Reviving an old topic just to show the results of an experiment that lasted many months.
I was trying to find a simple soft start for incandescent lamps and in the end I used in series
thermistors. I made a simple 555 timer circuit that was turning on and off a relay every
five minutes and switching between two bulbs. One of them had two thermistors in line.
Its hard to find thermistors with exactly the value you want, that is why I used two in series.
The time interval was chosen in order for the thermistors to return from 90 degrees Celsius,
back to the ambient temperature. Each 40W bulb was 5 minutes on and 5 minutes off.
All bulbs were from the same batch. OSRAM 40W / 400 lumen.
(Image has been processed in order to show everything equally exposed.
The bulb is way brighter from what it looks like in the photo.)
The bulb with the thermistor has a nice soft start and was reaching full brightness after 90 seconds.
The results:The protected bulb has already outlasted two unprotected ones and it keeps going strong!
Each bulb lasted for exactly 2 months. (or 720 hours of continuous operation and 17280 switchings.)
So, for the life of the incandescent, inrush current and thermal stress is just as important as filament
evaporation. I will continue the experiment to see how much longer this bulb will last because I think
that inrush current is more important. Its been already on for at least 1440 hours. I will report back.
Bulb Miser and NASA were right I think.