Igniting and starting discharge lamps. Osram HQI-E 150 W/NDL CL
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4 Igniting and starting discharge lamps
Some discharge lamps do not require an external ignition unit, as the supply voltage is sufficient to ignite the lamp or because the lamp has an integrated ignition unit. These lamps must not be used in installations with an external ignition unit or they will fail prematurely due to internal arcing.
4.1 External ignition units
4.1.1 Parallel ignition unit
Pulser ignition unit choke Luminaire
All other discharge lamps must be ignited by an additional unit. Ignition units or circuits of varying types are used for this purpose.
U
S
Capacitor for
PFC
Lamp
At room temperature, the filling particles are still present in solid form (metal halides or amalgam) or in liquid form (mercury). The arc tube contains the start gas, usually an inert gas such as argon or xenon, between the electrodes. The insulating gas filling in the arc tube must be made conductive in order to generate hot plasma. This is carried out by high-voltage pulses generated by a separate ignition unit or by the ignition unit in an electronic ballast. Constantly available free charge carriers (electrons) are accelerated by high voltage, providing them with sufficient energy to ionize atoms on impact and generate more free charge carriers. This process, similar to an avalanche, finally produces conductive hot plasma within which the current flow excites the partly evaporated metal halide filling such that light is radiated.
Fig. 19: Simplified circuit diagram for conventional operation of high intensity discharge lamps with pulse ignition unit
With a pulse ignition unit, the choke must be insulated for its surges. The pulse ignition units can normally take a load of 1000 pF, permitting lead lengths of about 15 m between lamp and choke. During ignition, the lead carries high voltage from the choke to the lamp so that care must be taken to ensure that the supply lead, socket and luminaire are adequately insulated for the corresponding high ignition voltage.
This type of ignition unit is used in single phase grids.
4.1.2 Semi-parallel ignition unit
The ignition voltage required to generate a breakdown between the electrodes depends on the spacing between the electrodes, the filling pressure of the gas between the electrodes and the type of gas. Examples for using these principles include the use of auxiliary electrodes or the use of Penning gases (see chapter
14.4 “Ignition at low ignition voltage (Penning effect)”).
U
S choke
Capacitor for
PFC
Semi-parallel ignition unit
Luminaire
Lamp
The sockets and cables must be suitably designed for the high ignition voltages. In particular with the E27 sockets for single-ended screw base discharge lamps, care must be taken that a similar socket (E27) for incandescent lamps is not used, which does not meet the requirements.
Fig. 20: Simplified circuit diagram for conventional operation of high intensity discharge lamps with a semi-parallel ignition unit
When lamps are defective or no lamp is inserted, continuous operation of the ignition unit can possibly damage the ignition unit or the luminaire. It is therefore advisable to switch the ignition unit off for a period of time after failed ignition, or to use an ignition unit with timer function.
Preference should be given to using a timer ignition unit.
In the semi-parallel ignition unit, part of the choke windings is used to transform the ignition pulses. This means the choke must be adequately insulated for the high voltage and have a tap for the ignition unit. As with pulse ignition units, the ignition unit can generally take 1000 pF or approx. 15 m lead length, and the connection lead between choke and lamp must be insulated for the corresponding voltage levels. A capacitor with minimum capacitance depending on the unit must be provided for compliance with the EMC of the ignition unit.
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