You’ve all seen them, the bulbs that are supposed to help the environment, with swirly tubes, short and fat, long and thin. Their efficiency comes from the fact that, unlike filament bulbs, the tubes don’t draw their current direct from the mains, instead they use a kind of inverter, known as a “ballast”, very similar to the ones used in laptops for backlights. Except these run on high voltage inputs, unlike a laptop inverter which will run off 9 to 15v DC and provide 1000v AC ignition voltage, with 300 to 800v run voltage depending on brightness setting.
Unlike a laptop, though, fluorescent lamps aren’t variable brightness. The ballast puts very little load on the AC input, instead it itself provides the current to drive the tubes, like a middleman.
Compact fluorescent lamps have some benefits in comparison with standard filament light bulbs:
1. Lower power consumption (as much as 80%) and
2. Much longer life expectancy when used in the correct environment with airflow (5 to 15 times)
1. Longer warm up times (mainly only experienced with cheaper bulbs)
2. Cannot be run off a dimmer switch.
3. Cheaper bulbs tend to be failure prone under heavier use more than 3 hours per day, or if not provided with adequate cooling around it.
4. More expensive per bulb than a filament one, but cost savings are made over its life to offset initial cost.
5. Depending on the colour temperature of the bulb, lower colour temps are not suitable for use as backlighting when using a camera.
Available colour temperatures
Fluorescent lamps are available usually in these color temperatures:
- Warm white (2700K)
- Cool white (4000K)
- Daylight (6000K)
The most common colour temperature is “warm white”, which is close in brightness to a classic 60W filament bulb and also is most pleasant to people, but cannot be used as ambient light for use with a camera.
Principle of construction and operation
Compact fluorescent lamps use a vacuum tube similar to classic strip lamp, the principle of energy transformation to visible light is the same. On either end of the tube are two electrodes coated with Barium, the tube is filled with Argon and Mercury. The cathode runs at high temperature (about 900 degrees Celsius) and generates many electrons which are accelerated by voltage, bouncing between electrodes, hitting the atoms of Argon and Mercury. This gives rise to low temperature plasm. The mercury energy radiates in a UV light form. The inside of the tube is coated with luminophore (phosphor), which transform UV light in to the visible light that you see.
The tube is powered by alternating current, provided by the ballast, so the electrodes (cathode and anode) switch on and off, alternating rapidly. Because of the use of a switched converter in the ballast, which runs on tens of kilohertz, the CFL lamp doesn’t flicker in comparison to a classic strip tube lamp. The converter, which is present in the screw or bayonet cap, substitutes the starter found in traditional classic strip lamps (which are wired direct to AC line), making CFL’s more efficient.
Here’s a look inside a Philips Genie 11W, for the curious electronic nerds out there 🙂
To help understand that little circuit board a little more, here’s its schematic diagram:
Theory of Ballast operation
The lamp requires a current to preheat the filaments, a high-voltage for ignition, and a high-frequency AC current during running. To fulfill these requirements, the electronic ballast circuit first performs a low-frequency AC-to-DC conversion at the input, followed by a high-frequency DC-to-AC conversion at the output.
The AC mains voltage is full-wave rectified and then peak-charges a capacitor to produce a smooth DC bus voltage. The DC bus voltage is then converted into a high-frequency, 50% duty-cycle, AC square-wave voltage using a standard half-bridge switching circuit. The high-frequency AC square-wave voltage then drives the resonant tank circuit and becomes filtered to produce a sinusoidal current and voltage at the lamp.
During pre-ignition, the resonant tank is a series-LC circuit with a high Q-factor. After ignition and during running, the tank is a series-L, parallel-RC circuit, with a Q-factor somewhere between a high and low value, depending on the lamp dimming level.
When the CFL is first turned on, the control IC sweeps the half-bridge frequency from the maximum frequency down towards the resonance frequency of the high-Q ballast output stage. The lamp filaments are preheated as the frequency decreases and the lamp voltage and load current increase. The frequency keeps decreasing until the lamp voltage exceeds the lamp ignition voltage threshold (up to 400v) and the lamp ignites. Once the lamp ignites, the voltage drops and the lamp current is controlled such that the lamp runs at the desired power and brightness level.
Common failures are faulty output capacitors, a major fault in cheaper bulbs, where cheaper components are used. When the tube doesn’t light up on time, or fully, there is a risk of destroying the transistors and their resistors. Lamp startup is very stressful on the ballast circuit, transistors usually don’t survive overloading at high temperatures, taking out the transistors fed by them. When the tube fails, the electronics are usually destroyed too. When the tube is old, the filaments become worn, causing high resistance to the circuit and either tube doesn’t lights up anymore. Normally in this case the electronics usually survive because the ballast will shut down if there is a loss of load caused by death of the tube. Sometimes the tube can be wrecked due to internal tension and temperature difference. Most frequently a stressed tube fails, when powered on, making it look like the whole lamp has failed.
Failure of the whole lamp at its worst is normally limited to a little bit of smoke, and/or a bad smell, and a small pinging noise. They are not allowed to “POP!” or cause direct shorts on the AC line, the input fuse on the ballast will prevent that.
Repair of the electronics usually means change of capacitors. When the fuse has popped, this signifies possible damaged transistors and resistors. Failures can be multiplied. For example, when there is shorted capacitors there can be thermally overloaded transistors that will be destroyed. The best transistors for replacing of original types are MJE13003, but they are not easy to find recently. I replaced them with BD129, but they are not available now. There exists other variants like 2SC2611, 2SC2482, BD128, BD127, but I am not sure if they will be long-life.
A fluorescent lamp is usually comprised of two parts. One is the plastic cover with holes for the tube and vents, and the plastic clips to attach to the bottom section. The tube is glued in using high temperature epoxy or cement glue. The bottom section has slots for the clips from the inner side. Inside is the printed circuit board with components and wires from the tube. From the upper side of the PCB are wires to top of the lamps, which are soldered or stamped to the contacts on the PCB, normally metal posts. Both plastic parts are clicked together and sometimes glued. Usually you can carefully leverage the casing with a small screwdriver sequentially to release the glue. Next you must leverage more to open the lamp. To close the lamp housing after repair you can only click both plastic pieces together.
Sometimes opening these lamps up is harder than the repair as the housing often gets damaged, lamps that have been heated and cooled regularly tends to lead to the plastic becoming brittle and hard to separate!