer I was hoping for, but definitely an excellent explanation of what my problem is and what to move towards next. Unfortunately I was hoping to push the lamp life much farther than double, and by my calculations the LED lights I was considering (if, for instance, I didn't need them to dim and could just run them on/off) would multiply the lifetime before general replacement by about 9-10.

 So back to the old system for now with a couple tweaks, and I'll be looking at a whole different install LED system when we get the time and funds to fix our problem.

 Thanks again for your response!

-Mark