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!!

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!!

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....

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 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...

I was going to post something on this, but refrained. However...

Many years ago, when transistor amplifiers were still new and exotic, Allied
introduced the KG-870, an integrated amp using germanium alloy transistors (you
know, the ones that barely got past 5kHz).

At that time, a lot of attention was paid to protecting the output devices.
(Germanium transistors were prone to thermal runaway.) Allied had an interesting
solution -- the emitter resistors were actually 12V automotive lamps! If "too
much" current passed through the transistor, the bulb's resistance would
increase, restraining the flow.

The bulb was also supposed to be a fuse. The writer of the Electronics World
article explained that the life of a tungsten lamp varied as the 12th power of
the applied voltage. Get the voltage high enough, and the lifetime becomes a
fraction of a second.

He didn't say where he got the 12th-power rule. Anybody know?

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...

I was going to post something on this, but refrained. However...

Many years ago, when transistor amplifiers were still new and exotic, Allied
introduced the KG-870, an integrated amp using germanium alloy transistors (you
know, the ones that barely got past 5kHz).

At that time, a lot of attention was paid to protecting the output devices.
(Germanium transistors were prone to thermal runaway.) Allied had an interesting
solution -- the emitter resistors were actually 12V automotive lamps! If "too
much" current passed through the transistor, the bulb's resistance would
increase, restraining the flow.

The bulb was also supposed to be a fuse. The writer of the Electronics World
article explained that the life of a tungsten lamp varied as the 12th power of
the applied voltage. Get the voltage high enough, and the lifetime becomes a
fraction of a second.

He didn't say where he got the 12th-power rule. Anybody know?

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.

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.

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