# Wien Bridge Oscillator

#### sparky 1

Joined Nov 3, 2018
648
I am not sure about the source not using grain of wheat, (grain of rice bulb) it talks about T1 rated 5V 45mA it may be useful in determining which bulb and what current. The size, vacuum and the positive temperature coefficient of tungsten being at play.
The page below shows a chart 1V 45 Ohm being in the middle for current on an LM324. I remember hunting and giving up with the unobtainable 14V 71mA ..
The size of the grain of wheat makes sense, but what amount of glow ? probably want to have very dim glow if any in order to cool fast enough to not lag ?

Thanks AG, the datasheet for that bulb, size 3mm and the power rating 225mW so what power makes it barely glow ?
I am guessing an inexpensive grain of wheat 6V 40mA is about 270mW cold resistance 133Ω. A potentiometer would give a close approximation.
I think when the filament gets hot the resistance rises. Some say it can be 5X even 10X or 1.33K and the correct gain is approx 3.1 for starters
The website shows a startup of oscillation on oscilloscope is fast settling at 35 mS. The right amount of tungsten might be found to be 200-300 mW
https://static.rapidonline.com/pdf/29267_v1.pdf

I used this online calculator to find the bulb entering the following values 6V and 37.5 mA
I will report back when the grain of wheat arrive which are approx 6V 40mA which calculates to 270mW cold resistance 133Ω
https://ohmslawcalculator.com/ohms-law-calculator

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#### Audioguru again

Joined Oct 21, 2019
4,103
Sparky 1, you are correct, I was wrong. The resistance of the bulb is much less when cool than when hot. An incandescent bulb is about 1/5th its hot resistance when it is cool acting like a thermistor. The LM324 and most other opamps are shown working well when the load is 2k ohms or more.

The resistor in series with the light bulb must be a little more than double the resistance of the cool light bulb, so that the gain is 3. When I made the circuit many years ago, the light bulb glowed very dimly. The dim brightness bounced as the output level bounced when the frequency was changed.