The Mysteries Of Low Voltage Dimming (Part 2)

In part 1 we discussed problems with low voltage dimming caused by electrical noise.
Electrical noise is interference which causes the fixtures or dimmers to react in very strange ways. Now we will review the bigger nemesis - transformer noise or audible noise.

Noise that is heard when dimming lighting can come from several possible sources. First is the sound of filament vibration caused by the enormous inrush of current created as the dimmer is engaged. The vibrating noise continues while the fixture is in a dimmed mode. This vibration can actually be heard in some environments. The best way to avoid this problem is to use a dimmer with the highest rise time, measured in milliseconds. "Higher" means longer, and when the dimmer takes longer to allow the current to flow, less stress is applied to the filament and the lamp operates more quietly.

A second source of noise is the dimmer itself. Dimmers that are poorly filtered can be noisy when they are not in the "full on" position. The answer to this problem is to purchase a brand of dimmer that is well constructed and fully warranted by a reputable manufacturer.

The most common source of audible noise is simply the transformer itself. It is the nature of a magnetic transformer, as an electrical device, which causes audible noise. Too often, low voltage lighting is specified for environments that are not compatible with the application of a magnetic transformer. A very quiet environment, such as a bedroom, is a likely candidate for objectionable noise. The same fixture in the family room or a public space would not be heard. Environments with hard surfaces such as kitchens are also not good applications for low voltage. In these spaces, noise from the transformer will be bounced around the room which resonates and acts as an amplifier compounding the problem.
Table 1, below, shows the decible levels and room types appropriate for low voltage lighting.

Does this mean that low voltage lighting can never successfully be used in quiet environments?
No, but there are two ways to improve the odds. Remote transformers may be used in these environments, a move which places the source of potential noise at a greater distance from the occupant. Also, the use of electronic transformers, reduces the risk of audible noise from the transformers. Whether using magnetic or electronic transformers, make sure the dimmer is compatible with the type of transformer. It is best to consult the dimmer manufacturer's literature to determine which dimmer is appropriate.

Often, the noise you hear may not be the transformer, but the compartment that the transformer is housed in. The natural vibration of the transformer can cause the compartment to vibrate and, in turn, be noisier that the transformer itself. This can be corrected by making sure the compartment is tight and does not have loose fittings or joints.

Finally, there are situations in which all precautions have been followed - including the type of environment, the decor in the room is favorable to noise absorption, the dimmer type matches the transformer - and there is still noise. The magnetic transformer may have loose laminations causing a vibration which is the source of the objectionable noise. The best solution in this situation is to replace the transformer.

Dimming low voltage fixtures can be applied successfully when you know the pitfalls to avoid and how to troubleshoot job problems when they arise.

Copyright 2001 VISUAL TERRAIN, INC.