KNX Dimming Actuator Guide: Leading Edge, Trailing Edge, and 0–10V
Compare leading-edge, trailing-edge, and 0–10V KNX dimming actuators. Learn how to match drivers, avoid flicker, and commission reliable lighting.
Reliable dimming starts with the lamp driver, not the KNX actuator. The actuator can generate an excellent control waveform and still produce flicker, drop-out, humming, or uneven low-level output when the driver is a poor match.
For each circuit, identify the driver model and its supported dimming method before choosing the actuator. “Dimmable LED” is not a complete specification.
Three methods, three different electrical jobs
| Method | How control works | Good fit | Main risk |
|---|---|---|---|
| Leading edge | Cuts the front of each AC half-cycle | Resistive and some inductive/legacy loads; compatible LED drivers | Acoustic noise, high stress on incompatible electronic drivers |
| Trailing edge | Cuts the end of each AC half-cycle | Many electronic transformers and dimmable LED drivers | Driver quantity, minimum load, and thermal derating |
| 0/1–10V | Separate low-voltage signal commands a compatible driver | Larger driver groups and commercial luminaires | Control direction, signal polarity, cable routing, and mains isolation |
Universal phase-cut actuators may automatically detect load type or allow an ETS selection. That does not eliminate the need to check the driver manufacturer’s compatibility list.
Leading edge and trailing edge in plain language
Both methods vary the energy delivered by switching part of the mains waveform. Leading-edge control turns on after the zero crossing and remains on to the end of the half-cycle. Trailing-edge control turns on at the zero crossing and switches off before the end.
The load sees very different electrical stress. Traditional inductive transformers often expect leading-edge control. Many modern electronic LED drivers behave better with trailing edge. Some drivers accept both, and some accept neither.
Never connect inductive and capacitive load types to the same dimming output unless the product documentation explicitly allows that combination. A universal actuator normally selects one operating mode per channel.
0–10V and 1–10V are not always interchangeable
In everyday specifications the terms are often mixed. In practice, verify:
- whether the controller sources or sinks the control current;
- whether the driver’s minimum command is 0 V or 1 V;
- what the luminaire does at the minimum signal;
- whether a separate relay is needed to disconnect mains;
- the permitted number of drivers and total control current;
- required cable type, polarity, and separation from mains wiring.
A 1–10V driver often dims to its minimum at 1 V but does not switch fully off through the control pair. A relay in the KNX actuator or a separate switching channel may be required. Confirm the exact driver data rather than assuming the signal pair provides isolation.
The compatibility calculation
For phase-cut LED circuits, check five limits:
- permitted LED wattage per channel;
- driver inrush current and maximum driver quantity;
- minimum connected load;
- ambient-temperature and multi-channel derating;
- manufacturer restrictions on parallel outputs.
The lowest applicable limit governs the design. For example, a 300 W channel may be restricted to ten of a particular 12 W driver because the driver count or inrush limit is reached first. The connected load would be only 120 W, yet adding an eleventh driver could still be outside the tested condition.
| Data to collect | Source |
|---|---|
| Dimming method and compatible dimmers | Luminaire/driver datasheet |
| Rated and standby input power | Driver datasheet |
| Inrush peak and pulse width | Driver datasheet |
| LED load limit and driver-count table | Actuator manual |
| Minimum load and permissible load mix | Actuator manual |
| Derating by temperature and channel use | Actuator manual |
ETS parameters that affect what users see
Minimum level should be set above the point where any lamp in the group flickers or drops out. Maximum level may be limited to reduce glare or suit the luminaire. Dimming time should feel consistent across circuits; a user notices immediately when one group reaches 50% much faster than another.
Also review:
- switch-on value and last-value memory;
- relative dimming speed;
- absolute value object (0–100%);
- actual brightness feedback;
- staircase and warning functions;
- scene values and whether users may store them;
- behaviour after bus or mains recovery;
- fault objects for overtemperature, overload, or short circuit.
Status feedback should come from the actuator, not from the button’s command object. That keeps panels and visualizations synchronized when a circuit is changed by a scene, sensor, central command, or another interface.
A commissioning sequence that catches real faults
Do not commission dimming at noon and sign it off from a laptop. Low-level problems are easiest to see under the actual room conditions.
- Confirm driver part numbers against the approved schedule.
- Test each circuit at 100%, 50%, 10%, and its intended minimum.
- Fade slowly through the full range and look for steps, flash, or drop-out.
- Switch the group from OFF to a low preset repeatedly.
- Run all channels in the module together for a thermal check.
- Call every scene and verify both level and feedback.
- Restore mains and KNX bus power separately.
- Record the final minimum level and curve for each circuit.
If one lamp in a parallel group behaves differently, swap the lamp or driver before changing ETS parameters for the whole room. A weak driver can make a correct actuator setting look wrong.
When DALI is the better answer
Phase-cut and 0/1–10V control remain useful, but they are not the best choice for every project. DALI is often preferable when individual luminaire addressing, driver-level diagnostics, emergency-lighting data, flexible regrouping, or tunable white control is required.
| Requirement | Often suitable method |
|---|---|
| Small room with a few retrofit dimmable lamps | Phase cut, after compatibility test |
| Several commercial drivers controlled as one group | 0/1–10V or DALI |
| Individual luminaire status and regrouping | DALI through a KNX-DALI gateway |
| Simple ON/OFF only | KNX switch actuator |
| RGBW or tunable white | DALI DT8 or a purpose-built constant-voltage controller |
The decision should be made before cabling. Changing from phase cut to DALI after installation affects drivers, wiring, gateways, cabinet space, and commissioning scope.
FAQ
Is trailing-edge dimming always best for LED?
No. It is common for electronic LED drivers, but the driver documentation and an actual compatibility test decide the result.
Why do LEDs flicker only at low level?
Common causes include an incompatible driver, a minimum level set too low, insufficient load, poor neutral connections, or interaction between lamps in the group.
Can two dimmer channels be paralleled for more power?
Only when the actuator manufacturer explicitly permits it and explains the required configuration. Do not assume relay-style paralleling is safe.
Does 1–10V switch a luminaire fully off?
Not necessarily. Many drivers need a separate mains relay for true OFF. Check the driver and controller interface specifications.
Technical references
- KNX product documentation, universal dimming actuator with leading- and trailing-edge control
- KNX product documentation, 0/1–10V dimming and switching actuator
- KNXmart, KNX dimming flicker problems and fixes
- KNXmart, KNX and DALI lighting integration
The practical rule is simple: approve a dimming channel only after the exact driver, exact quantity, full dimming range, thermal condition, and recovery behaviour have been tested together.