Figure 1. The 13.51 step relay dimmer from Finder can operate either as a simple step relay, with or without memory, or as a sophisticated dimmer.

There are times when 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, and one solution to this is a step relay dimmer based on MOSFET devices, which offers a softer transition when turning the light on or off, and also a memory feature which returns the lighting level to a preferred value each time it is switched on. Additional benefits compared with conventional triac-based dimmers include a 54% reduction in power switching losses and 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, shown in Figure 1, 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 (Live In), another connects to the lamp load (Live Out) 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
Within the step relay function there are two possible modes of operation. 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 using the dimmer and memorised – the user does not need to adjust the level with the dimmer 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. The desired mode can be selected using a simple 'set-up' routine.
The advantage of using this type of module even in this straightforward mode is that the transition between the ON and OFF states is softer, giving an effect that is more comfortable and pleasing to the eye than an instantaneous change in light level.

Figure 2. Using the 'with memory' mode allows the load to return to the previously memorised ON state.

Figure 3. When operated in the 'without memory' mode the load returns to the fully ON state each time the relay is activated.

Dimmer function
By exerting pressure on the push-button control for a longer interval the dimmer function can be activated. This causes the lamp output to progressively increase or decrease alternately from the previous level, over a maximum of 10 incremental light levels between fully off and fully on. However, for safety reasons the dimmer is not able to reduce the level to complete darkness: prolonged pressure on the button when decreasing the lighting level will only reduce it to the 10% level. A subsequent momentary pulse on the button will then turn the light off completely, if that is required.

Figure 2 shows the step relay dimmer operation in 'with memory' mode - a long control pulse causes the light level to be progressively raised or lowered through a maximum of 10 incremental steps, while a short control pulse toggles between ON and OFF with the ON light level resuming the value set during the previous ON state.

Figure 3 shows how the operation differs in 'without memory' mode, where a short control pulse toggles ON or OFF between the maximum light level and the OFF state.

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 (which can often cause an annoying buzzing noise).
• 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.

Conclusion
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.