The Mysteries Of Low Voltage Dimming (Part 1)

The fixtures arrive on time, the architect and the contractor actually get along, the client is out of town for a month and nobody is waiting to get paid! Does this sound like the dream job, or what? Well, it probably is, until the phone rings and you hear that the low voltage fixtures are making the clients feel as if they are living in the twilight zone! The dimmer in the family room is causing the hallway to dim and the dining room fixtures don't dim at all.

With any low voltage system there is potential for any number of strange occurrences. Typical problems are either audible noise (humming) or electrical noise. Electrical noise is not something one hears. Instead, it is interference that distorts the electrical current causing the fixtures or dimmers to react in very strange ways. There may be a cure.

Conventional incandescent dimming is accomplished either by changing the amplitude of the current or by changing the amount of time during a cycle that the current is allowed to flow.
Figure 1 shows a typical sine wave for an electronic dimmer. Notice that during the cycle the current is shut off for a brief instant. This "chopping" of the wave is a potential source for creating electrical noise on the line.

Low voltage operation requires a transformer to convert the voltage from 120 volts to something less, usually 12 volts. A transformer is one of two types - magnetic or electronic. Magnetic transformers use copper windings around a steel core. They are inductive by nature, which means they store the energy as a magnetic field. Magnetic transformers are very sensitive to D.C. voltage. For this reason it is not wise to use standard incandescent dimmers to dim magnetic transformers. Standard dimmers contain a small amount of D.C. current and can cause a transformer to overheat and eventually fail. Voltage spikes and current surges may also cause a failure in the dimmer or the transformer.

Electronic transformers, however, are capacitive by their design. They are a device that stores an electric charge. Therefore, dimmers should be matched to the type of transformer being used.

Now to the problem at hand - interference when dimming the fixtures, or electrical noise on the line. Chances are that the problem is caused by a missing neutral. Most wall box dimmers deigned for magnetic loads have only two wires. Figure 2 shows a typical wiring diagram for most commercially available "magnetic low voltage single pole" dimmers. The problem here is that the neutral is not connected to the dimmer itself. The circuit is not "latched" and noise or "crosstalk" occurs between different circuits while activating the dimmer control.

The better solution is to use a dimmer wired as shown in Figure 3. Notice this dimmer has the black wire on the line side, the yellow wire attached to one side of the load and the white wire attached to the other side of the load. This configuration closes the circuit and prevents interference from other wiring in the building. This is extremely important when a number of dimmers are "ganged" in the same box or in close proximity, such as a home or a restaurant.
Central dimming systems usually do no t experience these problems because they are most often wired with separate neutrals to each circuit - at least they should be. The important information here is to tell the dimming equipment manufacturer whether the transformers are magnetic or electronic. To avoid problems occurring on the jobsite the specification should clearly call out the type of transformer that is being used.

It may be possible to completely eliminate all problems associated with low voltage systems, but with careful planning and proper wiring you can improve the odds.