# Which is more efficient? 12V, 20W bulb or 6V, 25W

Discussion in 'Electronic Basics' started by Tibur Waltson, Jan 15, 2004.

1. ### Tibur WaltsonGuest

Today's spotlight is so bright that when I shine a light at close range, for
example looking for a black widow, I could be blinded temporarily. The
spotlight uses a 6 volt lead-sealed battery and puts out 5000,000 candle
power and uses a 25W bulb. Will the 12V-bulb compared to the 6V-bulb
last two and a half times longer using 6V as a power source?

12V-Bulb 1.67A, 7.2-Ohm, 20-Watt
6V-Bulb 4.17A, 1.44-Ohm, 25-Watt
Thanks.

2. ### Bob MastaGuest

If you use a 12V bulb with a 6V battery it will last
"forever". Bulb life is inversely proportional to
some high power of the rated voltage (like the
11th power or something), so even a slight
reduction results in tremendous life increase.
HOWEVER, you will get a lot less light output
than simply half!

Bob Masta

D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com

3. ### happyhobitGuest

A 6 volt lamp on 6-volts is more efficient than a 12-volt lamp on 6-volt
because of the temperature it's running at. 10-percent less voltage results
in 30-percent less candlepower.

A Halogen lamp would be more efficient but would probably have a shorter
life.

If you include your 'application' into the equation (Is it more light than
you NEED?) then a 12-volt 10 watt might be more 'efficient'.

4. ### happyhobitGuest

I'm sorry I meant a 6-volt 10 watt not a 12-volt 10 watt.

5. ### Tibur WaltsonGuest

This would mean that a 4.7-volt 10 watt Halogen would be more efficient.
If only I can figure out how to prevent the filament from shorting out.
Thanks all.
Tibur

6. ### Ian StirlingGuest

You need to find a 6V 10, or 5W light.

7. ### happyhobitGuest

Interesting idea. If you could pulse it, it might work. I know that an LED
pulsed with a higher current appears brighter than one running on a steady
but lower current.

I was wrong when I said that the halogen lamps had a shorter life. Newer
halogen lamps last longer than conventional vacuum-incandescent lamps.

8. ### Ian StirlingGuest

Which arn't that conventional.
Most lamps are inert gas filled.

9. ### Don KlipsteinGuest

Vacuum lamps are rather common.

Conventional incandescent lamps with a wattage near or over something
like 10 watts per centimeter of visibly-aparent filament length usually
get the traditional argon-nitrogen fill. The gas slows down filament
evaporation, and that allows designing for a higher filament temperature
that is better for radiating visible light.
But if the power per unit of visibly-apparent filament length (or maybe
length plus diameter) is less than something like 10 watts per centimeter,
then a vacuum is better than the usual argon-nitrogen mixture. A gas fill
in these lamps would conduct enough heat from the filament to reduce
efficiency of producing visible light more than redesigning for a higher
filament temperature (with argon-nitrogen) would increase the efficiency
of producing visible light.

As it turns out, heat conduction from a wire (or a rod) to a surrounding
gas is usually nearly enough proportional to the length of the wire, but
does not vary proportionately with the diameter. If you increase the
diameter of a hot wire, you also increase the thickness of the "boundary
layer" of hot gas around the wire, and that decreases the temperature
gradient within the boundary layer, often (not always) nearly as much as
the circumference of the wire is increased. So heat conduction by a
gas from a long thin filament is more than from a shorter, thicker or
wider (even if tightly coiled and wider) filament that has the same
exposed surface area. This explains why 15 watt 120V incandescent lamps
have a vacuum while 12V lamps of the same wattage have a gas fill.

- Don Klipstein ()

10. ### happyhobitGuest

Are you saying that there is no such thing as a 'conventional
vacuum-incandescent lamp"? I don't personally know, or care, if there are
more inert gas or vacuum incandescent lamps. That's not what I was saying.