Dimming Methods

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Dimming Methods

• Types of Solid State Dimming
• Types of Fluorescent Dimming
• HID Dimming

By Jon Limbacher of Spectrum Lighting
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Technology Comparison

• Professional dimming has come a long way from the
  candle snuffers of the 18th century to the gas
  tables of the 19th century and resistance dimmers
  of the early 20th century. Forward phase dimming
  with SCR technology has reigned over conventional
  loads since the early 1960s and is still today
  the most cost-effective choice for dimming
  installations.
• When forward phase technology emerged, electronic
  components made it possible to allow only
  portions of the AC cycle through to the load. The
  familiar SCR and Triac devices were used to
  control the intensity of light, varying the
  switch ON point of the lamp current each half
  cycle (forward phase). The convergence of
  improvements in transistor technology, lower
  technology costs and more improved processors
  brought forth reverse phase dimming, also called
  trailing edge dimming, which switched OFF the
  current flow at a predetermined interval.
• Patented techniques for producing a pure sine
  wave output with variable amplitude to control
  lighting levels use transistors to slice the
  mains into pulses, vary the current using pulse
  width modulation, and average the result, which
  produces a continuous, variable amplitude smooth
  sine wave.

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Forward Phase Control(SCR)

• The basic SCR dimmer will remain the choice for
  budget-conscious dimming installations for the
  foreseeable future. The simple technique of
  varying the switch-on point of the lamp current
  each half-cycle is established in tradition and
  is very cost effective.
• Most forward phase technology is the best in its
  class and boasts high quality choke designs on
  all dimming products. Advanced options take
  dimming a stage further with high-risetime chokes
  to reduce the filament noise by slowing the rise
  time of the curve even more.

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Forward Phase Control
5
Forward Phase Drawbacks

• The disadvantages of the technique include noisy
  filaments that can buzz audibly, and the
  possibility of cross-interference between dimmers
  and audio systems.
• SCR dimmers are actually quite efficient, and
  very little gets lost in the dimming circuit
  itself. But when operated at anything less than
  full output, the SCR dimmer presents a distinctly
  non-linear load, creating what is known as
  triplen harmonics. This means that the phase
  currents in a three-phase system do not cancel
  out as intended, but, rather, add up. In the
  worst possible case, the neutral current can be
  up to 73 higher than any one-phase current. The
  harmonics also produce audible noise and
  overheating in the distribution wiring and feeder
  transformers and can lead to penalties from
  utility companies.
• With its distorted waveform, SCR dimming is
  simply unsuitable for many loads, including most
  electronic transformers and electronic ballasts
  for fluorescent and metal-halide sources. In some
  cases, the load will perform badly in other
  cases, the load and the dimmer may sustain
  permanent damage.

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Reverse Phase Control

• . The IGBT is a semiconductor that is fast
  replacing both regular transistors and the
  trusted SCR in many power control applications.
  IGBT stands for Insulated Gate Bipolar
  Transistor. Invented in the late 1970s by Frank
  Wheatley at RCA and currently in its fourth or
  fifth generation of development, IGBTs are the
  preferred component for power control
  applications. They are significantly more
  efficient and easier to control than most other
  power semiconductors. IGBTs are commonly
  available with ratings up to 1200 amps and about
  1700V, making them suitable for use in just about
  any dimming application imaginable.
• The commercially-practical implementation of
  reverse phase dimming uses IGBT transistors. A
  transistor differs from an SCR in that it can be
  controlled to gradually vary the current, not
  just to switch it on. By gradually turning off
  the current rather than turning it on, a reverse
  phase angle dimmer reduces the filament noise in
  a similar fashion as a forward phase SCR dimmer
  without the need of a choke.

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Reverse Phase Control
8
Reverse Phase Drawbacks

• RPC dimmers can be very quiet and compact. They
  work well for filament loads or electronic
  low-voltage transformers but are unusable with
  inductive loads. Neon, ballasted loads, fans, and
  small electric motors will generate destructive
  inductive kickback energy when dimmed by an RPC
  dimmer. RPC dimmers either should not be used
  with these loads or should switch to
  forward-phase control (FPC) to dim such loads. In
  both cases, FPC or RPC, the dimmers are producing
  triplen harmonics.
• Using a transistor as a switch (on or off)
  produces minimal heat, while using a transistor
  to directly control the voltage to a load creates
  a lot of heat. The more time spent by the
  transistor in the analog mode, the more heat it
  produces. Specifications for 800µS rise or fall
  time require the transistor to operate in an
  analog mode the entire 800µS. This can raise the
  operating temperature past the limits set by the
  transistor manufacturer. Some designs prevent
  failure of the IGBT by turning it off early,
  thereby producing less heat, and importantly,
  less fall time. Reduced fall time means increased
  lamp noise!
• The reverse phase dimmer is still chopping the
  waveform at the line frequency, so harmonic
  currents and electrical interference are still
  present. Acoustic noise for forward and reverse
  phase dimmers has been evaluated and is
  quantitatively the same for equivalent rise and
  fall times.
• More importantly, reverse phase dimmers that
  modify their behavior to become forward phase
  dimmers at will, in order to handle certain
  loads, are capable of creating harmonic neutral
  currents well in excess of the design
  specifications for either a completely forward
  phase or reverse phase system. A sine wave dimmer
  creates no harmonic currents.

