How HID headlights work and they fail

Learn how HID headlights work and how they fail

High Intensity Discharge (HID) headlights are commonly used in high end car models because they appear brighter and are more energy efficient than standard halogen headlight bulbs. HID headlights are also referred to as Xenon headlights because they bulb is filled with xenon gas.

How HID headlights work

An HID bulb is completely different than a traditional light bulb. Instead of using a glowing filament, an HID bulb creates light by generating a high voltage arc between two electrodes. In some ways, it’s much more like a fluorescent tube than a traditional light bulb.

There are three components to an HID headlight:

HID bulb
Igniter
Ballast

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 stikes an arc in the Xenon gas in the bulb

When you turn on the headlights, the ballast receives 12-volt power from headlight switch or body control module.

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.

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.

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. They’re all illegal. 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 bulb 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 an 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 igniters job is done. If your HID bulbs light up, but flicker or turn off completely, it’s not caused by the igniter. It 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 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 an pink color. Flickering is the most common symptom of a failing HID bulb.

That’s because the worn electrodes that can’t maintain the arc. Swap a working bulb into that position. If the working bulb stays lit, that proves 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