I have a few questions regarding HID lamps, their ballasts and their replacement options and strategies. I have been spending a large amount of time researching this subject and I still have unanswered questions.

I am looking for a deeper understanding of the underlying technological designs and principles of operation to have a more informed guess about whether I would be expecting compatibility issues with a confidence of 80% or 10%. (If someone has references to good documentation and detailed principles and compatibility principles, feel free to share)

I am also trying to tap into the experience of others with such technologies to get a broader understanding.

  1. First off, can a bulb of a lower wattage can be used with a ballast of higher wattage? For example, install a 250W HPS bulb into a 400W HPS light fixture with integrated ballast.

    • I am pretty sure the reverse option (400W HPS bulb into 250W HPS fixture) is not adequate, notably based on temperature levels reached in the fixture and the increased current drawn from the ballast/ignitor.

    • I have gathered than 250W into 400W HPS fixture could also be an issue. However, I don't have a good understanding of where the issue is coming from.

    • I have also seen ignitors with wattage ranges, for example "for use with 200-400W HPS lamps". Are those specially designed to handle such a range, what difference does it makes?

  2. Can a different HID type bulb (such as Metal Halid or Mercury) be used with a HPS ignitor/ballast?

    • Can standard 200W MH bulbs be fitted in a fixture designed for 200-400W HPS lamps?
    • Are all ignitors/ballast basically designed for/compatible with handle any type of HID bulbs? Even old ones?

      • I guess modern/fancy electronics ignitors/balast designs are quite universal.
      • But for old ones, if not compatible, what difference is there between ignitors for various HID types bulbs?
    • Are special MH bulbs required/exist which would "appear" or work like a HPS bulb for the ignitor/ballast, although using MH gas and technology?

  3. Now, there are a number of more "greener"/less energy hungry alternatives, such as Induction Lamps and LED bulbs. When switching to those technologies, a lower wattage bulb can be used to get a similar light output (example: 220W HPS bulb replaced a 100W LED or induction bulb). Given the answers to the previous question about the ballast wattage and ignitor compatibility, can a 100W LED retrofit bulb (such as GE LED80) be used with a ignitor/ballast designed for 200W-400W HPS?

    • I know that the final answer to this has to rely on having all manufacturers specifications (ignitor/ballast/bulb) to agree with each other regarding compatibility.
      • But, when working with retrofitting 20-30 years old stuff, it can get quite complicated to get the exact specifications
      • And getting a manufacturer to commit to such a statement in the first place is quite a challenge in itself, especially for small installations with not much of a "sale" incentive.
      • And on what specifications should they agree to in the first place?
    • I am looking for general rule of thumbs and expected issues if there would have any, to be able to reach some kind of conclusion like:
      • It may work, but that will be a lucky shot!
      • It should work, don't worry too much!
      • It may appear to work at first, although bulbs may get progressively damaged and expected life time of 50000h may get much lower or light output may be much less than expected.
      • It is a fire hazard in the works, never do that!
  • How come that question was indeed declared off topic in electronics.stackexchange.com which covers electrical engineering. I am myself an electrical P.Eng and it goes much beyond 24V and electronics. Lighting & power are a big and integral part of Electrical Engineering! Aug 20, 2017 at 22:11
  • especially since diy is for home improvement, and the size you're talking isn't really used in homes. 50-150 watt sizes for barn lights, but those are on the outs. Ah, who am I kidding? The main use is pot growing :) Aug 20, 2017 at 22:53

1 Answer 1


1. With HPS, no for smaller bulbs on bigger ballasts.

Discharge lights are current devices. They are non-linear (behave almost like a dead short) and so, must be externally current-limited. That's the whole point of a ballast or driver.

HPS bulbs work at a specific current in one of four working voltage ranges (page 9): 52V, 55V, 100V or 250V plus or minus 15%. To swap, the bulbs would need to be in the same voltage range. Too small a bulb, the bigger ballast would force the higher current through the bulb, burning it out. The reverse, the bulb might work, but might not warm up properly.

By comparison, if you stick a VHO 8' fluorescent tube in an HO fixture, you get HO light output from the VHO bulb.

2. Nope! Different and incompatible, especially ignitors

You absolutely cannot interchange various types of discharge lighting - not least because the way the ignitors work is very different.

Here, HPS is an odd duck. It has an ignitor which fires a 2000-3000 volt spike to ignite the bulb. The supply is constant-current, so it will increase voltage under no load. This triggers a VBO, which flows a burst of current into a winding on the ballast transformer, inducing a spike to the bulb.

They make special MH bulbs made to run on HPS fixtures; these are resistant to the spike and are tuned for HPS voltage and current. They cost 50-100% more than a stock HPS or MH bulb.

Other than that, facility managers don't intentionally use the wrong bulb. Nobody will support that, so if you want to try it, you're on your own. It will either not work, give poor life, burn out the ballast, or blow bulbs - and these suckers run very hot and you don't want molten sodium raining on your head. Some are even reputed for starting fires.

3. LED replacements must be designed for that.

You linked a compromise screw-in LED meant to go into an HPS or other HID light without removing the ballast. If they are designed, (UL) listed, and labeled for that use, it's safe and legal. Otherwise, it is not. The operative word is "compromise".

HPS is more lumen-efficient than any but the most efficient LEDs on the market - if you like yellow light. And if you're trying to light the inside of a sphere, which you might if you're growing pot. Reflectors suck. If you really want a cone of light (and most people do), LEDs win big, because they inherently emit a cone of light. They are far more lumen-efficient inside that cone, and weak reflectors don't help enough.

Of course, if arranged like a "corn cob", the advantage is utterly lost. SMH... Those are for people in a hurry looking for a screw-in replacement that uses their same reflectors and lensing. But wait. Why keep the old fixtures? Do you really want to keep the old ballast? No, it wastes energy and is a maintenance item. You want to direct-wire existing line voltage to the LED device. Do you honestly foresee rolling back to actual HID lighting? Never gonna happen. Throw the fixture in the attic, get a modern LED fixture for 1/3 the cost - and don't look back.

Standard practice is replacing a 400W HID with a 6-tube F32T8 fixture. 4' LED replacement tubes are a commodity item, and prices are in free-fall -- $8 last I checked for a top brand. $48 and you have a HID light's worth of LED light. Hit up Craigslist for a used fluorescent fixture, yank the ballast and rewire direct - you're in business. Given how cheap LEDs have gotten I wonder why anyone would fool around with HID lighting anymore.

Real fluorescents are even cheaper if you get eBay ballasts, and the superb tubes now available.

  • 2 is a truth with modification. Once ignited, the Vf of different types of bulbs are so close you are limiting to approximately the right current anyway. For 110 V service with 90 V vs 80 V Vf the difference is of course bigger than for 230 V service, but I wouldn't think too much of it.
    – winny
    Aug 20, 2017 at 9:41
  • Thank you for your notes. My fixtures are retro "pole top" lampadaires / street lamps to illuminate the grounds. Changing the fixture is definitely not my first option. However GE (and other manufacturers I guess) have what they call Type B replacement lamps where they are mentioning in specs that Ballast bypass is required. Since my fixtures are 120V powered, it would be a rather simple matter of throwing out the ballast and ignitors, rewiring the socket and installing line-powered LED lamps.
    – Philibert Perusse
    Aug 20, 2017 at 13:39

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