A simple solution for using traditional dimmer circuits with high power LED lights
No doubt that LED light has quite some advantages over tungsten lighting even in the realm of theatre and professional lighting nowadays. In theatres however, most often the lighting equipment consists of a large dimmer pack controlled by a DMX light console of some kind. The light circuits themselves do not have DMX connectors and thus the use of LED light appliences available on the market would involve rewiring almost the entire theatre... If these are used on the traditional dimmer outlets, they loose their functionality if they dont fail altogether.
Dimmer packs invariably make use of triac or thyristor circuits operating by chopping the line voltage under variable phase angles. If we want to use existing circuitry in combination with modern high power LED's, some inventivity is involved. A reasonably simple solution consists of applying the dimmer controlled voltage to a suitable transformer, rectifying the secondary low voltage and with some current limiting device -a resistor being the easiest solution- loading with the LED cascade. Here is a sketch for this utmost simple circuit:
The transformer used has to be an audio type transformer. The types that have been in use in old vacuuum tube amplifiers are generally possible options. Of course their power rating has to correspond to the power required for driving the LED's. Transformers for audio applications have an air gap such that they also pass high frequencies without overheating. With normal mains transformers there is a risk of burning out the transformer even when it is properly rated. The series/parallel combination of the LED's should be choosen such that we make optimum use of the available voltage from the secondary. If it's a standard 8 Ohm winding, a rectifier bridge can be used. Note that there is no smoothing capacitor here. As a consequence, the light will flicker at the rate of twice the mains frequency, just like with gas filled lamps. Of course a capacitor can be used, but it tends to slow down the reaction time of the light output to the dimming quite a bit. A VDR to block off inductive spikes is placed in parallel with the LED's to protect them. Do not forget that power LED's need cooling and have to be mounted on a sufficiently large heatsink.
Ghent, december 3th 2010
naar index Godfried-Willem Raes