Take Apart a Compact Fluorescent Bulb




About: Middle aged geek username also works at yahoo.com, mac.com, comcast.net, wharton-10.arpa

Compact Fluorescent Light Bulbs (CFLs) are increasingly popular as a way to save some energy. Eventually, they do burn out. Some seem to burn out annoyingly quickly :-( Even if not burnt out, CFL bulbs have become very cheap, especially if you live in an area where they get subsidized by your local electrical utility.

Are there any hobbyist usable electronics parts inside a CFL? How do they work, anyway? And when they burn out, why have they burnt out?

Let's take some apart and see!

(This Photo by PiccoloNamek from Wikipedia. Hopefully this is sufficient to meet the requirements of the license; I didn't have my lawyer review the Gnu Free Documentation License)

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Step 1: Take It Apart 1: Cut a Pry-slot

Most of the CFLs I've seen have a seam where they can be pried apart without too much difficulty. Sometimes the seam is glued or "welded" together, other times it's just where two pieces have been "press fit" together.

Unfortunately, even if only press-fit, the two pieces are usually too securely attached to just pry them apart with your hands, if only because one of the halves has only the glass tube to get a grip on. Sometimes the joining seam is loose and/or large enough to fit in a flat-blade screwdriver, but it is easiest (assuming you don't want to re-use the bulb casing) to cut a shallow slot at the seam with a hacksaw. Just hold the housing securely (in a small vise as pictured, or not), and saw a slot just barely through the casing - about 4mm.

CautionTry REALLY hard not to break the glass fluorescent tube. Aside from sharp edges, fluorescent lights contain phosphors of unknown and possibly dangerous composition, and a small amount of mercury that you'd rather not have released in your home or workshop.

Step 2: Take It Apart 2: Pry It Apart!

Now that you have a slot, you should be able to insert a flat-blade screwdriver. With a bit of a twist, the rest of the seam will separate (even if glued or welded.) (Hold on glass tube, or it may fall loose and hit something and break.)

(the dangerous (?) mercury is contained withing the glass tube portion, which is sealed entirely separately from the electronics section. As long as you don't break the glass, the mercury stays nicely sealed away...)

Step 3: So What Have We Got?

I THINK the three CFL "Ballasts" shown here are from a 60W-equivalent IKEA quad-tube lamp, an anonymous 75W-equiv spiral lamp, and a 100W-equiv spiral lamp. The circuits seem to be relatively similar (see next pages), and they have similar components. Other CFLs may have different internals; Vendors are making IC-based CFL Ballast circuits with assorted improved qualities. These three seem to have pretty "dumb" circuits.

(moderately) High Voltage diodes
(moderately) High voltage capacitors - some of these have nice long leads so they can be clipped off without even needing to unsolder them.
Big Inductor - on the order of 2.5 milli-Henries for a 20W lamp.
Smaller Inductor - exact value unknown.
Toroidal Transformer (useful for Joule Thief!)
High Voltage Transistors or Mosfets
Assorted resistors.
High-voltage, High-temp "spaghetti" - this is usually silicone coated fiberglass; useful stuff in certain applications, and hard to find and expensive if you have to buy it.
The Fluorescent Tube itself - if this is still good, you can do things like replace the ballast with a DC inverter and have a battery-powered CFL.

Step 4: What Does All That DO - How Does a Fluorescent Light Work, Anyway?

A fluorescent light is a gas discharge tube. It works a little like a strobe tube, and a little like an LED. Once it's running, it will happily allow very large electrical currents to flow through some ionized gas. To prevent it conducting so much power that it burns out or blows fuses, you have to limit the current with some sort of external circuit (this is the part that is similar to LEDs.) This is the main purpose of the fluorescent ballast. (The other function of the ballast is to get to that "once it's running" state. This can involve filaments, high(er) voltage pulses, and stuff like that.)

The picture shows a simplified fluorescent tube and ballast. You'll notice that the ballast is an inductor. This is because an inductor can act as a current limiter 'for AC current without actually using up any power the way a resistor (as used for LEDs) would. A neat trick. The current through the inductor (and thus the lamp, since it's a series circuit) is proportional to the AC frequency, and the inductance of the inductor. If you've ever seen the magnetic-only ballast from a standard fluorescent light, you'll have an idea how large an inductor is required at the 60Hz AC that comes out of the wall.

