Using Dimmer Switches with Toroidal Transformers

Use of dimmer switches in lighting control is commonplace. In their basic form, these dimmer switches progressively
delay the switch-on point in each half cycle of the mains waveform by use of a triac, thereby reducing the power
delivered into the load and thus the heating of the bulb filament. This fast switch-on part way through the mains halfcycle
gives rise to a current surge in the load and is the cause of the all-too-familiar dimmer ‘buzz’ that can sometimes
be heard from the light fitting and/or the dimmer unit itself.

The buzzing problem is further exacerbated in the case of transformer driven low voltage lighting. The transformer is
made of turns of wire around magnetic material. The rapidly rising current pulses created by the dimmer unit can cause
the transformer to vibrate in sympathy, rather like a loudspeaker.

In order to reduce this effect it is necessary to slow down the current pulse. This can be achieved by introducing a filter
comprising an inductor (choke) and a capacitor into the circuit. This topology is used in some of the more expensive
conventional triac dimmers.

Using Dimmer Switches with Toroidal Transformers

Typical component values would be L = 10mH and C = 0.1uF (x-rated) 300v. The inductor should be rated appropriately
for the circuit (eg 1000w = >4A)

There are also some more sophisticated (and more expensive) dimmer topologies which try to address this problem by
either creating a lower voltage sine wave or by controlling the load by switching off part way through the half-cycle.

All Airlink low voltage lighting transformers are dimmable. In the event that there is a requirement for the units to be
used in a ‘critical’ environment, we can customise most of our transformers to include a filter system similar to that
described above at modest cost.