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On Tue, 11 Dec 2012 21:47:27 -0800, John Larkin
wrote:

On Tue, 11 Dec 2012 12:04:13 -0500, z wrote:

On Tue, 11 Dec 2012 08:35:17 -0800, John Larkin
wrote:

On Tue, 11 Dec 2012 10:17:58 -0500, z wrote:

On Tue, 11 Dec 2012 12:47:53 -0000, "john stone"
wrote:

Have a 10Watt, 12Volt Hologen lamp for a christmas decoration. But the
mains tranformer I have is only 6 Volts.

Apart from being a bit dimmer, would there be any other consequences running
at the lower voltage? Thanks.

At 6V the halogen cycle won't operate so you'll probably get
discoloration of the bulb after some time and perhaps the bulb won't
last as long (to the degree that it's a trade-off of lower power vs.
the elimination of the halogen cycle). Other than that, it'll
probably be a very yellow light at pretty low intensity.

We run a halogen (over our dinner table) from a dimmer. We seldom use it at full
brightness. It's about 10 years old and still on the original bulb.

The envelope is probably black by now.

No. Why should it be, if we seldom run it full brightness?

Because you DON'T run it at full brightness the envelope isn't cleared
by the halogen cycle. This is a common failure of halogen bulbs. They
get black, reducing their light output and fail early.

Filamant evaporation
goes as some insane power of voltage. And when we do occasionally run it at full
voltage, won't the halogen cycle do its thing?

But you just said you rarely run it at full power. It can't if you
don't.

Incandescent life goes with
something like the 12th power of, er, power, so it's not surprising.

Exactly. Running it at half voltage, or even 3/4 voltage, is almost equivalent
to turning it completely off.

s/power/voltage, as pointed out earlier.

Various references say that incensescent lifetime goes as voltage to the -13 to
-16 power.

12/16, who's counting.

You'll probably find that it won't last all that long, now, at full
power. It's filament is now on the inside of the envelope.

Can't see why. The evaporation rate is 0.5^13 as much at half voltage as at
full. That's essentially no evaporation at all.

The halogen cycle stops.

Anyhow, it's over 10 years old and works fine. I may never get to use that spare
bulb.

There are carbon filament bulbs still operating from a hundred years
ago. shrug