Step 5: How Is a Compact Fluoresent Different?

So what's different about a Compact fluorescent?

A CFL tube is pretty much the same as a straight fluorescent; it's just folded up.

To make the ballast smaller, we have to shrink the inductor somehow. Since the current is proportional to the inductance AND the frequency, we can make the inductor smaller just by increasing the frequency! Basically, the electronics in a CFL (or in an "electronic ballast" for conventional fluorescents) contains a circuit that will make HIGHER FREQUENCY AC from the normal 60Hz input.

Typically, the AC input is rectified and filtered to High Voltage DC (HV diodes, electrolytic caps), and then some sort of oscillator (other caps, toroid, small inductor) is used to drive some HV transistors to produce a final output that is still about the same voltage, but at a much higher frequency than the original. This way, the final current-limiting inductor ("big inductor") can be much smaller.

Step 6: What Breaks?

Having looked at the guts quite a few dead CFL bulbs, I feel somewhat qualified to point out a few of the reasons that they go bad.

First, of course, the tube itself can go bad, having leaked too much vacuum, or evaporated too much metal internally, they just stop working. When manufacturers quote you extreme lifetimes for CFL bulbs, this is the failure mode that they have in mind.

Unfortunately, a large number of CFLs seem to go bad in the ballast electronics. I've seen them smoke, emit bad odors, and even spark (scary, given the probable flammability of lamp shades.) I've taken them apart and seen obviously burnt components. I'd like to blame this on "cheap imports", but I've had a fair number of brand name CFLs with similar problems. Even some electronic ballasts in circleline fluorescent fixtures. Sigh. (It does seem to be getting better.)

Unfortunately, just because a component on the circuit board is burnt, doesn't mean that that's the component that went bad initially.

The major suspect seems to be the electrolytic capacitors that filter the HV DC. I've seen these with bulging and even burst casings. If you read capacitor spec sheets, you'll discover that such capacitors have a finite lifetime to start with, and that lifetime goes down relatively dramatically as operating temperature goes up. Inside a poorly ventilated casing with 20W of power being dissipated nearby makes for some pretty high temperatures. There ARE high-temp capacitors, but I've never seen one inside a CFL :-( Once the cap goes, the HV oscillator is getting pulsing current instead of DC, which I suspect it doesn't like, and it's not surprising that other things go wrong too.
Some, but not all, CFLs contain a fuse...

The inductors are pretty hardy things; they're probably good unless they show obvious signs of being burnt. The non-electrolytic caps are probably the same, and you can easily test them for shorts using a multimeter. I've never tested any of the transistors...

Step 7: What Can I Do With the Parts?

If the tube is still good, you can power it with other types of ballasts or inverters. The picture shows a cheap surplus CCFL inverter mounted inside the spiral of a CFL; the bulb now operates on 5V (and runs about 3W...)

If the inverter as a whole is still good, you may be able to use it to power other types of fluorescent bulb. Search the internet for more detailed instructions.

The capacitors, resistors, and diodes may have general purpose applications, if they're good.

To me, the valuable parts are the inductors; it can be difficult to find inductors in typical hobbyist marketplaces, especially in the sort of high-current versions found in CFLs. The toroid can easily be stripped of its original windings and re-wound for other purposes, such as the classic Joule Thief single-cell LED driver. The small inductor looks like it would fit in many "low tech" switching power supply applications, like The Roman Black Switching regulator or this other white LED driver. The large inductor I'm not sure; in the worst case it also provides a compact core that could be re-wound for special purpose applications.

If you don't use the tube, try to dispose of it at a recycling center that accepts fluorescent lights. They may not be too happy to get ... pieces, but they shouldn't mind TOO much as long as the glass is intact.

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263 Discussions


1 year ago

Isn't it the other way?

"The current through the inductor (and thus the lamp, since it's a series circuit) is proportional to the AC frequency, and the inductance of the inductor."


2 years ago

Westfw, I know it's a bit late to say something (I just ran across this instructible), but there is a bit of a typo under "Section 6: What Breaks?" where it says this:

Unfortunately, a large number of CFLs seem to go bad in the ballast electronics. I've seen them smoke, emit bad odors, and even spark (scary, given the probably flammability of lamp shades.)

