DALI or phase dimming: choosing the right LED control
Do we need DALI, or will a dimmer on the wall do? We are asked this regularly on the phone. There is no general answer, but there are clear criteria.
This article explains how the two methods work, why dimmed LED luminaires flicker and at what point DALI is worth the extra effort.
Leading edge or trailing edge: what "TRIAC dimmable" actually means
Dimming through the mains supply works by cutting a slice out of every half-cycle. There are two ways of doing this, and for LED the difference matters. Both are commonly sold as "TRIAC dimmable", which is only accurate for one of them.
Leading-edge dimming, the method behind the term TRIAC, switches on with a delay after the zero crossing and removes the beginning of the half-cycle. A triac does the switching, which is why these units are usually called TRIAC dimmers. They date back to incandescent lamps and halogen transformers.
Trailing-edge dimming works the other way round: it conducts from the zero crossing and switches off before the half-cycle ends. Transistors do the work here instead of a triac. These dimmers are also known as RC dimmers, and are usually sold as universal dimmers.
For LED drivers, trailing-edge dimming is normally the right choice. The input of an LED driver behaves capacitively, and the steep switch-on edge of leading-edge dimming produces current spikes there. There is a second problem on top of that: a triac needs a minimum current to stay conductive. An LED load draws little current, particularly at the bottom of the dimming range. This is precisely where the flicker comes from that many people know from dimmed LED lighting.
What to look for when buying: almost everything on the market is labelled as "TRIAC dimmable", whether it is or not. What counts is the driver data sheet. If it lists leading edge only, you need a matching dimmer, otherwise the light will flicker. Many LED drivers handle both, in which case a universal dimmer is the safe option. We state this on the product page, for example for our 90 mm recessed spotlights with CRI 98.
Where phase dimming works well
Operation is self-explanatory. A rotary dial or a push dimmer in the wall, the luminaires behind it, done. Nobody has to programme anything or open an app. It can be retrofitted into an existing installation without additional cabling.
Typical settings are individual rooms without any higher-level control: group rooms, treatment rooms, meeting rooms, retail and work areas, living spaces.
Where the limits are
- No common standard. Dimming behaviour and compatibility depend on the particular combination of dimmer and driver. A dimmer that runs cleanly with one luminaire may flicker with another, or fail to reach the bottom of the range.
- Limited load per dimmer. How many luminaires one dimmer can take is stated on the dimmer. With LED, the figure to go by is not the connected wattage but the LED load given by the manufacturer, which is often considerably lower.
- Every circuit is wired separately. Dimming groups independently means a separate cable and a separate dimmer for each one. This gets more involved with every room.
- The bottom end comes sooner. How far a luminaire can be dimmed is down to its driver. Many stop somewhere in the single-digit percentage range, some considerably higher.
What DALI does differently
DALI stands for Digital Addressable Lighting Interface. Instead of altering the mains voltage, a control cable sends digital commands to the drivers. The luminaire still receives full voltage and dims itself.
Every driver has its own address. A single DALI line can address up to 64 devices individually, plus groups and scenes. Which luminaire belongs to which group is a matter of programming, not of wiring. Regrouping later means changing software rather than pulling cables.
DALI is specified in the IEC 62386 series of standards. Devices carrying DALI-2 certification are tested by the Digital Illumination Interface Alliance and listed in a public database. This makes combining equipment from different manufacturers far more predictable than with phase dimming. It is not a guarantee that every manufacturer's special functions will work with every controller.
Two further points speak for DALI where they are needed. First, dimming down to very low levels: many DALI drivers go down to around one per cent before they switch off. Restaurants, bars and cinemas need this, and the command for a smooth fade to off is part of the protocol. Second, feedback: a DALI driver reports its status, so a failed unit shows up in the control system instead of someone having to walk the building to find it.
Side by side
| Criterion | Phase dimming | DALI |
|---|---|---|
| Method | cutting into the mains voltage | digital commands over a control cable |
| Standard | none, the combination has to suit | IEC 62386, DALI-2 certified |
| Devices per run | governed by the dimmer load | up to 64 per line, individually addressable |
| Wiring | one cable per dimming circuit | two extra cores, groups set in software |
| Lower dimming limit | driver-dependent, often stops sooner | driver-dependent, often around 1 % |
| Flicker at low levels | occurs, above all with leading edge | down to the driver, not the dimming method |
| Scenes, schedules, sensors | only with additional actuators | built into the protocol |
| Fault reporting | no | yes |
| Commissioning | wire it up | address and programme |
Which system when
The question is not which method is more modern, but which one suits the building and the people who operate it.
