Electrical Control Gear for Lamps
Device(s) in the electric lamp circuit of gas discharge and solid-state lamps which limits the lamp current to the required value and ensures that the lamp can be started and once started can be operated stably over a longer period.
The devices needed are called igniters, ballasts, and, for LEDs, drivers.
Most of the gas discharge lamps have such a high internal resistance that they need a voltage peak, higher than the normal operating voltage, to initiate the discharge.
Igniters for Fluorescent Lamps
In most fluorescent lamp circuits, the electrodes are preheated for a few seconds before a high-voltage peak is applied across the lamp to initiate the discharge. Preheating of the electrodes facilitates the emission of electrons. Glow-switch starters were often used for this purpose. However, their harsh peak voltages have a negative effect on lamp life. Electronic starters with exactly the same functions as the glow-switch starter, but with a better-controlled voltage peak, are more and more used. They always ignite after one ignition pulse, so eliminating flickering during ignition. Because of the better control of the ignition peak, electrodes suffer less damage so that lamp life increases.
Igniters for HID Lamps
Ignition of HID lamps is initiated by a high peak voltage without preheating of the lamp electrodes. Igniters for HID lamps are normally started using an electronic igniter that generates a series of high-voltage pulses of the required magnitude (varying for the different lamps between some 200 and 5000 V). The electronic circuit is so designed that these pulses cease after ignition has taken place. Because of differing requirements for ignition voltage, shape of voltage peak, and number of voltage pulses within a certain period, each type of HID lamp (and often also each different wattage) needs its own type of igniter.
If mains voltage is supplied to electrical devices, the current has to be limited; otherwise, it will keep increasing until the device or circuit in which it is flowing breaks down. In many electrical devices, the internal electrical resistance of the device limits the current. These are devices with a so-called positive-resistance characteristic in which the current is automatically limited to a small range. Incandescent lamps have a positive-resistance characteristic and thus need no separate current-limiting device. Gas discharge lamps have a negative-resistance characteristic, meaning that the unlimited current available will increase until the lamp breaks down.
By introducing an external resistor in the lamp circuit, the current is stabilized. Simple resistors can thus be used as current-limiting ballasts for gas discharge lamps. They do, however, dissipate a lot of power. Instead, inductive coils are used, viz., copper wire wound around an iron core. They have the same effect as a resistor, but at much lower power losses in the ballast. The ballast also ensures that the lamp continues to operate despite the fact that twice during each frequency cycle of the mains voltage (in Europe 50 Hz and in the USA 60 Hz) the current is zero, the lamp is off and thus has to be reignited. An inductive ballast system helps to reduce the time the lamp is “off” during each zero passage of the current. This is because an inductive system shifts the phase between mains voltage and lamp current.
Until the 1980s, all ballasts consisted of inductance type of ballasts also called “electromagnetic ballasts” because of the electric and magnetic fields their coils generate. Since then, more efficient electronic ballasts have been developed.
Electronic Ballasts for Fluorescent Lamps
Electronic ballasts for fluorescent lamps electronically transform the sinusoidal 50 Hz mains frequency into a square-wave voltage of higher frequency, hence their name HF electronic ballasts. The high-frequency voltage lies between 25,000 and 105,000 Hz (viz, 25–105 kHz). HF electronic ballasts work on the same inductive principle as conventional electromagnetic ballasts. At the higher frequencies, much smaller coils with correspondingly smaller losses can be used than in conventional electromagnetic ballasts. When a fluorescent lamp operates on a frequency higher than some 10 kHz, the efficiency with which the radiation is created increases. The more efficient light production combined with the lower ballast losses means that the system efficacy of fluorescent lamps operated at high frequency increases by some 20–25 % compared to that of the same lamps operated on electromagnetic ballasts.
The electronic ignition function is incorporated in the ballast. With electronic operation the exactly required starting voltage pulse can be accurately supplied to the lamp. This minimizes electrode damage and so considerably increases lamp life. Relative to lamps operated on electromagnetic ballasts, the lamp life is increased by 30–50 %.
Electronic Ballasts for HID Lamps
Electronic ballasts for HID lamps have become available for some types of HID lamps. In contrast with HF electronically operated fluorescent lamps, there is hardly anything to be gained in terms of improved efficiency in the light-creation process by operating HID lamps on an electronic ballast. Nevertheless, energy savings are obtained, because the power losses in electronic HID ballasts are lower. The advantages of electronically operated HID lamps often lie in the better and easier control of certain lamp properties.
Drivers for LEDs
Instant starting is no problem with solid-state lamps, but they are low-voltage rectifiers that allow current to pass in one direction only. This means that the AC mains supply has to be transformed to low voltage and then rectified into a DC supply. Although a solid-state light source has a positive resistance characteristic, the voltage–current dependency is exponential in the area of operation. Small fluctuations in supply voltage therefore cause large variations in current that can damage the light source. A simple series resistor in the electrical circuit stabilizes the current to create, in fact, a “constant” current supply. In practice only miniature indicator LEDs use such a resistor. Because so much energy is lost in resistor-type drivers, all high-power, high-brightness LEDs employ electronic drivers that provide for the transformation from high voltage to low voltage, for rectification, and for the constant current supply.
Power losses in electronic drivers vary, according to their quality, between something like 10 and 30 %, with really -poor-quality drivers, as much as 50 % of the nominal wattage of the LED itself.