Fluorescent Lamps & Electronic ballasts

A fluorescent lamp or fluorescent tube is a gas-discharge lamp that uses electricity to excite mercury vapor.

It is divided into the Traditional Fluorescent Lamps and the Electrode less Fluorescent Lamps.

Traditional fluorescent or low pressure mercury lamp, is the use of low-pressure mercury vapor ultraviolet radiation during the discharge process, so that the principle of light-emitting phosphors to emit visible light, so it belongs to the low pressure arc discharge light sources.

 

Classification of fluorescent lamps

Present common fluorescent lamps:

(1) Straight tube fluorescent lamps. This is a double-ended fluorescent lamps. Common nominal power: 4W, 6W, 8W, 12W, 15W, 20W, 30W, 36W, 40W, 65W, 80W, 85W and 125W. Diameter: T5, T8, T10, T12. Lamp holder: G5, G13.T5 and T8 are widely used in current.T5 is applied to clothing, department stores, supermarkets, food, fruit, pictures, display windows and other colorful occasions. T8:Its shade, brightness, energy saving, life is better for hotels, offices, shops, hospitals, libraries and family simple but requires high brightness, the color of occasions.

(2)   Color straight fluorescent lamps. Common nominal power:20W, 30W, 40W. Diameter: T4, T5, T8. Lamp holder: G5, G13. Low luminous flux, for shop windows, advertisements or similar place decorations and color display.

(3) Circular fluorescent lamps. In addition to shape, the circular and straight fluorescent lamps not much different. Common nominal power of a 22W, 32W, 40W. Lamp with G10q.. Primarily to ceiling lamps, chandeliers and other light sources for supporting, for families, shopping malls and other lighting.

(4) Single-ended compact fluorescent lamps. The fluorescent lamp, ballast and lamp closely into one (ballast on the bulb holder), in addition to devastating blow, not demolish them, it is known as "compact" fluorescent lamps. Since no external ballast, the ballast drive circuit also, so this is self-ballasted fluorescent lamps and the start fluorescent lamps. The light directly through the E27 and other head for grid connection, can easily replace incandescent lamps directly.

Operation

Starting

The mercury atoms in the fluorescent tube must be ionized before the arc can "strike" within the tube. For small lamps, it does not take much voltage to strike the arc and starting the lamp presents no problem, but larger tubes require a substantial voltage (in the range of a thousand volts).

Switchstart/preheat

This technique uses a combination filament/cathode at each end of the lamp in conjunction with a mechanical or automatic switch that initially connect the filaments in series with the ballast and thereby preheat the filaments prior to striking the arc. Note that in North America, this is referred to as Preheat. Elsewhere this is referred to as Switchstart.
These systems are standard equipment in 200–240 V countries (and for 100–120 V lamps up to about 30 watts), and generally use a glow starter. Electronic starters are also sometimes used with these electromagnetic ballast lamp fittings.

Instant start

In some cases, a high voltage is applied directly: instant start fluorescent tubes simply use a high enough voltage to break down the gas and mercury column and thereby start arc conduction. These tubes can be identified by a single pin at each end of the tube. The lamp holders have a "disconnect" socket at the low-voltage end to isolate the ballast and prevent electric shock. Low-cost lighting fixtures with an integrated electronic ballast use instant start on preheat lamps, even if it reduces the lamp lifespan.

Rapid start

Newer rapid start ballast designs provide filament power windings within the ballast; these rapidly and continuously warm the filaments/cathodes using low-voltage AC. No inductive voltage spike is produced for starting, so the lamps must be mounted near a grounded (earthed) reflector to allow the glow discharge to propagate through the tube and initiate the arc discharge. In some lamps a "starting aid" strip of grounded metal is attached to the outside of the lamp glass.

Quick-start

Quick-start ballasts use a small auto-transformer to heat the filaments when power is first applied. When an arc strikes, the filament heating power is reduced and the tube will start within half a second. The auto-transformer is either combined with the ballast or may be a separate unit. Tubes need to be mounted near an earthed metal reflector in order for them to strike. Quick-start ballasts were more common in commercial installations because of lower maintenance as no starter switches need to be replaced. They are also used in domestic installations due to the virtually instant start. Quick-start ballasts are only used on 240 V circuits and are designed for use with the older, less-efficient T12 tubes, T8 retrofits will not start when used with quick-start ballasts.

