Sven Franklyn Weil wrote:

In article , Lizard Blizzard wrote:



eventually burn out, too. So it would be wise to do the right thing and
replace them with LEDs, and they will last tens of thousands of hours.



I replaced a grain-of-wheat lightbulb for a clock-radio dial with one from
Radio Shack. The supply voltage is 5 volts so I bought a 12 volt bulb.

Imagine my surprise when these damned things are only rated for 15
hours!!!!



Well, the life of a bulb is roughly (rated voltage/operating voltage)^6
so (12/5)^6= 191 * 15 hours = 2865 hours. I'd call these 5V bulbs
myself....

But the light output goes down with at least as strong a function, I
believe. So although you can greatly extend the life of a bulb by
reducing the voltage, you also greatly reduce the light output. And
without nearly as much of a reduction of the input power. So you end up
with a dim, very inefficient bulb. That might be ok for some
applications (say, if it's at the top of a tower where it's a really
major pain to change) but not others (like all the bulbs in your house,
unless you love to pay that electric bill).

Roy Lewallen, W7EL

Robert Casey wrote:

Well, the life of a bulb is roughly (rated voltage/operating voltage)^6
so (12/5)^6= 191 * 15 hours = 2865 hours. I'd call these 5V bulbs
myself....

In article , Robert Casey wrote:
Well, the life of a bulb is roughly (rated voltage/operating voltage)^6
so (12/5)^6= 191 * 15 hours = 2865 hours. I'd call these 5V bulbs
myself....

Rob, I'm probably being dense but I don't follow the math. Then again
I was never really good at math.

what is the ^6? Elevated to the 6th power? So is this bulb going to
last a while?

P.S.: If it blows in my lifetime I'm not replacing it again. The only
reason I did so the first time was because the original had blown its
glass envelope (I _don't_ know why). Yikes!!

--
Sven Weil
New York City, U.S.A.

Sven Franklyn Weil wrote:

In article , Robert Casey wrote:


Well, the life of a bulb is roughly (rated voltage/operating voltage)^6
so (12/5)^6= 191 * 15 hours = 2865 hours. I'd call these 5V bulbs
myself....



Rob, I'm probably being dense but I don't follow the math. Then again
I was never really good at math.

what is the ^6? Elevated to the 6th power? So is this bulb going to
last a while?

That's right.


P.S.: If it blows in my lifetime I'm not replacing it again. The only
reason I did so the first time was because the original had blown its
glass envelope (I _don't_ know why). Yikes!!

In article , Robert Casey wrote:
That's right.

Awesome! I like the sound of that!! :-)

Thanks.

I assumed that if since it was rated for 15 hours that it would last for
30 hours if run at approx half the rated volts.

--
Sven Weil
New York City, U.S.A.

my recollection of the equation for life as a function of voltage is that
it's the 13th power, not 6th. - unfortunately, I can't find the GE lighting
handbook I got this out of to confirm.

by the way, if anyone wants some optoelectronics catalogs from the early
70s, contact me off the list
snip
Imagine my surprise when these damned things are only rated for 15
hours!!!!



Well, the life of a bulb is roughly (rated voltage/operating voltage)^6
so (12/5)^6= 191 * 15 hours = 2865 hours. I'd call these 5V bulbs
myself....

I found graphs in a 1978 GE miniature lamp data book, giving life,
candlepower, and current as a function of lamp voltage. It has a bunch
of qualifications:

"Calculations of characteristics shown in Chart I are approximate only
between 95% and 110% of rated voltage for lamp types with 5,000 hours
life or less. Certain lamp types will vary widely from calculated
values. This chart will not apply to lamps with lives in excess of 5,000
hours. This chart does not apply to halogen cycle lamps."

