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?


3 Answers 3


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.


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:


  • 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.


  • 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.


  • 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.
  • \$\begingroup\$ Aha, thank you! I think this: Mercury 400 watts or less and the H34 1000 watt mercury: 250 V, and 220 V usually works. answers my question. So if I understand correctly, even though the supply voltage to the external floodlight is 240V, that 240V is required to start the MH bulb which will generally run at ~130V? Great. I was concerned that inserting a 130V bulb into a 240V supplied floodlight would ruin the bulb. \$\endgroup\$ May 2, 2022 at 23:06
  • \$\begingroup\$ @RoelVandePaar, ALL Discharge Lamps need an appropriate Ballast. Don’t waive this. Connecting one Lamp Without a Ballast, could be dangerous: In an eventual occurrence (lightning storm, etc) that could reach a transient high voltage or ionize the gas, the existing lower voltage would continue to maintain the lamp energized, but without any current limiting feature. \$\endgroup\$
    – EJE
    May 3, 2022 at 0:53
  • \$\begingroup\$ So, that mentioned Metal Halide lamp at 130VAC rms and without a ballast most probably will not turn on (+ 99% chance), but if it does by any odd, the overcurrent may be enough to overheat and explode the lamp bulb. A Fuse may protect the wiring from further damage, but would not be difficult to fine tune it to protect from such uncontrolled overcurrent. \$\endgroup\$
    – EJE
    May 3, 2022 at 0:55
  • \$\begingroup\$ In 240V countries, often the ballast is connected as an auto-transformer. It is both. It acts as a ballast, and it acts as an auto-transformer to efficiently change the supply voltage. \$\endgroup\$
    – david
    May 4, 2022 at 8:34

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.

  • \$\begingroup\$ All voltage-triggered negative-resistance arcs have a current density that increases with current and thus lower negative resistance according to geometry. Faraday discovered and recorded that. Current thus affects both negative incremental resistance and heat losses in positive resistance. Any trigger must exceed inception voltage (IV) for Partial Discharge (PDIV) and sufficient number or duration to raise temperature to sustain the arc. But to my knowledge all the arc bulbs discussed so far are bipolar until you mentioned "thermionic LEDs". Did you make this up? are there real examples? \$\endgroup\$ May 18, 2022 at 19:00

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