Chris Carr wrote:
>
> Jinx wrote
> > A couple of months ago I had a big b*tch about my spotlights
> > regularly blowing. I've done some testing with series resistors.
> > Very happy with the results
> >
> > http://home.clear.net.nz/pages/joecolquitt/0bulblife.html
> >

> If it is the switch on surge that is the predominant failure mechanism, then
> instead of a resistor have you considered using a NTC (Negative Temperature
> Coefficient) Thermistor. Some (considerable) time ago these used to be
> marketed, for insertion in the bottom of an ES type lampholder before you
> screwed in the lamp bulb.

Generally it is. After long hours of use the filament
gets metal fatigue, then the "russian roulette" factor
of turning it on cold and not knowing how many volts
exists on the mains at that part of the AC cycle causes
the filament to break.

If the mains is near full value (about a 50% chance)
there is a HUGE current into the cold filament limited
only by it's cold resistance. With a 60w globe this is
about 60 ohms. 340v into 60 ohms = 1900w. Ouch!

Adding a 50 ohm resistor in series with the bulb
only loses about 5% of total running volts, which loses
5-10% of light output. No big deal. BUT it can give
many times the running life of the bulb due to greatly
reduced cold turn-on surge current. When there is a
5% volts drop series resistor the bulbs usually run
for many years before blowing.

The resistor gives about half the cold turn on current
and half volts, so reduces cold turn on power to
about 1/4.

I learned this from an old electrician when I was an
apprentice, they used this 5% resistor trick for some
ceiling bulbs that were a real pain to get to. :o)

As for the NTC thermistor, my experience with
thermistors in the repair industry tells me they
are a LOT less reliable long term than a large
resistor is. I suppose a series thermistor + resistor
might give the best of both worlds?
-Roman

PS. The chart on Jinx' web page is great, but I would
really like to know if the "life" of the bulbs was
measured continuous or under what on/off cycling.
And does 110v/240v mains make a difference too??

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