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- The New Standard for Silence
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- Years ago, the standard dimmer for theatres was the autotransformer.=
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- Autotransformers were not as load sensitive and reacted much faster =
then
the saturable core reactor, and were a vast improvement on the
resistance dimmers of the ‘20s and ’30s… and they =
were
much safer.
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- With the advent of Transistors in 1947, the lighting industry began
working the first generation of solid state electronic dimmers which=
by
the end of the 1950s became the Silicon Controlled Rectifier (SCR).<=
/li>
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- No longer did you have a dimmer that output a sine wave.
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- SCRs have not changed much over the years, and even the newer SSRs s=
till
chop the sine wave, causing all kinds of electrical, acoustical, and
radio-frequency noise.
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- The advent of high capacity sine wave dimmers for the entertainment
industry introduces an alternative dimming method that reduces many =
of
the common problems associated with common theatrical dimming.
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- Sine Wave Dimming is a type of electrical dimming known to the
engineering world as Pulse-Width Modulated (PWM) Dimming.
- PWM dimming is a form of AC dimming which allows you to adjust the
current provided to the load downstream of the dimmer just as with t=
he
conventional method of using SSR dimmers, but by using a completely
different output waveform.
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- By modifying the waveform differently then SSR dimmers, PWM dimmers =
can
reduce the audible sound the physical dimmer makes itself, the sound
filaments make inside the lighting instruments lamps, and the
radio-frequency noise created by the chopped sine wave output of
tradition SCR and SSR dimmer types.=
This creates a quieter atmosphere in the theatre as well as
increases lamp life and decrease strain on the neutral leg going into
the dimmer rack.
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- The theory behind of sine wave dimming is simple:
- Chopping the sine wave causes problems
- These problems are reduced by the chokes, which blur the harsh edges
where the gate is opened and closed.
- Chokes currently walk the fine line between being very efficient but
not working effectively, and working effectively but reducing speed=
and
efficiency. Therefore,
making ‘better’ chokes is not really possible.
- So, make a dimmer that samples at a much higher rate.
- This dimmer will chop the wave so fast that the filtering will
virtually eliminate the harsh fluctuations associated with SCR dimm=
ing
and are known to cause many problems within the theatre.
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- Sine wave dimmers need both AC power, 120v at 60Hz in the US, and a =
data
input, such as DMX512.
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- The control module in the brain of the dimmer rack takes the control
input and translates it into instructions for the microprocessor loc=
ated
within each physical dimmer module.
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- The microprocessor translates the instructions from the control card=
a
signal pulse width modulated control signal that tells the IGBT when=
and
for how long to open the gate.
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- The IGBT, following the directions from the IGBT, opens and closes t=
he
gate. When the gate is=
open,
the AC voltage flows through
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- The AC voltage that flowed through the gate is then filtered into a
smooth sine wave.
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- This sine wave is then checked against what the dimmer expects the w=
ave
form to be and if it is close enough it does nothing. If the wave form varies in =
any
way, the dimmer is shut down.
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- …and the power continues on to your load.
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- Other dimmers open the gate when the wave form crosses zero, and clo=
ses
it again at the appropriate point through the wave.
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- This input signal from the control board goes into a controller, whi=
ch
translates it into a set of instructions determining the duty cycle =
(or
the ratio of ‘on’ to ‘off’) of the circuit.<=
/li>
- These instructions are input into a microprocessor, which takes the
instructions and creates a binary control signal which is then sent =
to
the insulated gate bipolar transistor (IGBT).
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- The microprocessor takes the control signal and translates it into
instructions that adjust the lengths of the ‘on’ and
‘off’ cycles (or duty cycle) in such a way so that the
output is a digital signal that allows only enough power through the
gate to create the desired voltage.
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- The microprocessor creates this binary control signal via its intern=
al
clock, which is reset at the end of each duty cycle period. The microprocessor is const=
antly
checking its clock against the current number given to it by the
controller. If the clo=
ck
exceeds the number in the instructions, it sends a control signal
telling the IGBT to turn off.
At the end of the duty cycle period, the microprocessors cloc=
k is
reset and is sends a control signal telling the IGBT turns on
again. After everythin=
g is
reset the process starts over again. This process happens about =
50,000
times per second.
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- Mathematically this operates like a multiplier. If the PWM duty cycle is .5=
(or
‘on’ for one half of the duty cycle period), then you si=
mply
have to multiply that by the input voltage to get the output
voltage. So, if the
controller’s instruction is “5” and the counter in=
the
microprocessor counts to “10” then the duty cycle is
“.5”. This=
means
that when the counter reaches “6” it tells the IGBT̵=
7;s
gate to close.