Semi-resonant start

Semi-resonant start was invented by Thorn Lighting for use with T12 fluorescent tubes. This method uses a double wound transformer and a capacitor. With no arc current, the transformer and capacitor ring at mains frequency and generate about twice mains voltage across the tube, and a small electrode heating current.This tube voltage is too low to strike the arc with cold electrodes, but as the electrodes heat up to thermionic emission temperature, the tube striking voltage reduces below that of the ringing voltage, and the arc strikes. As the electrodes heat, the lamp slowly, over 3-5 seconds, reaches full brightness. As the arc current increases and tube voltage drops, the circuit provides current limiting.
Semi-resonant start was mainly used in commercial installations because of their higher initial cost. There are no starter switches to be replaced and cathode damage is reduced during starting. Due to the high open circuit tube voltage, this starting method was particularly good for starting tubes in cold locations. Additionally, the circuit power factor is almost 1.0, and no additional power factor correction is needed in the lighting installation. As the design requires that twice the mains voltage must be lower than the cold-cathode striking voltage (or the tubes would erroneously instant-start), this design can only be used with 5 ft and longer tubes on 240 V mains. Semi-resonant start fixtures are generally incompatible with energy saving T8 retrofit tubes, because such tubes have a higher starting voltage than T12 lamps and may not start reliably, especially in low temperatures. Recent proposals in some countries to phase out T12 tubes will reduce the application of this starting method.

Electronic ballasts

Electronic ballasts employ transistors to alter mains voltage frequency into high-frequency AC while also regulating the current flow in the lamp. These ballasts take advantage of the higher efficacy of lamps operated with higher-frequency current. Efficacy of a fluorescent lamp rises by almost 10% at a frequency of 10 kHz, compared to efficacy at normal power frequency. When the AC period is shorter than the relaxation time to de-ionize mercury atoms in the discharge column, the discharge stays closer to optimum operating condition. Electronic ballasts typically work in rapid start or instant start mode. Electronic ballasts are commonly supplied with AC power, which is internally converted to DC and then back to a variable frequency AC waveform. Depending upon the capacitance and the quality of constant-current pulse-width-modulation, this can largely eliminate modulation at 100 or 120 Hz.
Low cost ballasts mostly contain only a simple oscillator and series resonant LC circuit. When turned on, the oscillator starts, and the LC circuit charges. After a short time the voltage across the lamp reaches about 1 kV and the lamp ignites. The process is too fast to preheat the cathodes, so the lamp instant-starts in cold cathode mode. The cathode filaments are still used for protection of the ballast from overheating if the lamp does not ignite. A few manufacturers use positive temperature coefficient (PTC) thermistors to disable instant starting and give some time to preheat the filaments.
More complex electronic ballasts use programmed start. The output AC frequency is started above the resonance frequency of the output circuit of the ballast; and after the filaments are heated, the frequency is rapidly decreased. If the frequency approaches the resonant frequency of the ballast, the output voltage will increase so much that the lamp will ignite. If the lamp does not ignite, an electronic circuit stops the operation of the ballast.
Many electronic ballasts are controlled by a PIC microcontroller or similar, and these are sometimes called digital ballasts. Digital ballasts can apply quite complex logic to lamp starting and operation. This enables functions such as testing for broken electrodes and missing tubes before attempting to start, auto detect tube replacement, and auto detection of tube type, such that a single ballast can be used with several different tubes, even those that operate at different arc currents, etc. Once such fine grained control over the starting and arc current is achievable, features such as dimming, and having the ballast maintain a constant light level against changing sunlight contribution are all easily included in the embedded microcontroller software, and can be found in various manufacturers' products.
Since introduction in the 1990s, high frequency ballasts have been used in general lighting fixtures with either rapid start or pre-heat lamps. These ballasts convert the incoming power to an output frequency in excess of 20 kHz. This increases lamp efficiency. These are used in several applications, including new generation tanning lamp systems, whereby a 100 watt lamp (e.g., F71T12BP) can be lit using 65 to 70 watts of actual power while obtaining the same luminous flux (measured in lumens) as magnetic ballasts. These ballasts operate with voltages that can be almost 600 volts, requiring some consideration in housing design, and can cause a minor limitation in the length of the wire leads from the ballast to the lamp ends.

End of life

The end of life failure mode for fluorescent lamps varies depending how they are used and their control gear type.









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