Electrical ballast Totally Explained

Everything about Electrical Ballast totally explained

An electrical ballast (sometimes called control gear) is a device intended to limit the amount of current in an electric circuit.
Ballasts vary greatly in complexity. They can be as simple as a series resistor as commonly used with small neon lamps. For higher-power installations, too much energy would be wasted in a resistive ballast, so alternatives are used that depend upon the reactance of inductors, capacitors, or both. Finally, ballasts can be as complex as the computerized, remote-controlled electronic ballasts used with fluorescent lamps.

Necessity for current limiting

Ballasts are used where a load doesn't regulate its own current consumption well enough. These are most often used when an electrical circuit or device presents a negative (differential) resistance to the supply. If such a device were connected to a constant-voltage power supply, it would draw an increasing amount of current until it was destroyed or caused the power supply to fail. To prevent this, a ballast provides a positive resistance or reactance that limits the ultimate current to an appropriate level. In this way, the 'ballast' provides for the proper operation of the negative resistance device by appearing to be a legitimate, stable resistance in the circuit.
   Examples of such negative-resistance devices are gas-discharge lamps.
   Ballasts can also be used simply to deliberately reduce the current in an ordinary, positive-resistance circuit.
   Prior to the advent of solid-state ignition, automobile ignition systems commonly included a ballast resistor to regulate the voltage applied to the ignition system.
   Although LEDs are positive resistance devices, they've insufficient resistance to regulate their current consumption when operated from a voltage controlled source, so ballasts are used to control the current flow through the LED. Because the power dissipation is minuscule, simple resistor ballasts are normally used.

Resistors

The term ballast resistor primarily refers to a resistor which compensates for normal or incidental changes in the physical state of a system. It may be a fixed or variable resistor.

Fixed resistors

For simple, low-powered loads such as a neon lamp or LED, a fixed resistor is commonly used. Because the resistance of the ballast resistor is large it dominates the current in the circuit, even in the face of negative resistance introduced by the neon lamp.
   The term also refers to an automobile engine component that lowers the supply voltage to the ignition system after the engine has been started. Because cranking the engine causes a very heavy load on the battery, the system voltage can drop quite low during cranking. To allow the engine to start, the ignition system must be designed to operate on this lower voltage. But once cranking is completed, the normal operating voltage is regained; this voltage would overload the ignition system. To avoid this problem, a ballast resistor is inserted in series with the supply voltage feeding the ignition system. Occasionally, this ballast resistor will fail and the classic symptom of this failure is that the engine runs while being cranked (while the resistor is bypassed) but stalls immediately when cranking ceases (and the resistor is re-connected in the circuit).
   Modern electronic ignition systems don't require a ballast resistor as they're flexible enough to operate on the low cranking voltage or the ordinary operating voltage.
   In some old AC/DC receivers (universal sets), the vacuum tube heaters are connected in series. Since the voltage drop across all the filaments in series is sometimes less than the full mains voltage, it was often necessary to get rid of the excess voltage. A ballast resistor was often used for this purpose, as it was cheap and worked with both ac and dc.

Self-variable resistors

Some ballast resistors have the property of increasing in resistance as current through them increases, and decreasing in resistance as current decreases. Physically, some such devices are often built quite like incandescent lamps. Like the tungsten filament of an ordinary incandescent lamp, if current increases, the ballast resistor gets hotter, its resistance goes up, and its voltage drop increases. If current decreases, the ballast resistor gets colder, its resistance drops, and the voltage drop decreases. Therefore the ballast resistor reduces variations in current, despite variations in applied voltage or changes in the rest of an electric circuit. These devices are sometimes termed barretters.
This property can lead to more precise current control than merely choosing an appropriate fixed resistor. The power lost in the resistive ballast is also reduced because a smaller portion of the overall power is dropped in the ballast compared to what might be required with a fixed resistor.
In times past, household clothes dryers sometimes incorporated a germicidal lamp in series with an ordinary incandescent lamp; the incandescent lamp operated as the ballast for the germicidal lamp. A commonly used light in the home in the 1960s in 220-240V countries was a circleline tube ballasted by an under-run regular mains filament lamp. Self ballasted mercury-vapor lamps incorporate ordinary tungsten filaments within the overall envelope of the lamp to act as the ballast, and it supplements the otherwise lacking red area of the light spectrum produced.

