"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