CFL (Compact Fluorescent Lamp) is a type of fluorescent lamp which is designed as a replacement for the incandescent lamp. Some CFL's can fit into normal light fixures (drop in replacements) while others need custom fixtures to work.
A fluorescent lamp is a gas-discharge lamp that uses electricity to excite a gas inside (usually a Mercury/Argon mix). Exciting the gas results in it emitting UV radiation, which is then absorbed by the fluorescent phosphor coating of the lamp. The phosphor coating then re-radiates the energy at a lower wavelength which is visible to the naked eye, resulting in useful light output.
The phosphor absorbs most of the ultraviolet light, but some does escape from the fluorescent lamp, which is why you can see plastic parts of CFL's become yellow and/or brittle with time. Most of the paints and plastics used in fluorescent fixtures contain UV-inhibitors so that they don't wear out as quickly. However, this doesn't stop the leaked ultraviolet light from slowly damaging other objects in the area.
Before we go any further, we are first going to go over some basics of the incandescent light. An important thing to know is that as an incandescent bulb is turned on, its resistance increases. For example when you turn on an incandescent light bulb, the current is at it's peak when started, but goes down as it gets hotter until it reaches some sort of equilibrium. At this point the incandescent bulbs resistance limits the current, preventing the filament to get so hot it gets destroyed. This makes the bulb inherently stable (as it's self regulating), and as such wiring a incandescent bulb is very easy. Just connect it to a power source and you are ready to go.
Unlike incandescent lamps, CFL's resistance actually GOES DOWN as they get hotter. They have a negative resistance. This means that you can't just wire them to a power source. If you do wire them directly to a power source, the current draw will keep increasing (due to the resistance going down) until something blows. I personally have had a CFL's glass body shatter when I first started using them (I didn't know how they work, so I blew a few of them while trying). Indeed it is because of this that I created this subsection of the site. To give people a basic idea on how they work how to get CFL's running cheaply, usually using junk/free components and (hopefully) some cool tricks which I found useful.
Anyway, back on topic. As the CFL's resistance goes down as it gets hotter, we need something to limit the current so that the CFL does not blow. The first method ever used was very simple, a fixed resistor in series with the CFL. This does the job, but is very inefficient and has long been since superceded.
Next development was the self-variable resistor. This resistor has the property that as current increases, so does it's resistance. This makes them a better option than the fixed resistor, as they are more efficient. This system is still in use for small CFL's (lower than 2W power) but has otherwise been phased out.
The basic structure of a fluorescent lamp:
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The white square is the tube, this is airtight and contains a mixture of Argon, Mercury and other gases. The green ends are the filaments. The operation of a fluorescent lamp is rather simple. The goal is to get an electric arc across the two filaments. This electric arc will give energy to the gas atoms, exciting them into radiating energy. The gas mixture in standard fluorescent lights emits short wave UV light when excited. This then strikes a fluorescent phosphor coating on the tube which absorbs the UV light and re-emits it as visible light (the colour is varied by varying the type and amount of phosphers used).
The hardest part is getting the arc to cross (strike) across the filaments. To get an arc to strike requires a very high voltage, which is difficult to do, so methods have been devised to reduce the voltage required. The most common involves the filaments. These can be used to heat the gas. As we mentioned above, the hotter the gas gets the lower it's resistance will be, thereby lowering the voltage required for the initial strike.
For the purposes of simplicity, my ASCII drawings portray fluorescent lights as long tubes (known as "striplights"), but they come in all sorts of shapes and sizes, some of which are shown below:
The bottom two tubes are the more well known "striplights" while the two top ones are known as CFLs (Compact Fluorescent Light/Lamp) and consist of tubes bent in on themselves in order to reduce the total length of the lamp. We will deal primarily with CFLs here, but most of it applies to striplights as well.
As mentioned above, CFL's need something to limit the amount of current going through them when operating. We have already talked about using resistors for this task, but they have their issues and are generally considered obsolete. More modern systems are known as "ballasts" and work on different principles.
Instead of using a resistor to limit the current, this system uses reactance. The system to use this is known as a Magnetic Ballast, a Reactive Ballast or a Inductive Ballast. These have been in use since about 1935, the time when commercial fluorescent lighting took off. The magnetic ballast is nothing more than a coil of wire wound around an iron core (essentially a choke). It works by using electrical current to generate a magnetic field. This magnetic field then induces electrical current in the wires which then opposes the flow of current that is trying to pass through the ballast to the lamps. This interaction achieves a balance and limits the total current flow to the lamps to a specific amperage.
As you might have noticed. Magnetic ballasts rely on AC current of a specific fequency. Most are "tuned" to work best between 50 and 60Hz, the standard line frequency used around the world. As a result of this, magnetic ballasts emit a low-frequency hum which is connected to the line frequency. They also are known to cause flickering of the CFL's (usually twice the line frequency, so a 50Hz frequency would result in 100Hz flicker). Saying that, as of 2007, magnetic ballasts have become very cheap (I bought a pack of five 11W magnetic ballasts for £1.50 off ebay), so that might be an option for you if you don't have the time/parts to tinker.
As for the noise, magnetic ballasts are given ratings from "A" to "D", "A" being the quietest, and the most efficient (as less energy is lost as sound). As a rule, magnetic ballasts tend to be heavier and bulkier than other ballasts, due to their method of operation. Here is an example picture of 3 magnetic ballasts (from my pack of 5, 2 are already in use):
The lamps used with with magnetic ballasts need what is called a starter. This is a temperature controlled switch that delays the striking of the arc by the ballast. The point of it is to allow time for the filaments to heat up the gas, thereby reducing it's resistance enough so that an arc can be struck. This form of lamp is called a pre-heat lamp, and is only in use with magnetic ballasts, due to their inability to do programmed start-up modes (unlike electronic ballasts, which have a few different methods, all mentioned below).
Electronic ballasts work on a different principle to magnetic ballasts. An electronic ballast is essentially a switched power supply. It takes an AC current, rectifies it to DC and switches it rapidly (between 10,000 and 20,000 Hz for CFL's) to generate a high frequency AC current, which is used for the lamps. The high frequency eliminates the hum associated with magnetic ballasts, resulting in electronic ballasts being virtually silent. Another advantage is that the higher frequencies allow for the use of a smaller transformer, meaning that electronic ballasts can be made much smaller than their magnetic counterparts.
In addition to this, Electronic ballasts allow for advanced features such as dimming (using PWM- Pulse Width Modulation), increased efficiency (~10% more than magnetic ballasts) and programmed start modes, to increase the life of the builb/or speed up the bulb start-up time.
There are three common start-up modes available for Electronic Ballasts:
Disadvantages of electronic ballasts is that they can intefere with TV/Radio reception (I know they do with me, but only when within 1 metre of the Tuner), plus some can intefere with remote control signals (never had this happen to me, I think this was the older systems). Also electronic ballasts are quite expensive, especially the ones with advanced features like Dimming. Thankfully old "energy-saving" CFL's provide ample supply of electronic ballasts which you can use in your projects. They are not as advanced as the ones you can buy, but they are cheap! (or free, if you get people to give you old energy-saving lamps). Look at the article "DIY CFL Ballasts" in this section for more info.