Reactive ballasts

Because of the power that would be lost, resistors are not used as ballasts for lamps of more than about two watts. Instead, a reactance is used. In an ideal or theoretically perfect reactance, no power would be lost while limiting the current; realistically, losses due to resistance can only be minimized, not eliminated entirely.
An inductor is very common in line-frequency ballasts to provide the proper starting and operating electrical condition to power a fluorescent lamp, neon lamp, or high intensity discharge (HID) lamp. (Because of the use of the inductor, such ballasts are usually called magnetic ballasts.) The inductor has two benefits:

Its reactance limits the power available to the lamp with only minimal power losses in the inductor
The voltage spike produced when current through the inductor is rapidly interrupted is used in some circuits to first strike the arc in the lamp.

A disadvantage of the inductor is that current is shifted out of phase with the voltage, producing a poor power factor. In more expensive ballasts, a capacitor is often paired with the inductor to correct the power factor. In ballasts that control two or more lamps, line-frequency ballasts commonly use different phase relationships between the multiple lamps. This not only mitigates the flicker of the individual lamps, it also helps maintain a high power factor. These ballasts are often called lead-lag ballasts because the current in one lamp leads the mains phase and the current in the other lamp lags the mains phase.
   In most parts of the world, the mains voltage is sufficient to strike and maintain an arc in the lamp. In the USA, the mains voltage isn't sufficient for the larger lamps so an autotransformer is used to step up the voltage. The autotransformer is designed with enough leakage inductance so that the current is appropriately limited.
   Because of the large inductors and capacitors that must be used, reactive ballasts operated at line frequency tend to be large and heavy. They commonly also produce acoustic noise (line-frequency hum).
   Prior to 1980 in the United States, PCB-based oils were used as dielectric in the capacitors contained in many ballasts (see transformer oil). Fluorescent lighting practices and terminology differ between USA and Europe, and some of this section doesn't currently apply worldwide.

Electronic ballasts

An electronic lamp ballast uses solid state electronic circuitry to provide the proper starting and operating electrical condition to power one or more fluorescent lamps and more recently HID lamps. Electronic ballasts usually change the frequency of the power from the standard mains (for example, 60 Hz in U.S.) frequency to 20,000 Hz or higher, substantially eliminating the stroboscopic effect of flicker (100 or 120 Hz, twice the line frequency) associated with fluorescent lighting (see photosensitive epilepsy). In addition, because more gas remains ionized in the arc stream, the lamps actually operate at about 9% higher efficacy above approximately 10 kHz. Lamp efficacy increases sharply at about 10 kHz and continues to improve until approximately 20 kHz. Because of the higher efficiency of the ballast itself and the improvement of lamp efficacy by operating at a higher frequency electronic ballasts offer higher system efficacy. In addition, the higher operating frequency means that it's often practical to use a capacitor as the current-limiting reactance rather than the inductor required at line frequencies. Capacitors tend to be much lower in loss than inductors, allowing them to more closely approach an "ideal reactance".
Electronic ballasts are often based on the SMPS topology, first rectifying the input power and then chopping it at a high frequency. Advanced electronic ballasts may allow dimming via pulse-width modulation and remote control and monitoring via networks such as LonWorks, DALI, DMX-512, DSI or simple analog control using a 0-10V DC brightness control signal.

Efficacy

Efficacy is the correct term and is the term used in this industry. Efficacy means "capacity or power to produce a desired effect" while "efficiency" refers to the ratio of power input vs power output. Since "lumen" is a measure of perceived light or the "desired effect" and not that of optical output of the lamp, term efficacy is used. A 60 W fluorescent lamp with red phosphor producing 30 W of optical power and the same power green phosphor lamp producing 30W of optical power would both have an efficiency of 50%, however the green lamp would have a higher efficacy due to the sensitivity of our eyes, as one watt of green light has greater luminous flux than one watt of red light.
Two different fluorescent lamps of the same power could be giving off the same amount of photometric power within the visible spectrum, but could have different efficacy depending on the way it conforms to the way our eyes respond. The modern polychromatic lamps are designed on this concept. White light is made by blending individual color phosphors that matches the sensitivity of our eyes instead of emitting light in broad spectrum including where our eyes are not so sensitive.

Instant start

Starts lamps without heating the cathodes at all by using a high voltage (around 600 V). It is the most energy efficient type, but gives the least number of starts from a lamp as emissive oxides are blasted from the cold cathode surfaces each time the lamp is started. This is the best type for installations where lamps are not turned on and off very often.

Rapid start

Applies voltage and heats the cathodes simultaneously. Provides superior lamp life and more cycle life, but uses slightly more energy as the cathodes in each end of the lamp continue to consume heating power as the lamp operates.

Programmed start

More advanced version of rapid start. Applies power to the filaments first, then after a short delay to allow the cathodes to preheat, applies voltage to the lamps to strike an arc. Gives the best life and most starts from lamps. This is the preferred type of ballast for applications with very frequent power cycling such as vision examination rooms and restrooms with a motion detector switch.

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