Here is another fun application of lightbulbs to add to this growing collection:
http://www.diyaudio.com/forums/chip-amps/254384-automatic-resonance-frequency-finder.html
http://www.diyaudio.com/forums/analog-line-level/254076-one-more-volume-control.html
http://www.diyaudio.com/forums/equi...-bulbs-into-rms-dc-converter.html#post4521860
They can also serve as a voltage reference.
You simply need to include it in a bridge, and wrap an opamp round: the huge gain of the opamp changes the initial non-linear characteristic into a much steeper one.
How it works?
At start-up, when the lamp is cold, the bridge creates positive feedback around the opamp, because the ratio of R1 to the lamp is larger than R2/R3.
The small initial voltage present at the output of the opamp is amplified, and sends the output in saturation to the V+ rail.
This heats up the lamp, and when the ratios of the dividers equalize, the positive feedback changes into negative, and the circuit stays at that equilibrium point.
What about the performances?
For a change of the supply voltage from 5V to 10V, the output voltage increases by 2mV.
With a better opamp than the 324, this could be vastly improved.
There is certainly a temperature coefficient, but it might not be problematic, or even used to advantage
Apart from fun, why would anyone use such a circuit instead of a classical bandgap for example?
There might be more or less valid reasons: the circuit is not inherently limited to some arbitrary voltage: bandgaps can normally not go below 1.2V, but this one could generate much less.
And I'm sure some people would find it advantageous: as a reference for a DAC, it certainly sounds "warmer" 😉
As shown, the noise characteristic must be pretty atrocious, but this could be eliminated by bypassing R1 and R3 with large caps.
Note that the principle is not limited to lightbulbs: any non-linear element can be used that way, including NTC's, PTC's (the lamp is one), VDR's, etc.
For elements having a convex I/V characteristic, you just have to include it in the other branch.
The circuit can generate indifferently positive or negative reference voltages.
http://www.diyaudio.com/forums/chip-amps/254384-automatic-resonance-frequency-finder.html
http://www.diyaudio.com/forums/analog-line-level/254076-one-more-volume-control.html
http://www.diyaudio.com/forums/equi...-bulbs-into-rms-dc-converter.html#post4521860
They can also serve as a voltage reference.
You simply need to include it in a bridge, and wrap an opamp round: the huge gain of the opamp changes the initial non-linear characteristic into a much steeper one.
How it works?
At start-up, when the lamp is cold, the bridge creates positive feedback around the opamp, because the ratio of R1 to the lamp is larger than R2/R3.
The small initial voltage present at the output of the opamp is amplified, and sends the output in saturation to the V+ rail.
This heats up the lamp, and when the ratios of the dividers equalize, the positive feedback changes into negative, and the circuit stays at that equilibrium point.
What about the performances?
For a change of the supply voltage from 5V to 10V, the output voltage increases by 2mV.
With a better opamp than the 324, this could be vastly improved.
There is certainly a temperature coefficient, but it might not be problematic, or even used to advantage
Apart from fun, why would anyone use such a circuit instead of a classical bandgap for example?
There might be more or less valid reasons: the circuit is not inherently limited to some arbitrary voltage: bandgaps can normally not go below 1.2V, but this one could generate much less.
And I'm sure some people would find it advantageous: as a reference for a DAC, it certainly sounds "warmer" 😉
As shown, the noise characteristic must be pretty atrocious, but this could be eliminated by bypassing R1 and R3 with large caps.
Note that the principle is not limited to lightbulbs: any non-linear element can be used that way, including NTC's, PTC's (the lamp is one), VDR's, etc.
For elements having a convex I/V characteristic, you just have to include it in the other branch.
The circuit can generate indifferently positive or negative reference voltages.
