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Dimmers are devices used to vary the brightness of a light. There are two types of dimming methods: Leading Edge Dimming - where the early part of each sine half wave is chopped off. Trailing Edge Dimming - where the latter part of each sine half wave is chopped off. Clear as mud? See our Introduction to Dimming Methods at the bottom of the page for more information.
There are two main dimming methods today. Each of these is suitable for different types of loads (see below):
Leading Edge Dimming - the early part of each sine half wave is chopped off. These are suitable for:
You cannot use leading edge dimmers for capacitive load transformers unless the transformer states, "Suitable for leading edge dimmers" although "dimmable with most dimmers" is a good sign too!
Trailing Edge Dimming - the latter part of each sine half wave is chopped off. These are suitable for:
Do not operate inductive loads on this kind of dimmer - this will usually destroy the dimmer. Resistive loads are suitable for both dimming methods. However, Trailing Edge Dimming offers a softer mode of operation for the lamps and will extend lamp life. Please note that some of the cheaper trailing edge dimmers in the market are only suitable for resistive loads. Please also refer to the recommendations by the manufacturers of lamps and other controls.
So what type of load do I have? Resistive loads = mains voltage incandescent lamps and halogen lamps Inductive loads = 'Old School' heavy wound or laminated transformer used for low voltage halogen lamps
Capacitive loads = electronic transformers or converters for low voltage halogen lamps
Dimming of Low Voltage Lighting Wire wound transformers - All wire wound transformers are dimmable. Electronic Transformers are not so simple to classify. Some are not dimmable at all, some need a leading edge dimmer, some need a trailing edge dimmer. Check manufactures details with your product or present your electrician with the transformer and ask them to provide a suitable dimmer module.
Why does dimmed lighting sometimes hum, and how can it be corrected? Because of the way all dimmers deliver power at settings other than full brightness, the filaments inside a light bulb may vibrate when lighting is dimmed. This filament vibration causes the hum. To silence the fixture, a slight change in the brightness setting will usually eliminate bulb noise. The most effective way to quiet the fixture is to replace the light bulb. As far as the `bulb singing` concerned, a bulb consists of a series of supports and, essentially, fine coils of wire. When the amount of current flow abruptly changes the magnetism change can be much stronger than it is on a simple sine wave. Hence, the filaments of the bulb will tend to vibrate more with a dimmer chopping up the wave form, and when the filaments vibrate against their support posts, you will get a buzz. If you have buzzing, it is always worth trying to replace the bulb with a different brand. Some cheap bulb brands have inadequate filament support, and simply changing to a different brand may help. Note buzzing bulbs are usually a sign of a "cheap" dimmer. Dimmers are supposed to have filters in them. The filter`s job is to "round off" the sharp corners in the chopped waveform, thereby reducing EMI, and the abrupt current jumps that can cause buzzing. In cheap dimmers, they have economized on the manufacturing costs by cost-reducing the filtering, making it less effective.
Buzzing problems with dimmers Each good dimmer has a filter choke inside. The chokes also help to eliminate `lamp singing` that can cause audible noise to come from the lighting fixtures. In providing those filtering functions, the chokes themselves can generate a slight buzz. A little bit of buzzing is normal with filtered dimmers. If the buzz from dimmer can be a problem it is recommended that the dimmer is placed in the area where this buzz will not be a problem.