I recently installed some track lighting and put GE reveal 120V lamps in it. One out of six didn't light up, so I went and got the trusty Fluke, and lo and behold, instead of a near dead short, it showed continuity but it was ~12MegΩ. That's weird, I thought, when lamps are missing a filament, they show ∞Ω.

Not satisfied, I went and tested there other new lamps, and lo and behold, they all showed ~3 to 8megΩ right out of the box, and when installed in the fixture, lit up.

So they all show a very high resistance, but the one with the highest resistance is the only one that didn't light up.

What is going on inside these things? GE says they are basically an incandescent, but with a special filter in the glass. If they have a filament, why don't they show a cold resistance of a ~100Ω?

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Mike ML.

 

Sounds like crappy manufacturing. There is probably a poor connection in the bulbs that the 120Vac will breach, but the low voltage of the ohm meter will not. What does a hot one, freshly removed from the socket, measure?

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Inside every little problem, is a big problem trying to get out.

 

Your multimeter is faulty. Get a good-old analogue meter for $10.00
My digital meter reads 13.6v from a 9v battery.

 

Sounds like a bad meter to me, or something wrong with the way you're measuring it somehow.

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Nope. The original test was done with a Fluke model 87. I redid the test with a Fluke model 75: Identical results!

More info: I took a virgin lamp (that measured ~10megΩ) and connected it across an HP power supply. I slow ramped up the voltage, and at about 40V, it began drawing lots of current, and glowed dimly. I disconnected it from the supply and measured its resistance, and now it was ~25Ω.

The plot thickens: Letting the lamp sit for about 5min, remeasuring the resistance, it is back to 12megΩ. Went and repeated the test with three more new lamps and they all do it...

I also tried "firing" the lamp with 120VAC, and they drop resistance when they light, and then jump back up to high resistance after a few minutes. Probably depends on how hot they are.

Maybe I have found a new "component". We should be able to design a circuit around this behavior

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Mike ML.

 

It's probably a weird property of the filament alloy, looks like it has a semi conductor like effect at low temperatures. Out of curiosity does it exhibit the same resistance in both directions, and is the resistance reliably reproduced?

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It's most likely the crimp area of dissimilar metals. A very fine oxide layer there, or even some anodised type layer with a high resistance.

Here's the million dollar question... Can you use one as a high power zener? Or a DIAC?
Maybe you could make a cool oscillator out of one.

 

As he gets a low resistance value obviously when the bulb is lit up it won't work under power. It may work as a diac at very low currents though, as long as the heat is low enough to not mess up whatever exactly is causing the high impedance state. It would be unreliable, thermally unstable and very sensitive to vibration. But still highly amusing if you could use it as a circuit element in a demonstration project.

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GE incandescent light bulbs also did not light for me.
The wire that was supposed to be welded to the metal base had the weld a few mm away and the joint arced on some bulbs which caused the light to work.

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Uncle $crooge

 

I've only ever used one of those, personally I don't like the light quality.

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I don't like the light quality either.
I have been using 100w 130v bulbs lately.
They are not frosted, and only give the illumination of a 75w bulb, but the estimated life is 14000 hours.
That's about ten times the life of a 120v lamp
A lot more cost effective is you do not need the full light output of a normal 100w 120v lamp.

 

I've taken a shine to CFL's, the phosphor blends they use now make them as good or better than incandescents for general lighting, and the rectification and increased frequency of a modern electronic ballast eliminates flicker, though I don't like the fact that they have mercury in them and they tend to be bulkier especially for three way 150 watt bulbs. I have a 3 way 150 watt that works just fine, but it sticks up out of the light fixture by about 4 inches =P It's gonna be a while before LED technology can compete with CFL's in the commercial market, right now the cost is just too high.

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