Lighting control
Dimming LED lighting
The part of a LED conversion that causes most of the phone calls. Why dimmable and non-dimmable lamps behave so differently, the difference between leading-edge and trailing-edge dimmers, what 1-10 V and DALI control actually are, and which manufacturers let you dim from more than one switch position.
Dimming LED — and why it goes wrong
Dimming a filament lamp simply reduced the voltage across a resistive element. An LED is not resistive: it is a driver feeding a semiconductor, and it only dims if both the lamp and the dimmer are designed for it. Three variables have to line up — lamp type, dimmer type, and total connected load.
Dimmable LED
Marked 'dimmable' on the box and the lamp. The driver is designed to interpret a chopped mains waveform and hold a stable output. Even so, dimmable does not mean universal — the lamp and the dimmer must both be suited to each other, and manufacturers publish compatibility lists for exactly this reason.
Non-dimmable LED
The majority of budget lamps. The driver expects a clean 230 V sine wave. On a dimmer they flicker, buzz, refuse to strike, glow when off or fail early. Fitting them to a dimmer circuit is the single most common cause of 'faulty' LED complaints.
Leading-edge (TRIAC) dimmers
The old rotary dimmers built for filament lamps. Typically 40-400 W minimum/maximum, designed around a resistive load. With an LED load of 20 W they sit below minimum load and misbehave. Legacy plates should be changed when a house is converted to LED.
Trailing-edge (leading standard for LED)
Electronic dimmers that switch on the falling half of the waveform. Quieter, smoother, and derated for LED — a plate marked '0-120 W LED' will state a much lower LED rating than its filament rating. This is the normal choice for a domestic LED retrofit.
0-10 V / 1-10 V analogue
A separate low-voltage control pair runs alongside the mains to drivers with control terminals. The mains stays on; the signal sets the level. Robust and popular in commercial and architectural schemes, and needed where deep, flicker-free dimming of large arrays matters. It requires the extra control cable to be installed at first fix.
DALI and digital control
A two-wire addressable digital bus. Each driver has an address, so scenes, groups and individual fittings can be re-configured in software without rewiring. Standard in commercial work and increasingly used in high-end residential.
The load trap
A dimmer plate marked 400 W may be rated only 100 W with LED, and will also have a minimum load of perhaps 10 W. Six 5 W GU10s total 30 W — fine on a modern trailing-edge plate, hopeless on a 1990s leading-edge rotary. Where an LED circuit sits below minimum load the usual symptoms are flicker at low level, a dead zone at the bottom of the range, or lamps that will not strike at all.
Converting halogen or incandescent to LED?
Traditional dimmers designed for 40–400 W of filament load often misbehave when the lamps are swapped for low-wattage LEDs. The reduced load can sit below the dimmer's minimum, causing flicker, buzzing, drop-out or a very narrow dimming range. In many retrofits the best solution is to replace the old dimmer with a dedicated LED dimming module rather than trying to find lamps that tolerate an incompatible plate.
Paul’s preference is the Varilight V-Pro range. These are programmable, British-made trailing-edge dimmers that work with the widest variety of dimmable LED lamps and can be configured to the exact load on the circuit.
Dimming from more than one switch position
This surprises most clients. In a conventional two-way circuit — hall and landing, or a room with switches at each door — only one position can be a dimmer. The others are plain two-way switches that turn the circuit on and off; the level is set at the single dimmer plate. Fitting two ordinary dimmers to the same circuit does not work and can damage both.
Several manufacturers solve this with a master-and-slave or smart approach, where every position can raise and lower the level. The systems below all offer it in some form. Ratings, master/slave pairings and maximum numbers of positions vary by product, so please verify the specific model against the manufacturer's current instructions before ordering:
Varilight — V-Pro Multi-Point
A Multi-Point master dimmer paired with any number of matching Multi-Point slave plates, so the same lights can be dimmed from every switch position on a two-way or multi-way circuit. Wired system, standard back boxes, British made.
Enkin — PDM250
A 250 W LED intelligent multi-way dimmer that allows the same lights to be dimmed from multiple switch positions without a separate master/slave arrangement. Designed specifically for LED loads.
Lutron — Caséta and RA2 / RA3 Select
A single dimmer module plus Pico wireless companion controls. The Picos can be surface mounted anywhere, so a stairway or open-plan room can be dimmed from several points without extra wiring.
Rako
Wired and wireless scene-based control. Multiple keypads control the same circuits, with scene recall as well as raise/lower — commonly specified on larger residential projects.
Schneider Electric — Wiser
App and hub based dimmer modules and plates; any number of paired controls or app devices can dim the same circuit.
Hamilton — Mercury
Multi-way dimming using a master plate with linked companion plates, aimed at the decorative plate market.
Aurora — AOne
Zigbee smart dimmer modules and wall controllers; multiple controllers can be bound to one lighting group.
LightwaveRF
Retrofit smart dimmer plates linked over RF to additional 'remote' plates and the app, giving dimming from more than one position without re-cabling.
Note on wiring: multi-point and smart dimmers frequently need a neutral at the switch position. Many older UK lighting circuits are wired with switch drops that carry no neutral, so an installation may need re-cabling, a neutral-free model, or an in-ceiling module behind the fitting instead of a plate.
Common questions
- Why does my LED flicker or buzz on a dimmer?
- Almost always a compatibility problem. Old leading-edge (TRIAC) dimmers were designed for 250-400 W of filament load; an LED circuit may draw only 10-30 W, which is below the dimmer's minimum load. Fit a trailing-edge LED-rated dimmer, check the lamp is marked dimmable, keep the total load inside the dimmer's LED-derated range, and avoid mixing different makes or batches of lamp on the same circuit.
- What happens if I put a non-dimmable LED on a dimmer?
- The driver is not designed for a chopped waveform, so you typically get flicker, buzzing, a very narrow or jumpy dimming range, lamps that stay on at the bottom of the range or flash at switch-on, and greatly shortened lamp life. It is not a safe long-term arrangement — fit dimmable lamps or replace the dimmer with a switch.
- What is 1-10 V dimming and where is it used?
- 0-10 V and 1-10 V are analogue control systems. The mains supply to the driver stays on and a separate low-voltage pair of control conductors carries a 1-10 V signal that tells the driver how bright to run. It needs LED drivers with control terminals and an extra two-core control cable, so it is mostly found in commercial and architectural lighting rather than typical domestic rewires. DALI is the digital equivalent and offers addressable control of individual fittings.
- Can I dim lights from two switch positions?
- Ordinarily no — a conventional dimmer is a one-position control, so in a two-way circuit only one plate can be the dimmer and the other can only switch on and off. Several manufacturers now offer multi-point or smart systems that do allow dimming from every position, including Varilight V-Pro Multi-Point (master plus slaves), Enkin PDM250 (250 W LED intelligent multi-way dimmer), Lutron (Caséta and RA2/RA3 with Pico companions), Rako, Schneider Wiser, Hamilton Mercury, Aurora AOne and LightwaveRF. Always confirm the exact master/slave pairing with the manufacturer's current instructions before ordering.
- Do I need a neutral at the switch for a smart dimmer?
- Often, yes. Many multi-point and smart dimmers need a permanent neutral at the switch position. Plenty of older UK lighting circuits are wired with switch drops that carry only line and switched line, so the options are to re-cable the drop, choose a no-neutral model, or fit an in-ceiling dimming module behind the light fitting instead of a plate.
Dimmers not behaving?
Specifying the right dimmer plate, programming a V-Pro to the load, or adding a neutral at the switch position are all quick jobs when done properly. Paul covers Brighton, Hove and Sussex.