Why are metal halide bulbs always ~130V?

Question

I noticed that - even though the local voltage is 240V - metal halide bulbs are always sold as 130V bulbs, and their product sheets list the maximum voltage as 130V. Do perhaps all flood lights that can accept metal halide bulbs have an in-built transformer?

Answer 1

Metal halide bulbs are discharge lamps that require a ballast or other kind of power supply to control the discharge current and a high voltage (kV) to strike the discharge.

Here for example is a bulb that is 95V.

Answer 2

Metal Halide Lamps are classified as Gas Discharge Lamp, like fluorescent lamps too. The Ignition required a high voltage, easily going to KVolt level.
As gas discharge starts, current starts to circulate and the thermionic effect heats the gas and increases its conductivity.
This results in smaller voltages needed to maintain the discharge in the bulb. For smaller lamps (lower Wattage) the effective operating voltage in the lamp is lower than 130V RMS - see next section.
The lamp shows a negative resistance behavior and will need an adequate device to operate it.
Ballast is not a Transformer, as those used for 12V incandescent/halogen lamps - It is a device used to operate a discharge lamp (including metal halide) with this dual behavior:

• High voltage delivered during startup (lamp ignition).
• Current limiting under a lower voltage (lamp operation).

Several gas discharge lamps consume more current during their warm-up time than their stated operating wattage when fully heated. This results in a temporary lower operating voltage until fully heated.

Ballast types are inductive (like those early ones for fluorescent lamps) or electronic.

Examples of voltages:

from an excellent compilation here:

ARC VOLTAGE IN NORMAL OPERATION:

• Mercury and metal halide, 50 watt: 95 Volts.
• Mercury and metal halide, 100 watt: 115 V.
• Mercury and metal halide, 175-400 watt: 135-140 V.
• 35 Watt automotive headlight Xenon Metal Halide: approx. 80-90 V.

MINIMUM VOLTAGE ACROSS THE LAMP IN EARLY WARMUP:

• Mercury and metal halide 70 watts or less: 11 V.
• Mercury and metal halide 100-125 watts: 12 V.
• Mercury and metal halide 175-400 watts: 13 V.

PEAK VOLTAGE NECESSARY FOR STARTING:

• Mercury 400 watts or less and the H34 1000 watt mercury: 250 V, and 220 V usually works.
• Metal halide 175-400 watts: 500 V, 450 usually works.
• Metal halide 1000 watts or more: 600 V?
• Metal halide under 175 watts and other pulse start models: At least 1500 V, possibly as much as 2500 V.
• High pressure sodium 35-100 watts: 2500 V!
• High pressure sodium 150-1000 watts: 4000 V!
• High pressure sodium mercury retrofit 150-360 watts: 300 V

OPEN CIRCUIT OUTPUT VOLTAGE of ballast, besides peak of starting pulses:

• Mercury 400 watts or less as well as H34 1000W mercury: 180 V, preferably 200 Volts RMS, maybe 220 V sustained at least a couple milliseconds with substantial current.
• Metal halide 35/39 to 400 watts: May need to be as high as 280 V RMS with peaks possibly 450-500 volts sustained at least a millisecond or two with substantial current. Maybe a little less for lamps 100 watts or less.
• 1000W or higher metal halide: probably about 400 V RMS AC with 525 V sustained a couple milliseconds with substantial current.
• High pressure sodium 35-100 watts, most models: 110 V RMS AC.
• High pressure sodium 150-400 watts, most models: 220 V RMS AC.
• High pressure sodium 1000 watts: 400 V AC.
• 35 Watt automotive Xenon Metal Halide: over 300 V with a substantial current for cold start, 400 maybe 450 V with a substantial current for hot restart.

Answer 3

Driver-wise, metal halide bulbs behave like thermionic LEDs: they are current driven.

Nobody particularly cares about the exact forward voltage of a LED in most applications: you just stick a ballast resistor or a current source to drive it and call it a day.

Same with metal halide bulbs: you use a ballast that regulates the current. There's also a high voltage pulse needed to ignite them.