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| According to a GE lamp catalog I've had for decades changing the voltage applied to a tungsten filament lamp by 5% changes the life by a factor of two and the light output by 10%. Running the voltage up raises the light output but cuts the life. Lowering the voltage increases the life and lowers the light output. Years ago my dad and I did that to a slide projector and that lamp instead of lasting 25 hours lasted as long as we wanted to use it. I'm not sure where that projector is, might be in one of my closets next to big boxes of 35mm slides.
The changes in life and light output are changes from that increment in voltage change. I once figured a 240 lamp on 120 volts should last about a century. There was a dimly glowing lamp in a San Francisco firehouse that had glowed for a century. When I wired a corn dryer in 1973 for my landlord, I mounted a 240 volt lamp in the contactor box to keep it dry and I know that lamp lasted at least 30 years running 24/7.
So a 120 volt lamp on 125 volts has just over half the live it would have had on 120 volts. A 130 volt lamp will has more than twice the life on 125 volts than on 130 volts.
What happens is that if the filament resistance didn't change with temperature that a 5% change in voltage makes a 10% change in power. That changes the operating temperature and the light output. The higher the temperature the faster the tungsten evaporates. To complicate things in understanding the filament temperature changes the resistance. A typical tungsten lamp cold resistance is about 1/16th its hot resistance so the turn on surge current is 16 times the operating current. Hence relays and switches intended for use with tungsten lamps need to have a T rating to handle that surge current.]
Gerald J. | |
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Incandescent bulb, 120 vs 130 volt
