Interesting measurements, wonder if someone has used a PTC Fuse in place of the bulb for the WB Oscillator?
Thanks
those are measurements from a couple of years ago, posted now because somebody was asking in another topic about the SPICE model of light bulb, to simulate a WB oscillator, so I thought those two measured charts would help making a rough approximation of what to expect from a miniature incandescent filament (light bulb of about 5mm/0.2inch diameter and 3.5V nominal or so).
I would be curious, too, to see the comparison in terms of a Wien Bridge distortions when the amplitude of the oscillator is stabilized using a light bulb vs a PTC Fuse.
As a wild guess, I would expect to see better result with the light bulb, simply because most of the WB schematics I've seen use either a light bulb, a FET, or the more elaborated servo method of Jim Williams described in the
"Max Wien, Mr. Hewlett, and a Rainy Sunday Afternoon".
The advantage of the light bulb is that it has a big thermal inertia, while I was measuring I noticed that the filament needs 3-5 seconds to reach its final light intensity. I would expect a fuse to be optimized for fast time response, therefore low thermal inertia.
Another concern would be the I-V curve, where the light bulb is very close to a pure resistor (the filament resistance is in fact a temperature controlled resistor, and the temperature of the filament is constant for fast voltage variations) therefore a linear element (an LTI system can not change a spectrum, so no distortions added). I suspect that, temperature aside, a complex material like the one used in making a PTC fuse might have a V-I curve with more non linearities than the filament, a metal alloy, might have.
Again, no idea if the above guesses will hold when tested in practice.