How HID headlights work and they fail

Discover the Science Behind HID Headlights

High-Intensity Discharge (HID) headlights have become a popular choice in automotive lighting due to their superior brightness and efficiency compared to traditional halogen headlights. HID headlights offer enhanced visibility, longer lifespan, and a unique, crisp light output that many drivers prefer. But how exactly do these headlights work? This article delves into the technology behind HID headlights, explaining their components and how HID headlights work.

The Basics of HID Headlights

HID headlights, also known as Xenon headlights, differ significantly from the more common halogen headlights. While halogen headlights use a tungsten filament heated by an electric current to produce light, HID headlights generate light through an entirely different process—gas discharge.

HID headlights are based on the principle of an electric arc, similar to the technology used in arc welding. Instead of relying on a heated filament, HID lights create an arc of electricity between two electrodes housed within a bulb filled with Xenon gas and other noble gases. When electricity passes through the gas, it excites the gas molecules, producing an intense and bright light.

Key Components of HID Headlights

To understand how HID headlights work, it’s essential to know the key components involved in their operation:

Xenon Bulb: The HID bulb is filled with Xenon gas and other noble gases, along with metal salts. HID bulb contains two electrodes positioned closely together. When a high voltage is applied, an electric arc forms between these electrodes, exciting the gas and producing light.

Ballast: The ballast regulates the electrical current and provides the initial high voltage required to ignite the Xenon gas. Upon startup, the ballast generates a voltage surge to create the arc. Once the arc is established, the ballast then reduces and stabilizes the voltage to maintain consistent light output.

Igniter: The igniter works in conjunction with the ballast. It helps initiate the arc by sending the high voltage from the ballast to the electrodes within the bulb, ensuring the Xenon gas ignites properly.

How HID headlights work. The ballast boosts 12-volts to 70-100-volts. The igniter boosts the 70-100 volts to 15,000 – 25,000 volts. That strikes an arc in the Xenon gas in the bulb.

1) When you turn on the headlights, the ballast receives 12-volt power from the headlight switch or body control module.
2) The ballast boosts the 12 volts to 70-110 volts and sends it to the igniter. The igniter re-boosts the voltage to 15,000  to 25,000 volts.
3) The high voltage jumps across the gap in the bulb, establishing an arc. Once the arc is established, the igniter’s job is done. The ballast then maintains a constant 70-100 volts at 35 watts to the bulb to maintain the arc.

The Three Stages of HID Operation

The operation of HID headlights involves several stages, from ignition to steady operation:

• Ignition Stage— When the driver turns on the headlights, the ballast sends a high voltage surge to the igniter, which then passes it to the electrodes within the Xenon bulb. This high voltage is necessary to break down the resistance of the gas and create the initial arc of electricity between the electrodes.

• Warm-Up Stage— Once the arc is established, the gas inside the bulb becomes ionized, producing an intense light. However, HID bulbs do not reach full brightness immediately. They undergo a warm-up period, usually lasting a few seconds, during which the light gradually intensifies as the gas becomes fully ionized and the arc stabilizes.

• Steady Operation— After the warm-up period, the ballast reduces the voltage and maintains a consistent current to keep the arc stable. At this point, the HID headlights produce a steady, bright light with a color temperature typically ranging between 4,000K to 6,000K, depending on the specific design. This light output closely resembles natural daylight, providing excellent visibility and reducing eye strain for drivers.

Two types of HID headlights

Standard HID headlights that use an HID bulb for the low beams and a standard halogen bulb for the high beams

Bi-Xenon headlights use a single HID bulb and a moving shield to block off a portion of the light when the driver chooses the low beam setting.

Why carmakers install HID headlights

HID headlights are more energy-efficient

HID headlights produce more light than traditional halogen headlights while using less power. A standard halogen headlight bulb uses 55-watts, compared to only 35-watts for an HID bulb.

HID headlights have a much longer lifespan — but they’re very expensive to fix when they fail

A typical halogen headlight bulb has an average life of around 800-hours. The super-bright versions have a lifespan of only 200-hours. But an HID bulb has a useful life of around 5,000-hours.

HID headlights project a whiter color

Drivers prefer whiter light because it provides more contrast at night. Halogen headlight bulbs generate light in the 3,000°K to 3,500°K range, similar to a tungsten lamp bulb. HID bulbs are available in many colors, starting at 4,000°K and going up to 6,500°K. For reference, daylight is around 5,500°K.

Can you retrofit HID headlight bulbs into your existing halogen headlight assembly?

Lots of companies make HID retrofit kits. None of them are street-legal for retrofitting into the halogen headlight assembly. Legality aside, they don’t work well at all when retrofitted into a halogen headlight because the optics are all wrong. In fact, it’s optically impossible for an HID retrofit kit to produce the proper beam pattern when they’re installed in headlight designed for halogen bulbx.

Here’s why HID bulbs don’t work properly in a halogen headlight assembly

A halogen bulb uses a glowing cylindrical filament with the brightest portion located in the center of the filament. HID bulbs, on the other hand, produce two sources of light, one near each electrode. So, the brightest portions of HID bulbs aren’t in the correct spot for the headlight’s reflector. Because the light is in the wrong place, the reflector and lens can’t produce the proper beam pattern.

HID retrofit kits throw less light on the road and more glare into oncoming traffic.

How HID headlight components fail

Igniter

The igniter’s only job is to boost the 70-110-input voltage from the ballast to 15,000  to 25,000 volts to start the arc. Once the arc is lit, the igniter’s job is done. If your HID bulbs light up but flicker or turn off completely, the igniter does not cause it. If they don’t light up at all and the fuse and circuit are good, the problem may be the ballast or the igniter. If your system uses a separate igniter, swap in the known good igniter. If the bulb lights with that igniter, the problem is the igniter.

Ballast

The ballast’s job is to maintain a steady 70-100-volt supply to the bulb. It’s basically an electrical regulator, and in many ways, it’s no different than the ballast used in a fluorescent light fixture. As a ballast fails and can no longer maintain a steady 70-110 volts, the result can be either bulb flicker or cause the HID bulb arc to fail and just turn off with no warning. If you turn off the headlight and then turn the back on, the igniter will reestablish the arc, but since the ballast can’t maintain the arc, the bulb will turn off again. However, the problem can also be with the bulb.

There’s no way to test the ballast due to the high voltage. You must test it by eliminating the other possible causes. Start by swapping in a known good bulb. If the good bulb lights and then goes out, chances are the ballast is bad.

HID bulb

The bulb has two electrodes and an electrically conductive Xenon gas. As the electrodes wear, the bulb can flicker, or the light color can change to a pink color. Flickering is the most common symptom of a failing HID bulb.

That’s because the worn electrodes can’t maintain the arc. Swap a working bulb into that position. If the working bulb stays lit, the problem is the bulb and not the ballast. If a known good bulb continues to go out, then suspect a bad ballast.

HID parts cost

The igniter is basically a step up transformer that produces very high voltage. Over time, the windings fail and the igniter stops working. A new igniter can cost $50-$75.

An HID ballast provides the first boost in power and maintains constant power once the bulb is lit. When an HID ballast fails, a new one can $300 to $500 depending on the brand.

HID bulbs are much more expensive than traditional halogen bulbs. OE bulbs can easily cost almost $200.

Need to learn how to diagnose an HID system? See this post

©, 2023 Rick Muscoplat