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- Remembering that voltage is measurement of the average of the space
under the curve, sine wave dimmers are calibrated to output the same
amount under the curve as its analog counterpart of the same
voltage. However, as w=
ith
all that is digital, the dimmer only works in terms of no voltage or
full voltage. While th=
ere
are many different types of PWM, the kind used in dimmers for the
entertainment industry are such that when full voltage is given, it
simply opens the gate and allows all of the voltage input into the
system at that moment to pass through.
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- This happens all throughout the AC sine wave, so when the gate is op=
en
it is only outputting that which is coming into the circuit through =
the
company switch and when it is closed it is outputting nothing.
- But since it is on half the time and off half the time, and this hap=
pens
thousands of time per AC voltage period the resolution is such that =
the
output averages out to be 50% of the input, or the same percentage as
the PWM duty cycle.
- Since voltage is the average amount under the curve, there is less
voltage output.
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- When set to 50% output, autotransformers output 50% of the input, al=
l of
the time.
- When set to 50% output, PWM dimmers output all of the voltage for
1/50,000 of a second and then none of the voltage for 1/50,000 of a
second and then are filtered such that even the miniscule gaps creat=
ed
by this digitizing are smoothed out.
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- Traditional SCR and SSR dimmers ‘chop’ the AC sine wave =
in
such a way that the output waveform is noticeably deformed.
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- While this is an acceptable deformation when working with resistive
loads such as the tungsten-halogen lamps used in most theatrical
lighting instruments because they simply need power to give resistan=
ce
to so that they will heat up to the point of incandescence, this does
create a byproduct: harmonics.
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- Acoustic harmonics:
- Traditional AC dimmers that chop the sine wave create filament
noise. This is create=
d by
the chopped sine wave. The
speed at which the sine wave is chopped in SSR dimmers is fast enou=
gh
to not be noticeable because the filament cannot cool down quickly
enough to visibly flicker.
It is however at a speed that causes the filaments themselves
inside the lights to vibrate and this vibration is loud enough to be
heard by the audience.
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- Acoustic harmonics:
- This audible vibration can also be heard from the dimmer itself as =
the
abrupt fluctuations of current can cause the choke and other electr=
ical
components to vibrate inside the dimmer itself.
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- Acoustic harmonics:
- Since PWM dimmers do not chop the sine wave, the filaments do not
vibrate at all. Becau=
se of
this, there is virtually no noise created by lights dimmed by PWM
dimmers or from the dimmers themselves, even in the 50% range.
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- Electronic harmonics:
- If all three of the phases are simultaneously faded up, you will get
what is known as triplin harmonics. These harmonics are the re=
sult
of the power that is run through the neutral.
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- Electronic harmonics:
- Three phase power is designed so that the three phases cancel each
other out and in an ideal situation there is no load on the neutral=
at
all.
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- Electronic harmonics:
- Assuming you have an unencumbered company switch to your dimmers, a=
nd
your dimmers are at full and at maximum capacity; if you dim everyt=
hing
down to 50% you will be putting substantial strain on the neutral.<=
/li>
- This strain is not only present in your system, but also in the neu=
tral
run back into the grid putting strain on the entire grid.
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- Electronic harmonics:
- In the same scenario, if you were to fade up all the lights on the A
phase to full but leave all the lights on the B and C phases at 50%=
you
would find an even larger problem.
- You are now putting as much as 115% of its rated current on the
neutral.
- If in the same scenario you had two phases at full capacity and one=
at
zero capacity you could get up to 140% the rated current.
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- Contemporary lighting packages are designed to consider this and are
installed with larger neutral legs then hot legs.
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- Electronic harmonics:
- A sine wave dimmer in the same situation.
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- Electronic harmonics:
- A sine wave dimmer in the same situation.
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- This lack of triplin harmonics allows PWM dimmers to be installed wi=
th
neutrals that are much smaller then those in a system that might
encounter the extra neutral load of wave chopping dimming.
- This allows you to save money on installation and is less harmful to=
the
electrical grid in your neighborhood.
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- For additional examples of the harmonic advantages of sine wave dimm=
ing,
please see the full report, including:
- 60-cycle hum
- Electromagnetic induction into the sound system
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- Inductive Loads
- SSR dimmers are unable to dim certain types of loads such as induct=
ive
loads.
- Since sine wave dimmers exactly replicate the waveform of a lower
voltage, almost all inductive loads can be dimmed by a sine wave
dimmer.
- This includes ballast lighting fixtures such as fluorescent lights =
as
well as electrical motors.
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- Electrical Efficiency
- CD80 dimmers tout a 97% efficiency, but this efficiency is only
available by utilizing the with low rise time version of the dimmer,
which operate louder then the higher rise time versions.
- Sine wave dimmers are more efficient then SSR and SCR dimmers, they=
are
able to operate much faster, be much quieter, and still achieve the
same efficiency.
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- The New Standard for Silence
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