"William Sommerwerck" wrote in message
...
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?


Lamps and PTC thermisters are used as non-linear resistors. The diode knee
is also very non-linear. Lamps are sometimes used in the feedback loop or
good clean sine wave oscillators. They stabilize around the non-linearity.
Ghost

Bob wrote:
Looking for tiny incandescent lamps for my TM-451A Kenwood.

Apparently these are 6 V or so; there are four of them in
series parallel controlled by a regulator to vary intensity.
They light up the LCD and two of them are burned out.

I have searched many of the usual sources and can't seem to
locate replacement lamps; as a last resort I could go to
Pacific Parts but they are so expensive. These are just
inexpensively made tiny bulbs with wire leads.

Any suggestions?

Bob

Perhaps one of these might fit the bill, LED replacements for typical
incandesents...

http://dkc3.digikey.com/PDF/T033/1200.pdf

Bob wrote:
Looking for tiny incandescent lamps for my TM-451A Kenwood.

Apparently these are 6 V or so; there are four of them in
series parallel controlled by a regulator to vary intensity.
They light up the LCD and two of them are burned out.

I have searched many of the usual sources and can't seem to
locate replacement lamps; as a last resort I could go to
Pacific Parts but they are so expensive. These are just
inexpensively made tiny bulbs with wire leads.

Any suggestions?

Bob

Perhaps one of these might fit the bill, LED replacements for typical
incandesents...

http://dkc3.digikey.com/PDF/T033/1200.pdf

In article ,
"Mark Jones" 127.0.0.1 wrote:

Bob wrote:
Looking for tiny incandescent lamps for my TM-451A Kenwood.

Apparently these are 6 V or so; there are four of them in
series parallel controlled by a regulator to vary intensity.
They light up the LCD and two of them are burned out.

I have searched many of the usual sources and can't seem to
locate replacement lamps; as a last resort I could go to
Pacific Parts but they are so expensive. These are just
inexpensively made tiny bulbs with wire leads.

Any suggestions?

Bob

Perhaps one of these might fit the bill, LED replacements for typical
incandesents...

http://dkc3.digikey.com/PDF/T033/1200.pdf


You seem fixated on replacing the originals with incadescent bulbs. Here
is another source: http://www.lightbulbwarehouse.com/

And if that is too expensive, try the miniature bulbs used for model
trains or doll houses or christmas tree lights.

Al

--
There's never enough time to do it right the first time.......

In article ,
"Mark Jones" 127.0.0.1 wrote:

Bob wrote:
Looking for tiny incandescent lamps for my TM-451A Kenwood.

Apparently these are 6 V or so; there are four of them in
series parallel controlled by a regulator to vary intensity.
They light up the LCD and two of them are burned out.

I have searched many of the usual sources and can't seem to
locate replacement lamps; as a last resort I could go to
Pacific Parts but they are so expensive. These are just
inexpensively made tiny bulbs with wire leads.

Any suggestions?

Bob

Perhaps one of these might fit the bill, LED replacements for typical
incandesents...

http://dkc3.digikey.com/PDF/T033/1200.pdf


You seem fixated on replacing the originals with incadescent bulbs. Here
is another source: http://www.lightbulbwarehouse.com/

And if that is too expensive, try the miniature bulbs used for model
trains or doll houses or christmas tree lights.

Al

--
There's never enough time to do it right the first time.......

William Sommerwerck wrote:


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

I don't know where it came from, or if it is accurate.
(I don't doubt or dispute it - I just don't know.)
But in 1978 I wired the 2 bulbs in the each of the EXIT
lights in the church in series. They were burning out in
3-4 months before that. Since I wired them in series, we
haven't had to replace a single bulb. The bulbs are lit
24x7, so they don't go through any on/off stress.

William Sommerwerck wrote:


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

I don't know where it came from, or if it is accurate.
(I don't doubt or dispute it - I just don't know.)
But in 1978 I wired the 2 bulbs in the each of the EXIT
lights in the church in series. They were burning out in
3-4 months before that. Since I wired them in series, we
haven't had to replace a single bulb. The bulbs are lit
24x7, so they don't go through any on/off stress.

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

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.