Introduction: Take Apart a Compact Fluorescent Bulb

About: Middle aged geek username also works at,,,
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)

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.