Phase dimming suits
a manageable number of rooms operated independently of one another, with no central control and no automation. Operation has to be obvious without training, and there is nobody on site to look after a system.
DALI suits
a larger number of luminaires that need switching in changing groups, scenes and schedules, daylight or presence control, a link to building automation, and the wish to see failures centrally rather than hunt for them. Dimming to very low levels belongs here too.
From our projects: technology that nobody operates achieves nothing. If lighting scenes are programmed and then never called up, the installation ends up running at full output and the investment was wasted. The level of complexity should match the people who work with it every day.
Not sure what suits your project?
Tell us how many rooms and luminaires are involved, who will operate the lighting and whether a building management system is in place. That usually settles the question within a few minutes.
- advice from a person
- since 2014
- many DALI and KNX projects
Retrofitting an existing building
In existing buildings, running a control cable to every luminaire is often the most expensive part of the job. This is where the wireless option comes in: the luminaires are fitted with a radio adapter and form a network among themselves. Dimming, groups and sensors work as they do with wired DALI. A control system can therefore be retrofitted where no cable will go, for instance in a building that stays in use or in a high-bay warehouse, and extended later.
Control pays off most where light is burning without being needed: areas with daylight from rooflights or ribbon windows, circulation routes and aisles where nobody works for hours at a time, and areas whose use keeps changing. How much this saves on top depends on daylight and occupancy. Any blanket figure would be guesswork.
For industrial buildings we have gone into this in more depth in our guide to LED high bay lighting, together with illuminance levels by task and the planning mistakes we see most often.
DALI and KNX
In building automation the two systems often meet. KNX controls blinds, heating and scenes; DALI controls the light. They are joined by a DALI gateway, which brings the DALI lines into the KNX installation. This is the usual route when lighting and the rest of the building services are to be operated together.
There is a smaller-scale option as well. KNX universal dimming actuators are available that handle both leading and trailing edge. They allow ordinary dimmable LED luminaires to be brought into a KNX installation without every luminaire needing a DALI driver. The luminaires cost less, but individual addressing and status feedback are lost.
Frequently asked questions
Is DALI always the better solution?
No, only the more involved one. For a handful of rooms with simple operation, a dimmer on the wall is the better answer. DALI starts to pay off as soon as groups, scenes, sensors or a large number of luminaires come into play.
Why does my dimmed LED luminaire flicker?
Usually the dimmer and the driver do not match. A leading-edge dimmer on a small LED load is the most common case, because the triac does not get the holding current it needs. Compare both data sheets; a universal dimmer often solves it. The minimum load of the dimmer is worth a look as well.
Can you switch from phase dimming to DALI later on?
Technically yes, in practice it is a rebuild. The drivers have to be swapped or the luminaires replaced, and the control cable has to be run. If DALI is on the horizon, starting with it is the cheaper route.
Does DALI dim to zero?
It dims to off. How far it goes before that is down to the driver. Many models reach around one per cent and then switch off. If the transition needs to be smooth, put it in the specification, because not every DALI driver manages it equally well.
Which businesses benefit most from DALI?
Above all those with daylight from rooflights or ribbon windows, those running several shifts, large buildings with differently used areas, and anywhere that will be extended later. DALI can be built up in stages.
Does DALI need an electrician with additional qualifications?
Not for the installation, but for the commissioning. Addressing and programming call for software and practice. Anyone doing it for the first time should allow time for it, or hand it over.
Planning a lighting project?
We will work through the dimming question with you, from a single room to the link with your building management system. One call is enough, and we are happy to call you back.
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- advice from the owner
- samples on request
Sources and further reading
- DALI Alliance (Digital Illumination Interface Alliance): database of DALI-2 certified products. Use it to check whether a device genuinely carries the certification.
- IEC 62386, the "Digital addressable lighting interface" series of standards. Part 101 covers the bus system, part 102 the control gear and part 103 the control devices.
- For any given installation, the data sheets of the drivers and dimmers used remain decisive, in particular the figures for dimming method, minimum load and lower dimming limit.
DALI is a registered trade mark of the Digital Illumination Interface Alliance and KNX a trade mark of the KNX Association. Figures for dimming ranges and loads are based on experience; the data sheets of the drivers and dimmers actually used take precedence.
