Are you asking how the
dimmer is instructed to dim, or how it electrically dims the
circuit? I'm assuming you mean the former; in which case, the dimmers do actually receive a
conventional DMX512 signal from the control board. More specifically, the
dimmer control module (like the
ETC CEM) receives this information, which is then processed and passed to the individual dimming units. With the advent of control networks that comply to
IEEE 802.1/3/11 standards, however, there are "exceptions" to this. The fundamental idea remains the same, though. A
DMX universe is packetized and put on the
network with other traffic, such as
RDM or
ACN data. This is again received by the control module, like
ETC's
CEM+. Networks like ETCNet1/2/3 operate in this fashion, which allows for greater scalability and the use of
conventional networking equipment. Many
DMX universes can be carried to nodes by the same singular CAT-5(e) or wireless infrastructure. This replaces long, expensive
DMX runs and their respective opto-splitters.
If however, you mean the latter case, then to answer your question: traditional dimmers, like Sensor, use phase-angle control by means of SCRs (a type of
thyristor) to
send only portions of the AC
sine wave through to the load. This means that after every zero-cross (one every pi radians), instead of the next half of the period being completed, the
SCR delays firing until a certain
point in time such that the
intensity specified by the user will match with its respective
RMS value. For example, in an ideal world*, a user would specify 50% on his/her
console and 50% of the original
current would effectively would be delivered through the
circuit. This would be accomplished by only sending these intervals of the wave through to the dimmed
circuit (over each period): [pi/2,pi] and [3pi/2,2pi]. If connected to an oscilloscope, the waveform would look similar to the following:
For this reason, one should
never connect anything that is not a
conventional lighting
fixture to a dimmed
circuit. (Such as inductive loads or anything containing sensitive electronics.) Even if a
dimmer is set to full
intensity, the
current is still running through the same
phase control circuitry which can yield a less-than-perfect
sine wave.
Another method of dimming circuits that's relatively new to the theatrical world can be found in products like
ETC's "Sinewave Dimming". The name is slightly obscure however, and it's not too
clear what they're trying to suggest. (What about the
sine wave?) These dimmers allow for a full, variable amplitude
sine wave by means of pulse width modulation. (I suppose that's just not a very good name from a marketing standpoint.) These dimmers use insulated
gate bipolar transistors (IGBTs) instead of SCRs, and provide a
circuit with an effective
sinusoidal waveform by modulating the full 120VRMS
current with a rectangular wave. (Google this if you want to learn more about it - it's rather complex.) This results in the elimination of harmonic currents and "
lamp sing," and in practice it has also been reported to increase lamp life slightly.
PWM can also be used to dim low-voltage loads such as LEDs, and low-amperage loads without the need for ghost loads.
Hope that answers any questions you might have had. Maybe we should start a
wiki article on this...
*This 1:1 linear relationship between selected
level and actual output does not exist in practice, which is why
dimmer profiles or curves are used for some
level of correction.