Sometimes a simple dimmer switch just isn’t capable of providing the level or ease-of-use of lighting control that’s required in a particular room. Setting the lighting mood needs a more flexible approach. A step relay dimmer based on MOSFET devices offers a softer transition when turning the light on or off, as well as a memory feature which returns the lighting level to a preferred value each time it is switched on.

The energy-saving features of these devices compared to conventional triac-based dimmers include a 54% reduction in power switching losses, as well as the elimination of the familiar buzzing noise caused by EMC suppression filters. Step relay dimmers are smart lighting circuit components that can either be used as part of a wall-mounted dimmer unit, or more flexibly, as one of several controllers in a lighting console for a larger room or small building.

Multimode step relay dimmer
The 15.51 step relay dimmer from Finder can operate both as a simple step relay, where the lamp load is alternately turned on and off by a succession of short pulses on the push-button control, and as a sophisticated dimmer, by pressing for a longer time on the push-button. This latter mode causes the lamp brightness to either ramp up or ramp down.

The 15.51 is a three terminal electronic lighting control module for use on a 230V AC supply. One terminal connects to the live contact of the supply, another connects to the lamp load, and the third terminal connects to a controlling push-button. The other connection from the push-button can be returned to either the L or the N of the mains supply.

Step relay function
Two possible modes of operation exist within the step relay function. In the first mode – with memory – a short pulse on the button causes the load to alternate between the OFF state and the previously memorised ON state. The preferred lighting level can therefore be set and memorised using the dimmer – eliminating the need to adjust the lighting level each time the light is turned on.

In the second mode – without memory – pressing the button momentarily causes the load to toggle between the full ON state and full OFF state in the manner of a conventional light switch, but providing the advantage of a comfortable smooth and soft transition between the ON and OFF states.

Technical benefits
The step relay dimmer uses MOSFET switches to achieve the lighting control. These devices have the advantage that, unlike conventional triacs, they can be triggered to conduct at the zero voltage point in the waveform, and triggered to open after an appropriate time delay – a feature known as ‘trailing edge switching’ or ‘reverse phase control’.

Trailing edge switching gives the MOSFET dimmer a number of advantages over triac-based designs, including:

  • Lower electromagnetic emissions, avoiding the need for large EMC suppression inductors.
  • The ability to control alternative types of lighting load, such as low-voltage halogen bulbs operated through a transformer. Triac-based dimmers are restricted to use incandescent lamps and specifically exclude use with inductive loads.
  • Lower power dissipation – for a 1.5A load, a typical triac design will dissipate approximately 1.95W, while the MOSFET design dissipates just 1.05W.
  • Full output regulation – the output can be controlled over the full 0% to 100% span, in contrast to a triac design which has a restricted lower limit, and also demands a minimum load.
  • Higher OFF state – with higher loads, the triac design will permit a significant leakage current in the OFF state.
  • Better reaction to gate disturbance – any momentary disturbance on the gate of a triac will affect the whole of that cycle, while a similar disturbance on the MOSFET gate will only affect it for the duration of the disturbance.

The use of a MOSFET-based step relay dimmer for domestic or commercial lighting installations offers a number of benefits in terms of both energy-saving and convenience compared with conventional triac-based dimmer switches, and also provides a softer and more aesthetic effect when used as a step relay to turn lights on and off, and when dimming through intermediate lighting levels.