What Causes a Fluorescent Lighting Fixture to Make Noise?

How Fluorescent Lighting Works

• Fluorescent lights are made up of sealed glass tubes that contain mercury vapor. When voltage is applied to the fluorescent light’s electrodes, the mercury vapor is ionized. This ionized gas causes the phosphor coating inside the glass tubes to illuminate.

Why a Transformer is Used in Fluorescent Lighting Circuits

• Fluorescent lamps require thousands of volts to ionize the mercury vapor enough to produce visible light. One way to increase voltage without increasing power is by using a transformer. A transformer is able to “step-up” voltage to the level that the lamp requires; at the same time, the transformer “steps-down” the electric current by an equal factor.

Why a Transformer Hums

• Transformers contain two or more wires that are wrapped around a common core. Many transformer cores are made of magnetic materials such as steel or nickel. If these cores were made of one solid piece of magnetic material, the transformer would waste energy due to a phenomenon called “eddy currents.” These currents would circulate through the transformer core and would be converted into waste heat.

  To minimize the number of eddy currents through the transformer, many transformer designers use cores that are made up of thin laminated sheets of steel or other magnetic material. When the transformer is powered, these sheets vibrate against each other. This vibration -- called magnetostriction -- is due to the creation of transient electromagnetic fields between the metal sheets. This causes a hum at twice the frequency of the transformer’s power source.

Considerations

• Transformer hum is a normal occurrence when a transformer is powered. Fluorescent lighting circuits that contain laminated-core transformers will normally hum while in operation.