How to fix a fluorescent lamp

Sunday, February 10, 2013

How to fix a fluorescent lamp

In this video, I show the basic parts of a fluorescent lamp and how to replace the ballast.

More online resources:


Today in the House of Hacks, I'm going to try to fix the annoying buzz in this light fixture.

Hi makers, builders and do-it-yourselfers, Harley here.

I don't know if you noticed it or not in the last video, but there was a real irritating hum coming from this lamp fixture. And it got so bad as I was working in the shop over the holidays that I just turned the lamp off.

About fifteen years ago I installed four of these light fixtures and in the intervening years I've replaced all of the lights probably two or three times except for this one, probably four or five times. Each time I replaced it they work for about six months or so and then the light output dims down and they start humming until I just take the lights out.

Fluorescent lamps have been around since about the 1880s. They weren't commercially viable until the 1930s. It took this long to do the development of them because while conceptually they're really simple, you have mains lines coming in, going into a ballast, and then from the ballast, power going into the tube. Pretty simple, just two items. But from an engineering standpoint, what's going on in the ballast and what's going on in the tube, are fairly complex. Primarily, the ballast has to control a lot of details with starting the tube up. And then there's the physical design and chemical composition of the tube that's fairly complex also. So, while simple, they're also complex. That's why it took 50 years for them to go from research to commercially viable.

The purpose of the ballast is actually two-fold. First, it controls the rather complex startup of the lamp. And because of this complexity, there have been a number of strategies used over the years as fluorescent tubes have been developed. The second thing that a ballast does is simply deliver high-voltage, low current, alternating current to the tube to keep it running once it's been lit. For this reason, low voltage fluorescent lamps, like are found in RVs, and high voltage fluorescent lamps, like you might find in the shop, many times can use the same fixtures, the physical case, and the same tubes. The only difference is the ballast that's designed for different voltages to work from. But they all convert to the same output voltage for running the tube.

Interestingly, incandescent lamps start with a very low resistance and increases the resistance as they warm up. This creates a condition where it automatically limits the amount of current and keeps them from developing a short circuit. However, fluorescent lamps are exactly the opposite. They start out at a very high resistance and decrease resistance as they warm up. So, the job of the ballast is two fold. First of all it controls startup, warms the lamp up, gets it going. Then, as it does warm up, it decreases the current to the lamp to keep it from a self-destructive melt down.

There are two types of ballast: magnetic and electronic. The difference is kind of like the difference between a linear power supply, like you might find in a heavy component stereo amplifier, and a switching power supply, like you find in most consumer electronics these days. Older units like mine have magnetic ballasts. More recently, electronic ballasts have started to take over due to lower manufacturing costs and increased reliability. The rise of CFLs in the last decade or so is due in part to the development of these electronic ballasts.

The fluorescent tube contains a low pressure mix of mercury vapor and an inert gas, typically argon. The inside of the tube is coated with a mixture of phosphorus. The ballast creates and maintains an arc going through the tube. The arc going through the mercury causes an emission of ultraviolet light. The ultraviolet light is absorbed by the phosphorus which then turns around and emits visible light. This process of absorbing one frequency of light and emitting a different one is called fluorescence. Hence the term "fluorescent tube." And can be found in other places besides just lights. And finally the color temperature of the light is controlled by what other chemicals are mixed in with the phosphorus in that coating on the inside.

So that's a summary on the theory of operation of fluorescent lamps. A great resource online that has a lot more detail can be found at Edison Tech Center and I'll leave a link in the description below.

Since fluorescent lights are so simple, about the only thing that can cause this kind of constant failure is a bad ballast. So, that's what I'm going to replace in this fixture.

This is a new ballast I picked up at the big box store down the street. And this is an electronic version to replace the magnetic version that came originally in the lamp.

Before starting a project like this, if the fixture is wired into the circuit, make sure the power is turned off.

Or simply unplug the fixture if it's plugged in.

In this model of fixture, I have these little clips that hold the cover on. Some models have screws that you have to remove. But, in this case, all I have to do is a 90 degree turn and the cover comes right off.

Ok. I've got everything buttoned back up. The lights are in. The power is turned on. And we'll flip the switch and see what happens. Yay! They work. Brilliant.

That's pretty much it for this job. The proof will be in about eight months or so to see if the lights are still bright and the hum's still gone. But, I've done this before on other fixtures and it's worked pretty well. So I have pretty hight confidence that it'll work.

So, until next time, go make something. It doesn't have to be perfect, just have fun.