Dimming Methods

Types of Solid State DimmingTypes of Fluorescent DimmingHID Dimming

By Jon Limbacher of Spectrum Lighting

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.

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.

Forward Phase Control

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.

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.

Reverse Phase Control

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.

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.

Pulse Width Modulation

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 .

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.

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.

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.

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.

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)

How Ballasts work

2 Wire Fluorescent

Recommended Ballasts

Advance Mark XLutron Tu-Wire

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.

How Ballasts work

3 Wire Fluorescent

Recommended Ballasts

Lutron FDB

Lutron ECO-10

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.

How Ballasts work

4 Wire Fluorescent

Recommended Ballasts

Advance Mark 7

Motorola Helios

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.

Ballast Ratings

1%, 5%, 10% what does it mean?

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