I think the "probably" should be "probable"? Or did you mean "probably flammable lamp shades"?

1 reply

Reply 2 years ago

Fixed, even if it is a bit late.


3 years ago

The fuse is often shrink-wrapped in one of the wires going to the ballast from the plug.


11 years ago on Step 2

Mercury is very toxic, but its not so dangerous in small amounts like this if there is no strong airflow it stays near the ground.

12 replies

Reply 11 years ago on Introduction

u do realize that the amount of murcury in one of these is not nearly enough to kill you, it wont even make u feel sick


Reply 10 years ago on Introduction

Not the point--heavy metal contamination is cumulative. It accumulates in the body. If you eat shark too many times a week it can be hazardous for the same reason--them being near the top of the food chain we get the end results of their build-up. There are a lot of hazardous inputs that won't make you feel sick immediately.


Reply 4 years ago on Introduction

Your body takes in some and eliminates some so it is not cumulative. If it were cumulative, you could have only so much shark over your entire lifetime versus how much shark you can safely eat in a week. For health concerns, it is about how much you take in and how fast you eliminate it that results in either relatively safe or relatively toxic levels in your body.


Reply 4 years ago

Shomas - Your logic isn't correct: "Your body takes in some and eliminates some so it is not cumulative". The statement contradicts itself. Additionally, it is well established that residual tissue contamination from mercury ingestion is cumulative.There are several different chemical forms of mercury: elemental mercury, inorganic mercury, and methylmercury. The form of mercury associated with dental amalgam is elemental mercury, which releases mercury vapor. The form of mercury found in fish is methylmercury, a type of organic mercury. Mercury vapor is mainly absorbed by the lungs. Methylmercury is mainly absorbed through the digestive tract. The body processes these forms of mercury differently and has different levels of tolerance for mercury vapor and methylmercury.
Mercury vapor, is also absorbed in the sinus cavities and it is here where the behavior of the mercury becomes very strange. It has an affinity for nerve tissue and through the sinuses, attaches itself to nerve endings and steadily travels the nerve to the brain where it then permanently resides and causes damage.


Reply 4 years ago on Introduction

The accumulative argument is nonfactual and espoused by persons who do not know better. My comment above to Peeet explains why.


Reply 10 years ago on Introduction

I think some concern borders on hysteria and lack of common sense. If i should accidentally drop a fever thermometer (there i really dated myself) i don't think i would call the men in the space suits. The XEROX company tried to make a dental imaging machine based on xerographic principals. They found out that the residual mercury in the dentist's carpets "poisoned" the drum. The project was eventually dropped. The same also is said to hold true if somebody breaks a fluorescent tube in a room where a XEROX machine is installed. For us hoomons It is the mercury compounds that are dangerous and that is what accumulates in fish organs, etc. Mercuric chloride is a deadly poison. Years ago our chemistry teachers in high school would pass liquid mercury around the classroom. Now they fire a teacher if he even has a vial of the stuff.


Reply 6 years ago on Introduction

Concern about mercury in CFLs does border on hysteria. You could spend months shattering 100,000 CFLs in one spot on your carpet, and once you had finished shattering the 100,000th CFL, you would finally have some cause for worry about mercury poisoning as an adult.

The bigger problem with CFLs, where the hysteria is justified, is pets, children, and the elderly. Mercury affects these groups much more than adults, and children and pets might lick the carpet. So if you have a household with no pets or children, then there's not much to worry about with breaking a CFL.

For those who worry about breaking them anyway, here;s a simple comment: think about the number of times you have ever broken a standard incandescent light bulb. Now divide that number by 10. Since CFLs generally last ten times longer than incandescents, this is the number of times you will break a CFL. Chances are this new number will be less than zero.


Reply 8 years ago on Introduction

Yes, I remember being allowed to roll it around on our hands - that's really going back some!! I'm almost 60 - phew!!


Reply 9 years ago on Step 2

True. Also it's not that much mercury. There are higher level of mercury it some fish.

Akshat Ag

4 years ago

This is very usefull for my project


6 years ago on Introduction

to get mine apart I just whack the seam with a small hammer


7 years ago on Step 7

I use this circuit for my flyback transformer driver