Dimming That Doesn’t Dim Off

Recently a designer specified 0-10V dimming for a series of LED downlights.  The electrician powered the LEDs by connecting them to a nearby breaker panel.  The 0-10V control signal was generated by the lighting control system.  This was a simple system that should have worked with no problems.  Much to everyone’s surprise the lights would dim but they wouldn’t go off!  What happened?

Dimming light sources other than incandescent is a technical challenge.  It requires the ballast (for fluorescent, HID, cold cathode, etc.) or the driver (for LEDs) to precisely control the amperage and the voltage, and may also require converting the incoming AC to a DC output (for some LEDs).  This is difficult for fluorescent and HID lamps because the electricity must arc from one side of the lamp to the other, and at low power levels that arc simply fails.  Dimming LEDs often results in a visibly jittery dimming curve as well as a jump to zero when dimming down and a jump to on when dimming up. (This article in Electrical Construction & Maintenance is a good overview of the problems with dimming LEDs.)

In architectural lighting, however, the inability to dim all the way to zero is usually not seen as a problem.  Typical dimming applications such as classrooms and meeting rooms may want to dim lights for a presentation, but some light is still desirable so that attendees can see each other for discussion and see their desktop to take notes. Dimming is acceptable as long as the dimming is smooth down to a minimum light level.  In these installations the drop from, say, ten percent to zero isn’t an issue because it doesn’t happen until the room is empty and the lights are turned completely off.

With 0-10V dimming, though, the dimmer or driver is powered by the incoming line voltage, so its always operating.  As a result, its minimum operating capacity is also the fully dimmed state.  A 100-10% ballast or driver, for example, has a minimum output of 10% not zero.  When the fader is at the bottom of its travel we would normally expect the lights to go off, but they only go to 10%.  Here’s a graph showing the performance of representative LED fixtures dimmed with a 0-10V dimmer.

Source: U.S. DOE
Source: U.S. DOE

The solution is to provide separate switching of the line voltage delivered to the fixture.  Most wallbox dimmers have a toggle switch below the fader so that the fixture can be shut off at any time.    Here’s a schematic of a simple circuit.

Source:  U.S. DOE
Source: U.S. DOE

Other solutions come from other dimming techniques, including three-wire and four-wire dimming where the line voltage and the control signal come out of the same device.  This guarantees that at a minimum state power to the fixture is shut off.  Other control protocols, notably DMX512, and the electronics that utilize them can usually dim to zero.

A Challenge for LED Luminaires

Today I was at an LED “shootout” at the New York City office of Barbizon (special thanks to John Gebbe and Scott Hali).  We were looking at products that might be used in a specific application – that of lighting an auditorium or theatre.  The shootout was between 26 fixtures from 17 manufacturers, all installed at a height of 10′.

Architecturally, the designer is essentially lighting three conjoined rooms:  the orchestra, where the ceiling can be 35′ high or more; the balcony, where the ceiling can range from 12′ to 25′ because of the steep slope of the seating; under the balcony, where the ceiling may range from 12′ to 18′, again because of the slope of the seating.

The first part of the challenge is to find a set of fixtures that can provide even illumination in these three spaces, each one of which has a sloped floor and therefore a  varying throw distance.  The second part of the challenge is for all of the fixtures to dim simultaneously.  Unfortunately, I don’t think we saw success.  Here’s what we saw.

First, only one manufacturer had a product line for all three possible mounting conditions – pendant, surface, and recessed.  That manufacturer, though, didn’t have three beam spread and/or brightness options to meet the range of typical installation heights.

Second, LED manufacturing is maturing, but it’s not mature.  That means we still don’t have strong, industry-wide standards for things like color.  In many cases it was difficult to use fixtures from two or more manufacturers because the color of the light produced (visually evaluated, and measured in color temperature, peak wavelength and spectral content) clearly didn’t match.

Finally, getting fixtures from multiple manufacturers to dim simultaneously proved very difficult.  Each set of installed fixtures would need its own (perhaps custom) dimming curve just to get a close match, and identical performance seemed impossible.   The problem here is three-fold.  First, multiple control protocols would be required.  The fixtures demonstrated used line voltage dimming, three-wire dimming, 0-10v DC, and DMX protocols.  That’s not a deal breaker, but it is an unfortunate complication.  Second, some of the LED drivers produced unacceptable dips, flickering, or pulsing of the light as they dimmed.  Third, some of the LED drivers couldn’t make a smooth transition from darkness or light, or light to darkness.  We saw fixtures pop on and drop out, dim up nicely but not dim out well, and dim out well but pop on.  Eventually this might be as easy as working with incandescent lamps, but not yet.

The easy lesson was that, for now, the safest choice for smooth dimming from darkness to full light is still incandescent.  The color of the light from fixtures in all of the installation conditions will match, the dimming curves will be the same, and they’re easy to dim.

The complicated lesson was that it is absolutely essential to mock up the proposed lighting system, using the LEDs, drivers, control protocols, and dimming equipment that will be installed.  It’s the only way to be certain that the start and end of a show, when the house lights dim down and then back up, isn’t a light show of its own.