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Posted: Thu Sep 18, 2008 8:49 am
by ecerfoglio
MrAl wrote:Hi there,

There might be something to be said about resistance vs Triac phase
control vs Diode. ......

whereas the pure resistance method (two bulbs in series) would not. This could make a big difference.

These three techniques are all different and need to be evaluated
Puting two (or more) bulbs in series does not give the same lifespan as using a series resistance

The diference is that a hot bulb filament has a bigger resistance that a cold one.

If you put bulbs in series, when you apply the voltage there is allways one of the bulbs that heats up first, increases its resistance and recieves more than its share of voltage and power. As the other bulbs heat up they balance themselves, but as the "quick" bulb gets the (nearly) full voltage each time it will eventually fail.

This was a problem with the old "AC/DC radios" (circa 1920) that had the valve heaters in series, and also with the Cristmas tree light strings (the "classical" ones wired in series).

Posted: Thu Sep 18, 2008 11:33 am
by MrAl

Oh ok, good point. The bulbs would have to be somewhat matched
i guess, although people do report this to work without doing that.
Maybe the common bulbs are a little more matched up than filaments.
Filaments also take longer to heat up than bulbs.

Posted: Thu Sep 18, 2008 11:40 am
by ecerfoglio
It is allways better to use, for example, two 220 V bulbs in parallel on a 110 V line (at half their nominal voltage) than to use two 110V bulba in series on the same 110 V line.

Plus when one of the bulbs dies the other one stays lit.

Posted: Thu Sep 18, 2008 6:05 pm
by dyarker
:eek: "... , two 220 V bulbs in parallel ..." Why not one 220V bulb of twice the Watts? :???:

In the Exit sign in church and Mom's night light examples, the lamps are mostly continuously on. Slight mismatch between bulbs less of an issue. Also, finding 220V versions of bulbs with small screw in bases in the U.S. not easy.