9
Pulse Width Modulation (Sine Wave)

• SineWave dimmers offer complete silence and
  energy efficiency and represent the state of the
  art in dimming systems.
• Pulse Width Modulation (PWM) techniques are
  employed in SineWave dimmers for controlling the
  amplitude of the incoming power supply. In basic
  terms, the input supply is sliced at high
  frequency (40-50 kHz) and the transistors are
  switched, varying the ratio of on/off time within
  the sample period. The 'on' period of the
  'mark/space ratio' is proportional to the power
  needed to match the amplitude of a sine wave at
  that point in the mains cycle.
• The output current waveform is smoothed using a
  passive network to produce an output waveform
  that accurately represents the waveform profile
  of the incoming supply. The PWM process adds less
  than 1 distortion to the mains supply, resulting
  in a completely silent dimmer with a remarkable
  facility for dimming almost any load.

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Pulse Width Modulation
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Sine Wave Drawbacks

• Sine wave dimmers are more expensive than
  corresponding SCR dimmers a lot more expensive,
  in some cases. This is partly due to the
  complexity of sine wave technology but also to
  the relatively small number of dimmer channels
  produced and, perhaps, to a lack of serious
  competition in the marketplace. Right now, the
  market is dominated by a small number of
  specialist manufacturers .

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How Fluorescent Lamps Work

• Lamp Basics

Unlike an incandescent light source, where atoms
are excited by heat, in a fluorescent tube atoms
are excited by a chemical reaction.
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How Fluorescent Lamps Work

• The inside of the lamp is coated with a phosphor
  mix that illuminates when UV radiation comes in
  contact with the glass. Since light is not a
  direct result of filament glow, fluorescent lamps
  are inherently more efficient than incandescent
  lamps.

Once the tube is excited, the electrodes continue
to remain heated due to current transfer, but the
voltage required to maintain the gas excitation
drops down significantly from the strike voltage.
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How Fluorescent Lamps Work
When you turn the lamp on, the current flows
through the contact pins to the electrodes. There
is a considerable voltage across the electrodes,
so electrons will migrate through the gas from
one end of the tube to the other. This energy
changes some of the mercury in the tube from a
liquid to a gas. As electrons and charged atoms
move through the tube, some of them will collide
with the gaseous mercury atoms. These collisions
excite the atoms, bumping electrons up to higher
energy levels. When the electrons return to their
original energy level, they release light
photons.
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How Ballasts work

• A ballast slows down changes in current

• Electronic ballasts vary the frequency at which
  they run the lamps without changing the electrode
  voltage and are therefore able to get a much
  wider range of dimming.

• Fluorescent fixtures are dimmed using a special
  dimmable ballast. This is because standard (AKA
  Magnetic) ballasts typically do not have the
  ability to maintain electrode heat to the degree
  required for proper gas excitation when input
  voltage is varied.

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How Ballasts work

• 2 Wire Fluorescent

• These are very common ballasts and the easiest
  to install.

• They require a dimmed hot and a neutral (ground
  is understood)

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How Ballasts work

• 2 Wire Fluorescent

Recommended Ballasts
Advance Mark X Lutron Tu-Wire
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How Ballasts work

• 3 Wire Fluorescent

• These ballasts are also common and are usually
  quite inexpensive.

• They use two dimmers for control and power,
  requiring a dimmed hot, a switched hot, and a
  neutral (ground is understood).

• The 2 dimmers associated with this ballast must
  by code share a common breaker.

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How Ballasts work

• 3 Wire Fluorescent

Recommended Ballasts
Lutron FDB
Lutron ECO-10
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How Ballasts work

• 4 Wire Fluorescent

• These are not seen as often as the 2-wire and
  3-wire models.

• They use a hot (non-dim), neutral ,plus two
  low-voltage conductors for 0-10vdc control
  (ground is understood).

• Control current is sourced by the ballast and
  sinked by an external device.

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How Ballasts work

• 4 Wire Fluorescent

Recommended Ballasts
Advance Mark 7
Motorola Helios
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Ballast Ratings

• 1, 5, 10 what does it mean?

• The percentages are based on light output
  measured with a light meter.

• This is essentially a Linear Scale

• The human eye does not perceive light increase
  linearly but rather as a close function of
  square law

• When looking at the minimum light level output
  by a fluorescent fixture, the eye will see more
  light than the percentage touted.

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Ballast Ratings

• 1, 5, 10 what does it mean?

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HID Dimming

• Yes you can do it
• Sine wave or Reverse Phase
• Special Electronic ballasts with 0-10Vdc control
• But, you probably should not
• Not useful below 50-60 light output
• Poor lamp performance
• Cool purple or green color shifts