Halogen bulbs and home line

Thread Starter


Joined Apr 22, 2007
Hi, this is not directly an electronics question, but you fellows are probably the best of the best, so I just have to ask you.

I am doing some work at the house of a friend, changing switches, power outlets, painting, and generally fixing stuff...

Couple days ago i checked the halogen bulbs he has in the ceiling in the kitchen because they were all blown, one of them had even crushed the glass covering it!

I checked the connections of the transformers (220V to 12 V) and all and it all looked good. So i just changed the bulbs.
This morning most of them had burned and don't work anymore.

I thougt it was something about power spikes or something (I had problems with my PC at home, i even had to buy a UPS), so i wondered if there is some easy way to prevent power spikes from reaching the bulbs, some kind of surge filter that could be applied to the transformers, maybe.

Thanks a lot for suggestions.



Joined Jul 17, 2007
Did you measure the output of the transformer? Was it really 12VAC?

Yes, you could get some MOV's (Metal Oxide Varistors); they will absorb "spikes" if the voltage exceeds their rating.

Rather than protecting simply the lights, you might consider installing/having installed overvoltage protection at the circuit breaker. Square D Company used to have such an item that you had to wire in to L1, L2, and N - I believe they have a simple plug-in unit available now. It takes up either one or two breaker positions, I can't remember which. You just kill the main breaker, take the front panel off, plug the thing in, reinstall the panel and turn the breaker back on - presto, you're done.

Other manufacturers probably have something similar for their line of equipment. Whatever you do, don't mix such equipment from one manufacturer's panels to another; if there is ever an electrical fire, the insurance company won't pay! It's best to hire an electrician for such mods.

Back to your transformer; I suspect what might've happened is that a few of the primary windings have shorted to themselves, or it's become damaged in some manner. With some of the primary-side windings shorted, the secondary voltage will be correspondingly higher than it's original rating because the ratio of turns has changed.

One other thing - you should never touch halogen bulbs, as even if you've just washed your hands, some finger oils get transferred to the glass and it's darn near impossible to get it off. Contamination on halogen bulbs is a very bad thing, and it will lead to short bulb life.

Also, check the sockets; with the heat that the bulbs give off, the sockets may be corroded. That will lead to heat buildup at the bulb base, possibly getting hot enough for the glass to melt, leaking air into the bulb and causing the filament to self destruct upon exposure to oxygen.


Joined Jan 28, 2005

One other thing - you should never touch halogen bulbs, as even if you've just washed your hands, some finger oils get transferred to the glass and it's darn near impossible to get it off. Contamination on halogen bulbs is a very bad thing, and it will lead to short bulb life.

Early on I learned this lesson and I have exercised extra caution ever since.

I think that while the phenomena is not as pronounced when it comes to incandescent bulbs, it is still a good practice to avoid touching ordinary household light-bulbs.

The oils that are on the skin are acid enough that in the presence of the high heat from the bulb will over time etch the glass to the point that the integrity of seal it provides is compromised. The purity of the inert gas and vacuum within will be degraded resulting in a shortening of the life of the bulb.


Thread Starter


Joined Apr 22, 2007
I tried to read the output with my multimeter (it's cheap, but it has always worked) and i get really strange things: like rolling numbers 0, 7, 5, 0, 2, 4, 5, 2, 7. Completely random and pointless.
I checked with my screwdriver (you know the one used to see witch wire is HOT, with the wee lamp, i dunno what it's called in english :)) and both wire seem to be hot, but maybe that isn't important (if you could explain why they are bot hot nonetheless I would be grateful).

I will see with some manufacturers if these devices you speak of exist for DIN slots to buy in Italy (over here people can't even change a bulb by themselves :)).

Thanks a lot for the help

I will post the outcome of my quest :)

P.S.: i learned my lesson about halogen bulbs and fingers as a little boy, when my reading lamp exploded :)


Joined Jul 17, 2007
Hmm - it sounds like you might be trying to read AC using a DC setting on the meter? Or perhaps it's not a transformer at all - it might have an SCR or TRIAC circuit inside what you believe to be a transformer?

It would be interesting to see what kind of waveform is on the lamp sockets - but you'd need an oscilloscope. There are plans for building O-scopes and installing them in your PC, but from the sounds of what kinds of readings you're getting - you may actually be seeing very narrow spikes of high voltage, which your meter is having a tough time making RMS readings from - and using a "kit-built" O-scope could be quite hazardous not only to your PC, but to yourself. :eek:

"Wee lamp" - does it have an orange glow to it, and look like a fire glowing around two plates inside a bulb? If so, that's a neon lamp; it's filled with neon gas that glows when ionized at about 56 volts.


Joined Apr 24, 2007
Regarding halogen bulb lifespan: I have also heard (and don't remember where) that a halogen bulb's lifespan will be maximized if its filament is perpendicular to gravity. i.e. For longest life, if possible the bulb (and fixture, if necessary) should be installed so that the bulb's filament is horizontal. (I'm wondering if anyone can think of a reason why that might be correct.)

- Tom Gootee


Joined Jan 10, 2006
Most modern Halogen fittings use an electronic transformer (as Sgt wookie was hinting at). as they are more efficient, run cooler, take up less space, are lighter, and cheaper. The downside being they can be a little less reliable, generate lots of RF interference, and the outputs are almost impossible to measure with anything less than a scope... the output looking something like an AM signal modulated at 50 or 60 hz peaking at around 30 or 40 volts (some wont even supply this unless a load is connected). Most digital meters turn their noses up at this.
You can tell which yours are simply by looking and lifting. an electronic transformer is nornally housed in a cheap plastic enclosure, and weighs what a block of plastic might weigh. A wound transformer is bigger, and weighs what a chunk of metal and copper would weigh.
It would be odd for all transformers to fail at once, and I wouldnt discount burnt up pins inside the lamp holder sockets, a faulty light switch... or use of bargin basement halogen spots.

As for horizontal bulbs, this is correct for halogen strip lights, as the filiments can otherwise gather at one end of the bulb creating a hotspot that melts the glass, but not so true for halogen spots.