Valeo company, as one of the automotive lighting leaders, has been supplying both first assembly lines as well as the aftermarket with xenon headlamps. Today, as a part of technical support of its product users, Valeo specialists explain the operating principles of popular xenon headlamps.
Xenon headlamps are distinguished from other types of lighting by obtaining light from an electric arc formed between two electrodes placed in a hermetic bulb filled with gas (xenon). The formation of the arc requires the creation of high voltage, that’s why a special module with an igniter is needed. Depending on the lamp type, the igniter may be either placed inside the headlamp (D1 and D3) or be a separate module (D2 and D4). The igniter is responsible for the ignition voltage and the current in the lamp.
Switching on: stage 0
Before the ignition the headlamp resistance is indefinitely high. At this stage the igniter is needed for the ignition of the lamp: a cold lamp requires around 10 kV, a hot one around 25 kV. With the supply of voltage a conducting tunnel is formed between the tungsten electrodes, between which the current will flow.
Before the ignition the working frequency amounts to 1 kHz.
Ignition: stage 1
During this stage the ignition circuit sends the high voltage pulse across the lamp, as a result an arc is established in the tube and visible light is generated. After a successful ignition the lamp requires high voltage in order to maintain the arc. Since the arc generates a lot of heat, the temperature in the arc tube increases rapidly. The metallic salts vaporise quickly, the arc is intensified and made spectrally more complete. The resistance between the electrodes falls. The electronic unit controlling the ignition switches to continuous operation mode.
Stage 1 – ignition – 20 kV between the electrodes
Ignition: stage 2
After the ignition the operating frequency is switched to 20 Hz. Such value is needed during the warm-up stage.
Stage 2 – warming, salts evaporation, maximum intensity
Warm-up
At this stage the DC converter provides sufficient current in order to sustain the arc (depending on the lamp demand). The converter generates sinusoidal alternating current. At that time the frequency is low (20 Hz), but upon the warm-up stage is complete, the inverter is operating at 200 Hz frequency.
Run-up
It’s the key stage of the xenon headlamp start-up process. The initial transient power during the start-up is high. It needs to be lowered at a later stage, in order to comply with regulation R99. The igniter controls the power level of the lamp in order to lower it to the normal level, i.e. the steady state.
Steady state: stage 3
At the steady state all metal crystals are vaporised and the arc has attained its stable shape, which means the luminous efficiency has reached its nominal value. The igniter supplies now a stable electrical power so as to avoid the flickering of the arc. Stable operating voltage is 85V AC in D1 and D2 and 42V in the mercury-free D3 and D4 systems. The AC square wave frequency is typically 400 Hz or higher.
Stage 3 – light emission (3200 lm – 35W)
Upon each switching off and on of the lighting, the cycle described above is repeated. Valeo specialists will describe the most popular types of xenon lamps in the automotive sector and their adopted numbering in a subsequent article.
