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flourescent light Ask MAKE: Why do flourescent lights buzz?

A few weeks ago, we looked at why dimmer switches cause incandescent light bulbs to buzz. We subsequently received a number of questions about why flourescent bulbs vibrate, such as this comment by snarkyFish:

It might be nice to have another one of these that explains why fluorescent lights / ballasts hum and buzz as well. I imagine it’s a very similar explanation, but a much more complicated fix.

You are right, they do buzz for a similar reason, which is the mechanical components being vibrated by electromagnetic waves. In this case, however, it is probably not the bulb itself that buzzes, but the ballast.

Fluorescent bulbs are made up of a glass tube that is filled with a low-pressure inert gas and a small amount of mercury, have a phosphor coating on the inside, and have an electrode on each end. When a large enough voltage is applied across the electrodes, the gas begins to conduct, which allows an electric current to flow from one electrode to the other. This current causes collisions between gas molecules and the mercury, which creates UV energy that is then converted to visible light by the phosphor coating on the tube.

This is all well and good, however as the gas heats up, it’s resistance goes down, which means that it becomes a better conductor and subsequently draws even more current. If this were left unchecked, the bulb would quickly heat up and self destruct, so a ballast is placed in series with the bulb to limit the current draw. There are a number of different kinds of ballast design, but the simplest one is to just use an inductor. An inductor will certainly limit the rate at which current can travel to the light, however it does so by absorbing magnetic fields into it’s core. This absorption, which causes magnetostriction, is probably the source of your hum- it literally causes the inductor to expand and contract at twice the AC frequency, which creates an audible sound wave (at 120Hz in the US or 100Hz in most of Europe).

I don’t think there is a safe way to fix this besides getting a new ballast. Newer ballasts use an electronic controller instead of an inductor for the ballast, so they shouldn’t be susceptible to the same buzzing problem. Making your own is certainly a possibility, but as with any high-voltage project, you better know what you are doing first.

Any other tips for how to fix a buzzing fluorescent light? Let us know in the comments!

[CC licensed photo by adotjdotsmith]

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