It extends from 60% to 140% of rated voltage. The text accompanying the
graph says that, "as approximations", the light output varies as the 3.6
power of the voltage and the life varies inversely as the 12th power of
the voltage. It also says of the graphs that "Indicated values (except
for long life lamps) are reasonably valid, between 95% and 110% rated
volts. Beyond that, indicated characteristics may not be realized
because of the increasing influence of factors which cannot be
incorporated into the chart." I assume that long life bulbs are excluded
because they're already running a a considerably lower than "normal"
voltage.

At 60% of rated voltage, the graphs show that the current had decreased
to about 75% of rated current, while the candlepower has dropped to
something like 17% of rated output. (This means the efficiency is around
38% of normal.) Life is around 650 times the rated life.

In the other direction, at 140% of rated voltage, you get about 120% of
rated current, about 325% normal brightness, and about 0.016 times
normal life.

"Double life" bulbs are popular. You can make any bulb into a "double
life" bulb (according to the graphs) by running it at 95% of normal
voltage. Current will drop 3 or 4 percent, and you'll get about 85% of
normal light output.

I'm sure more information is available on the web for anyone who's
interested.

Roy Lewallen, W7EL

Thanks for the confirmation of 12th power.

It extends from 60% to 140% of rated voltage. The text accompanying the
graph says that, "as approximations", the light output varies as the 3.6
power of the voltage and the life varies inversely as the 12th power of
the voltage. It also says of the graphs that "Indicated values (except
for long life lamps) are reasonably valid, between 95% and 110% rated
volts. Beyond that, indicated characteristics may not be realized
because of the increasing influence of factors which cannot be
incorporated into the chart." I assume that long life bulbs are excluded
because they're already running a a considerably lower than "normal"
voltage.

At 60% of rated voltage, the graphs show that the current had decreased
to about 75% of rated current, while the candlepower has dropped to
something like 17% of rated output. (This means the efficiency is around
38% of normal.) Life is around 650 times the rated life.

On Thu, 28 Aug 2003 12:44:54 -0700, the renowned "William Sommerwerck"
wrote:

Thanks for the confirmation of 12th power.

The 12th power approximation does originate at GE Lighting, AFAIK, but
is only valid for voltages rather close to the rated operating
voltage, and for typical high voltage incandescent lamps. Long-life
and halogen bulbs WON'T behave the same.

http://www.eaoswitch.com/about/lamps.htm

Here's a rule of thumb for low-voltage halogens:

http://www.ndlight.com.au/low_voltage_lighting.htm

They claim a 5% voltage increase will reduce life by 50%, which is
more like the 13.5th power.

The one time I checked the 12th power approximation against actual
testing of low wattage high voltage (mains) lamps it was off by more
than an order of magnitude, so take the whole thing with a grain of
salt, IMHO, unless your lamp type matches the type used for testing.
I'm sure a real lamp specialist could go on for hours about this sort
of thing.

Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

The 12th power approximation does originate at GE Lighting,
AFAIK, but is only valid for voltages rather close to the rated
operating voltage, and for typical high voltage incandescent
lamps. Long-life and halogen bulbs WON'T behave the same.

The urban legend about halogen lamps is that reducing the voltage even slightly
causes the filament to burn out prematurely. The reasoning is that the slight
drop in temperature reduces the halogen self-healing effect much more than it
reduces the evaporation of the filament. I believe this is correct.

Thanks for the references.

Now... Does anyone know anything about helium reducing the life of incandescent
lamps? grin


http://www.eaoswitch.com/about/lamps.htm

Here's a rule of thumb for low-voltage halogens:

http://www.ndlight.com.au/low_voltage_lighting.htm

They claim a 5% voltage increase will reduce life by 50%, which is
more like the 13.5th power.

The one time I checked the 12th power approximation against actual
testing of low wattage high voltage (mains) lamps it was off by more
than an order of magnitude, so take the whole thing with a grain of
salt, IMHO, unless your lamp type matches the type used for testing.
I'm sure a real lamp specialist could go on for hours about this sort
of thing.