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.
john stone forklarede:
A bit dimmer?
If the resistance of the lamp was independent of the temperature, which it is not, the lamp would draw 1/2 the current at 1/2 the voltage, giving 1/4 the wattage, 2.5w.
However, the resistance _is_ dependent of the temperature. Se more at
But the bottom line: Nothing will fail catastropically, but you may get much less light than expected. But it may look nice and christmassy :-)
Leif (with a lot of but's ;-))
-- Husk kørelys bagpå, hvis din bilfabrikant har taget den idiotiske beslutning at undlade det.
A halogen lamp on the correct voltage runs the filament hotter than a standard incandescent tungsten lamp. Without the halogen gas filling, this would soon cause the tungsten to migrate from the filament to the inside of the bulb, where it would blacken the glass. The filament would rapidly become thin and then would break.
At a certain temperature, the halogen will combine with the tungsten on the inside of the glass. It then floats around until it meets the hot filament, where it deposits the tungsten back where it came from (approximately). This depends on having the glass hot enough to allow the halogen and tungsten to combine, which occurs at near dull-red heat (which is why quartz glass is necessary).
If you under-run the lamp, the filament will not blacken the glass as quickly, but the glass will not reach recycling temperature, so any blackening will remain. The relative temperatures of the filament and glass will determine whether the life if the lamp is extended or shortened. The light output will be very muchreduced and lacking the blue end of the spectrum.
-- ~ Adrian Tuddenham ~ (Remove the ".invalid"s and add ".co.uk" to reply)
To put it a bit more simply...
A /slight/ reduction in voltage will cause a halogen lamp to burn out prematurely, because the bulb doesn't get hot enough to initiate the "recycling" process -- but the filament is still extremely hot.
Half voltage is a huge drop. You should have no problems.
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.
-- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com
full
The envelope is probably black by now. Incandescent life goes with something like the 12th power of, er, power, so it's not surprising. 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.
Actually, it's the 12th power of voltage.
Even a slight drop in voltage causes a big increase in life. For example, a
5% reduction almost doubles the lamp's life (1.85 times). At a 10% reduction, the lamp lasts 3.5 times as long.
Of course.
"john stone" wrote in news:ka7a21$hdi$ snipped-for-privacy@dont-email.me:
Halogen has a build in cycle to keep the filament healthy. That does not work at a lower temperature. So running it at the wrong voltage will cause it to fail early. Other then that, for a decoration a dim bulb is no problem,
full
No. Why should it be, if we seldom run it full brightness? 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?
Incandescent life goes with
Exactly. Running it at half voltage, or even 3/4 voltage, is almost equivalent to turning it completely off.
Various references say that incensescent lifetime goes as voltage to the -13 to
-16 power.
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.
Anyhow, it's over 10 years old and works fine. I may never get to use that spare bulb.
-- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com
"John Larkin"
** The formula that predicts lamp life is only valid for a small range around the nominal voltage, about 10%. The simplifying assumptions it is based on are true only in this range.Reducing the voltage by large percentages extends lamp life by factor of 10 or possibly up to 100 - the halogen cycle has SFA to do with it.
... Phil
I guess the question becomes: is there some voltage a bit below normal, where the halogen cycle doesn't work and lamp life actually degrades? I'm guessing no.
-- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com
Why? If there is almost no filament evaporation (which is the case at, say, half or 3/4 voltage) why would it fail sooner? The halogen thing might not kick in at low voltage, but then it's not needed.
-- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com
I'm guessing yes. A friend had halogen lamps on a dimmer, which he dimmed only slightly -- and they burned out pretty quickly.
The halogen cycle requires a very high temperature. Ergo...
running
full
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.
full
But you just said you rarely run it at full power. It can't if you don't.
s/power/voltage, as pointed out earlier.
12/16, who's counting.
The halogen cycle stops.
spare
There are carbon filament bulbs still operating from a hundred years ago.
It's all a curve fitting exercise, and V**12 looks a lot like V**10.(*) It's often quoted as going like exp(10500/T(K)), where T is the filament temperature, which goes roughly like sqrt(V). That's a slightly more physicsy fitting function, but none of them work very well over any wide voltage range. For instance, the lifetime increase at 10% below rated voltage is variously quoted anywhere from 3x to 5x.
Cheers
Phil Hobbs
(*) Which is why the Hilbert matrix is so ill-conditioned.
-- Dr Philip C D Hobbs Principal Consultant
no.
The bulb envelope cleans up if you run it at full power for awhile anyway.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant
Nope. The halogen cycle is to keep the envelope clean. The high gas pressure extends the filament life, by causing tungsten to be selectively redeposited back on the same hotspots it came from. Otherwise the tungsten redeposits randomly, and there's nothing to keep the hotspots from necking down and failing just as early as in a low-pressure bulb.
(We keep coming back to this every few months lately.)
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant
If there's almost no filament evaporation, who cares if the halogen cycle isn't working? It's not needed. And it will work if full line voltage is applied occasionally.
Our kitchen-table halogen is dimmed and is over 10 years old, on the original bulb.
-- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com
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