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Light on the Subject
By Andy Ciddor
Hang Ten
Sine wave dimming:
what’s all the (lack of) noise about?
The Strand (Genlyte) C21
dimmer rack cabinet
Strand C21 sine wave
dimmer module
Swisson sine wave dimming
Sine wave technology is being touted as the solution to
all our dimming problems, and like any campaign to get
us to replace our perfectly good existing equipment, we
should look closely at its benefits.
Sine wave dimming is both the latest and the most ancient
of electrical dimming technologies. For the first few decades of
electric lighting, it was the only technology available. It wasn’t
called sine wave dimming at the time. That title only became
necessary to differentiate this method of dimming from the
type that changes the shape of the incoming power waveform
to control the level of our lights.
All the original methods of dimming lights involved putting
some form of electrical resistance in line with the lamps
to limit the current flowing through them, consequently
reducing their brightness. The output waveform of a resistance
dimmer has the same sine wave shape that arrives at
the input: it simply gets lower in height as we reduce the
current flow. But resistance dimmers had some significant
drawbacks: They produced a lot of wasted heat, dimmed
smoothly only with a particular load wattage, were rather
bulky and could not easily be remote controlled.
Thyristor Dimming
The thyristor (SCR or Triac) dimmer was developed in the
middle of the 20th century and remains the major dimming
technology in use today. It is compact, energy-efficient, dims
smoothly across its entire load range and is easily remotecontrolled
by a small control signal — just about everything
the resistance dimmer lacked.
As a thyristor is a type of high-speed electronic switch,
dimming is achieved using a technique known as “phase
control.” Rather than lower the height of the sine wave of
input power, phase control dimmers omit a variable-sized
chunk of the sine wave at the start of each half cycle of
power. The chunk is cut from the start of each half cycle
because, although it’s quite easy to turn a thyristor on, once
it’s carrying current, it is very difficult to switch it off again.
Phase control systems switch on, then simply wait for the
current to stop when the voltage returns to zero at the end of
the half cycle. The lower the output level required, the larger
the amount of each half cycle is omitted. The thermal mass of
the filament smoothes out the bumps in the current output
so it doesn’t flicker, it just gets cooler and less bright as the
missing chunks get larger.
But phase control brings with it some serious side effects.
The sudden surge of current in the lighting cables and luminaires
produces a burst of electromagnetic interference (EMI)
that finds its way into everything from unshielded audio
and video cables to someone’s hearing aid. Plus, the uneven
waveform of the current being drawn causes major imabalance,
harmonic distortions in the power supply network,
and overloads in neutral cables. The sudden inrush of current
to the filament of the lamp being dimmed also brings with it
mechanical stresses that not only shorten the working life of
the filament, but also cause it to vibrate at audible frequencies
— a phenomenon known as a “singing.”
To reduce these effects, the output of thyristor dimmers is fitted
with a choke coil (a lot of copper wire wound around a core
of ferrous material). The choke magnetically constricts the rapid
rise of current in the load circuit. The larger the choke, the longer
the rise time and the less severe the side effects. Unfortunately,
to have any significant benefit, the chokes required are large,
heavy, expensive and reduce the efficiency of the dimmers.
Consequently, budget dimmers have small chokes to keep the
cost down, while touring dimmers have the smallest and lightest
chokes possible to keep their weight and size down. Places full
of electronics, like TV studios, concert halls and recording halls,
have the largest chokes that money can buy.
Controlling Sine Waves
Electronics engineers have been working to develop a system
that would combine the benefits of electronic remotecontrolled
dimming, with the low impact of a dimmer using a
smooth sine wave as its output. The answer has been known
for a while, but the electronics needed to bring it to reality
have been a long time coming.
16 September 2007 • www.stage-directions.com