The case for the lonesome TRIAC

April 12, 2016

Chris C Blog PhotoThe case for the lonesome TRIAC

There are more phase-cut dimmers installed in the world than any other lighting control. Used for decades in both commercial and residential spaces to dim incandescent lamps, they have taken a lot of flak since solid-state lighting became viable. A lot of the discussion and complaints about LED fixture performance on these dimmers was warranted back in the late 2000’s and early 2010’s, but I think it’s time the professional lighting industry took another look at this humble control and what it enables.

TRIAC graphA little background first: phase-cut dimmers are controls that chop off a portion of either the front (leading/forward) or back (trailing/reverse) of the alternating current sine wave before it gets to the light source. How much of the sine wave is cut off is proportional to how much the source is dimmed: chop off a lot, the source will be very dim; chop off a little, the source will only be a bit dimmer than full output. Forward-phase TRIAC (TRIode for Alternating Current) and MLV (Magnetic Low Voltage) dimmers are the most common phase-cut controls, with reverse-phase ELV (Electronic Low Voltage) being less so.

The big knock on phase dimming for the last few years has been how poorly it works with LED luminaires or more accurately, LED drivers. Unless they’re designed to do so, drivers and ballasts as reactive loads (not resistive loads, like an incandescent lamp) do not play nicely with chopped-up AC waveforms and phase dimmers themselves don’t like trying to dim them. Even LED drivers & electronic fluorescent ballasts that are designed for these types of dimmers run into a low-end limit on the minimum amount of the AC waveform that they have to get in order to keep themselves running properly…this is the source of the “dead travel” at the bottom end of TRIAC dimmers when hooked up to LED sources.

The commercial lighting industry gravitated towards other methods during the fluorescent era, and continue to use these today. Protocols including 0-10V, DMX, DALI, and various proprietary systems use the separation of power and control to their advantage, adding features such as remote addressing, very deep dimming, and wiring through low-voltage methods.

TRIAC graph 2Not that this is without cost. A direct-AC, driverless, phase-dimmable LED source with <10% flicker delivering about 2,000 lm of 3500K, 80+CRI light all inside an electrically-safe housing accompanied by a list of qualified dimmers costs about $25. You will have to specify your input voltage (most driverless LED systems are voltage-specific), but you’ll be able to dim to 10% or lower using the same mains power wires and a dimmer that costs anywhere from $8 to $100, depending on how fancy you want to be.

A good-quality driver-powered LED source that produces similar output costs about $5. Add about $25 for a high-performance 0-10V dimmable driver and your source cost per fixture is just a bit over the phase-dimmable option above. Then add installation labor to pull the additional conductors (or add a lot more installation labor if the dimming wires have to be separated from the AC mains) and hook it up to a 0-10V dimmer switch that starts around $150.

For a single-fixture installation with these assumptions, the cost difference would look like this:

0-10V dimmable
Gear cost (LED source and driver if needed) $25 $30
Additional electrical labor $0 $20 (assume 15 minutes at $80/hr.)
Dimmer control $50 $150
Total $50 $200
Incremental cost increase $-- $125

For a 10-fixture install, the incremental cost increase for the 0-10V option would go up to $225, assuming it only takes 10 minutes to fish and connect the dimming conductors for each subsequent fixture. This number gets much greater in either case if a proprietary or digital control system is used.

Phase dimming is not a panacea, and systems with separate power and control wiring have many well-entrenched applications where their use has definite benefits: deep dimming and dim-to-off, status reporting, scene addressing, and many others. Phase dimming LED sources do, however, enable the use of high-quality sources and fixtures for general applications without the strong value engineering target that a fully separated control architecture brings. It may be worth a second look.