I have a lamp with the following known specs:

  • 32W
  • 6.6A
  • AC

Applying p = V x I, I expect the required input voltage to be 4.8V AC.

However, this seems to be an unusual input voltage for a lamp. For context: this is a MR-16 style quartz (possibly halogen) bulb, listed as part number 21127 here.

The only specs given for this lamp are watts and amps, above, so I cannot confirm voltage via their documentation. The typical application of this bulb is part of a significantly more complex circuit involving a constant current regulator.

I have tried looking for a 120V AC --> 4.8V AC 6.6A transformer without success.

My questions:

  • Is finding a transformer outputting 4.8V AC at 6.6A actually the simplest way to power this bulb? What alternatives exist?
  • Could the transformer output DC instead of AC without harming the bulb?
  • What are the consequences to the bulb of using a transformer which outputs a more standard 6V or 12V?
  • What are the consequences to the bulb of using a transformer which outputs less than 4.8V?

My guess is exceeding 4.8 volts would cause the bulb to burn out faster, whereas less than 4.8 volts would cause the bulb to emit less light (but would this be harmful to the transformer?). I also expect transformers outputting either DC or AC could be used.

Thank you for your guidance.

  • \$\begingroup\$ does the bulb have a negative-resistance region, that requires a current-limiter supply? bulbs with ARCS will need current limiting. \$\endgroup\$ Nov 17, 2019 at 2:50
  • \$\begingroup\$ analogsystemsrf, are you referring to these types of bulbs? en.wikipedia.org/wiki/Arc_lamp \$\endgroup\$
    – max
    Nov 17, 2019 at 17:03

2 Answers 2


The 2009 Cooper Crouse-Hinds catalog for airport runway lights indicates some of the lamps require a constant current power supply, i.e. a special transformer designed to prevent the inrush current spike that shortens lamp life. Look to the fixture manufacturer for a replacement.

On the other hand, if you're just experimenting with a lamp, likely surplused because of replacement by efficient, long-life LED's, try gradually increasing voltage on the lamp until 6.6 amps are drawn, and check the voltage at that point. You could then use an ordinary transformer of that voltage rating, but with shortened lamp life.

You also might try an old desktop PC power supply that has 5 V DC output, putting a one ohm variable resistor (rheostat) in series, gradually reducing the resistance. To make your own rheostat, use nichrome wire from a heating element, with a bolt and nut slider. About 30 cm of 0.65 mm (22 AWG), 50 cm of 0.80 mm (20 AWG) or 1 m of 1.0 or 1.2 mm (16 or 18 AWG) should be ample. Use thicker wire if possible to spread out the heat... don't burn the fingers.

BTW, don't touch the quartz lamp, or wipe with alcohol if touched, to avoid damage from sodium in sweat.


Tungsten Bulbs have a well known positive temperature coefficient or PTC where the filament resistance rises x10 from cold to hot. Halogen gas bulbs have higher MTBF at elevated temperatures with a metal vapor recycling mechanism than std. Tungsten filaments which just use less expensive inert Argon gas.

Thus when random startup occurs at peak voltage, the current can be 10x the rated hot current, which causes radial filament forces to possibly break a weakened filament at 1500 hrs MTBF such as conventional tungsten lamps with unregulated sources rather than a much higher MTBF rating with a constant current source for higher reliability.

You could use DC as long as it was constant current regulated. Operating at a higher voltage or constant voltage reduces MTBF. But I don't know about airport runway certification if that is allowed as it is not exactly the same performance with mismatched bulbs. AC method uses voltage transformers per bulb with current limiting/compensation.

Operating at a lower voltage raises MTBF as long as startup current is not exceeded but also reduces brightness.

This is why household bulbs tend to go poof when you turn then on after their normal lifetime. Also if they are enclosed without air flow and run hotter than normal, this will reduce MTBF significantly.

  • \$\begingroup\$ Thanks Tony, I appreciate the background on tungsten bulbs vs halogen. Regarding runway certification, this is just a personal project and won't be used in a real-world situation. \$\endgroup\$
    – max
    Nov 17, 2019 at 17:10
  • \$\begingroup\$ for giggles run any two equal bulbs in series thus half rated voltage if shared equally and try to understand why that never happens \$\endgroup\$ Nov 17, 2019 at 17:22
  • \$\begingroup\$ it's called thermal runaway due to PTC and mismatch \$\endgroup\$ Nov 17, 2019 at 17:23
  • \$\begingroup\$ You didn't ask for a simple solution ,so I let you think about the problems 1st \$\endgroup\$ Nov 17, 2019 at 17:27

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