How dimming actually works in a modern home

Two families, and everything else is a detail

Every dimming method in a house belongs to one of two groups.

The first group works on the power wire itself. A wall dimmer takes the alternating current coming from the panel and passes only part of each cycle. Less waveform, less light. Nothing extra has to be pulled to the fixture, which is why this approach has been standard in houses for sixty years.

The second group leaves the power alone. Full voltage goes to the fixture, and a separate low voltage pair, or a radio signal, tells the driver inside what level to run at. The driver does the dimming. That costs an extra wire and some planning up front, and it buys a far better result at the bottom of the range.

Forward phase, also called leading edge or TRIAC

A forward phase dimmer waits a moment after each zero crossing, then switches on and passes the rest of the half cycle. Turn the slider down and it waits longer.

This is what almost every old dimmer in an existing house does. It was built for incandescent and halogen filaments, which do not care how ragged the power looks on the way in. It is also the correct method for magnetic low voltage (MLV) transformers, the heavy iron core units behind older landscape and cove lighting.

Put a modern LED on a forward phase dimmer and you can get buzzing, visible flicker, and a light that snaps off a quarter of the way down the slider. An LED draws a fraction of the current a filament did, and an old dimmer needs a certain amount of current flowing to hold itself on.

Reverse phase, also called trailing edge or ELV

Reverse phase does the opposite. It switches on at the start of the half cycle and shuts off before the end, so the current ramps down instead of slamming on. That is much gentler on the electronics inside an LED driver or an electronic low voltage (ELV) transformer.

Reverse phase dimmers run quieter, hold a low setting better, and are what most line voltage LED drivers are built to accept. They do need a neutral wire in the switch box, and plenty of older homes do not have one.

Both phase methods share a ceiling. Whatever the brand, chopping the power line usually bottoms out somewhere between five and ten percent of full output. Below that the light gets unstable. If you want a dining room that sits just above candlelight, phase dimming will not get you there.

0-10V

0-10V is the simplest of the separate signal methods. Two extra wires run to the fixture, violet and gray by convention. The voltage across that pair is the instruction: ten volts for full output, five for roughly half, one volt for about ten percent or whatever the driver calls its floor.

It is analog, and it only talks one way. The control sends a level, the driver obeys, and nothing comes back.

Two details cause most of the trouble in the field. The first is sinking against sourcing. In the common commercial arrangement, described in IEC 60929, the driver supplies the ten volts and the control pulls it down. Theatrical gear often works the other way and supplies the voltage from the controller. Put two sourcing devices together and the lights sit at full or flicker.

The second is off. A true 0-10V driver may go dark at zero volts. A 1-10V driver will not, and needs a relay to actually cut power, which is why some 0-10V zones have a separate switching circuit alongside the dimming pair.

DALI and DALI-2

DALI stands for Digital Addressable Lighting Interface. It uses two wires like 0-10V, but instead of a voltage it sends digital messages, and every driver on the line has an address of its own.

One DALI line handles 64 addresses, 16 groups and 16 scenes. A driver can sit in more than one group, so changing which fixtures make up the dining room is a software edit rather than a rewiring job. The bus is polarity free, so it cannot be landed backwards, and it runs to about a thousand feet between the two farthest devices on a 16 volt supply rated at 250 milliamps.

The part homeowners notice is that DALI talks back. Each driver reports its level and says when it has failed. In a house with two hundred fixtures, that is the difference between hunting for a dead downlight and being told which one it is.

DALI dims to about 0.1 percent, on a curve shaped so the change looks even to your eye rather than even on a meter. DALI-2 adds certification for the control side and for sensors and keypads. D4i puts the power supply and the diagnostic data inside the luminaire.

DMX512

DMX512 came out of the theater and still behaves like it. Data travels down an RS-485 line at 250,000 bits per second. One run, called a universe, carries 512 channels, and each channel is a single number from 0 to 255. The whole set is sent again about 44 times a second, which is what makes fast fades and chases look smooth instead of stepped.

Fixtures daisy chain. Each one is set to a starting channel and reads as many channels as it needs: a red, green, blue and white fixture takes four. Set the next fixture to channel five and it picks up where the first one stopped.

Two rules keep DMX out of trouble. End the chain with a 120 ohm terminator, or you get flicker that only shows up once the last fixture is connected. And keep a run to around 32 devices before adding a splitter.

DMX used to be the only way to get color into a house, which is why older projects have a DMX controller sitting next to the lighting panel just to run a cove. That is no longer the usual answer. It still earns its place with third party color fixtures, landscape and facade work, and anything that needs theatrical timing, and it is almost always one layer of a system rather than the whole thing.

Manufacturer digital systems

Several manufacturers run digital protocols of their own. Lutron EcoSystem is built on the DALI communication standard with a Lutron dimming protocol on top, which is how Lutron can guarantee the performance of its drivers with its own controls instead of hoping a mixed pairing behaves.

The drivers are where the low end lives. Hi-lume 1% does what the name says. Hi-lume Premier 0.1% goes ten times lower, fades between off and 0.1 percent when you switch it on or off, and meets IEEE 1789 for flicker across the whole range. Ketra fixtures hold that same 0.1 percent while also tuning color temperature.

Color works the same way. Ketra tunes the full spectrum, so a room can follow daylight rather than sit at one color temperature all day. Rania uses a three channel light engine to cover 1800K to 5500K at 90 or better CRI, and on a location aware HomeWorks system it knows where the house is and shifts through the day accordingly. Lumaris tape covers white and RGBW. All three are controlled over Lutron Clear Connect, which means the color and the dimming arrive together, from the same keypad, without a DMX or DALI controller in the middle.

Crestron, Control4 and Savant have dimming modules of their own and talk to 0-10V, DALI and DMX through interfaces. That is how one house ends up running several of these methods at once behind a single keypad.

Where wireless fits

Wireless dimming is not a separate method. The radio replaces the control wire, not the dimming itself. A wireless dimmer in a wall box is still doing forward or reverse phase on the load behind it. A wireless fixture control is still a digital driver with the bus moved to the air. Radio is what makes real lighting control possible in a finished house where nobody is opening walls.

The methods side by side

What this means for your project

Dimming is a wiring decision before it is a product decision. Pulling a control pair to a fixture during rough-in costs very little. Adding it after the drywall is closed is a renovation.

Most houses end up mixed, and that is fine. Phase dimming in closets and laundry rooms. A digital system in the rooms people actually live in. Where the light has to change color or follow the day, that now sits inside the fixture instead of in a separate controller, which is the case for Ketra and Rania on a Lutron system. What matters is that somebody chose it on purpose.

The document to ask for is a dimming schedule that sits next to the fixture schedule, naming the dimming method, the driver and the control for every load in the house. Lighting designers produce one. Electricians usually do not. When nobody owns that document, the decisions get made one load at a time at the job site, by whoever happens to be standing there.

We design and install lighting control across South Florida and the Bahamas, and we work from architectural drawings so the dimming method, the driver and the control are settled before anything is ordered. You can see more of that work on our home automation and lighting page.