[Jeri] uses light bulbs in an oscillator

Way back when [Ms Ellsworth] was a kid, she kept seeing the same circuit over and over again in her various op-amp books. It was a Wien bridge oscillator, a small circuit that outputs a sine wave with the help of a light bulb. Now that [Jeri] is much wiser, she decided to play around with this strange oscillator and found it’s actually pretty impressive for, you know, a light bulb.

The interesting portion of the Wien bridge is the gain portion of the circuit. It’s just a simple resistor divider, with a light bulb thrown in on one of its legs. When the current increases, this causes the light bulb to warm up (not enough to glow, though). When the temperature increases, the resistance in the light bulb increases, making the oscillator reach an equilibrium.

It’s a clever setup, but what about swapping out a resistor in place of the light bulb? In the video, [Jeri] tries just that, and it’s a mess. Where the light bulb circuit is amazingly stable with very, very low distortion, the resistor circuit looks like a disaster on the scope with harmonics everywhere.

A very cool build that would be perfect for an audio synth, but as [Jeri] says in her YouTube comments, “This doesn’t have enough distortion for indie bands.”

Comments

  1. Wilfred says:
  2. http://www.alpharubicon.com/elect/3dollarbattggn.htm

    Using a light bulb in the exact same way to limit current for a battery charger. This is the absolute minimum possible battery charger and it will fry you or your battery over-easy if you forget about it, mess up the polarity or do something stupid. Unplug it before making any adjustments.

    You can everything to make this charger, except the diode, from any hardware store or wal-mart in the country, so it’s a good emergency trick. For regular use or to fully charge, use a smart charger instead.

  3. Alex Rossie says:

    I love these circuits, we saw them in our A-level physics classes, made by an inspirational teacher.

  4. qwerty says:

    The key is using the positive resistance property of the lightbulb to make a really simple almost-constant current regulator: current increases->heat increases->resistance increases->current decreases. Ordinary resistors also has this property, and often they are used as such in even more circuits (nearly every pre/amplifier in the world), but it’s much less pronounced than in a small lightbulb. There are more modern parts that can be used in this context like PTC thermistors, FETs, Vactrols, etc. yet the lightbulb remains an ingenious way to accomplish an important task in the simpler possible way.

  5. John says:

    Hi Jeri (if you’re reading these),

    First: you do great work. Thanks for all the good info you’ve provided over the years. Second: This is off topic but is that the Tektronix dual domain device? I was given a short presentation of one of these at an EMC/EMI seminar recently and it seemed really cool. How useful have you found it to be?

  6. ejonesss says:

    i remember from the 80’s bose made a speaker with built in protection that i think used a light bulb like used in the dome light in car that would light up and even burn out like a fuse if you tried to push too much power say connect directly to ac line or turned up the volume too high or if the amplifier driver shorted.

    the speakers was expensive like $1000 +

  7. Reg says:

    FWIW There’s a wonderful article in “Analog Circuit Design” by Jim Williams (who’s also the editor) entitled “Max Wein, Mr. Hewlett and a Rainy Sunday Afternoon” which narrates his efforts at removing the lamp which resulted in THD below the noise floor of the distortion analyzer (0.0003%). But even his first iteration w/ a lamp was only 0.0025% THD.

  8. cruster says:

    Clever stuff! Thanks for putting this together :-)

  9. Queeg says:

    Incandescents are so 1900’s. I gotta try this with a LED instead.

    ;-)

  10. Doihn says:

    Agreed, move the “Read comments” back under the article, which makes sense since all the social stuff is down there anyway. Otherwise the design is pretty good…

  11. Ken says:

    Neat video, thanks (Jeri, HaD) for showing me a very cool circuit.

    However, there is a misunderstanding about the role of the light bulb in reducing distortion. The light bulb serves as a kind of automatic gain control, changing its resistance so the circuit achieves unity gain (and thus stability) near a desired output amplitude. Compare this with other oscillators that use non-linear gain (i.e. distortion) to limit output and you see why the Wien bridge produces a good signal. The light bulb also allows for reasonably quick start up by providing greater than unity gain when it is cold.

    Now, notice that the light bulb changes its resistance slowly and does not appreciably affect distortion during any particular cycle after its temperature has stabilized. If it changed at that time scale, the output signal would be distorted. Replacing the bulb with an equivalent ideal resistor after stability is reached would improve signal quality for a short time, but output amplitude would soon drift, either extinguishing or distorting the signal.

    So why did Jeri’s video show so much distortion when the resistor was swapped in? Mostly because the op amp was clipping high output levels. But if you look closely, it still appears that the distortion was a little higher than the light bulb version even during those times the output level was moderate. This is not because of the light bulb’s magical resistance, rather it must be a consequence of non-linearity in the variable resistor. Keep in mind that the resistor swapped into the circuit was dissipating just as much energy as the light bulb, and was undoubtedly heating up. This can be easily remedied by swapping out both the light bulb and the negative feedback resistor for new resistors with proportionally higher values, aiming for perhaps a few milliamps of current.

    Again, thanks for an instructive video. I really enjoyed it.

  12. Galane says:

    Great, another reason to hoard incandescent light bulbs before laws are passed to ban them all. ;)

  13. Equinoxe says:

    I use a light bulb in series with the mains when I have to find a short-circuit in a device (which blows the fuse).
    This way I can do measurements while powering a device.

  14. ejonesss says:

    Equinoxe that’s one way or you could wrap foil around the fuse and put it back in and what ever is shorted then will heat up and smoke or explode.

    of course there may be sparks, smoke and fire witch would not be good.

  15. Ren says:

    IIRC, The first commercial product by Hewlett-Packard was an audio oscillator that used a light bulb. Walt Disney bought 200 to use in the making of Fantasia.

  16. Halexander9000 says:

    “This doesn’t have enough distortion for indie bands.”
    That made my birthday. I laughed so hard.

  17. Oscar says:

    plagiarism. nothing original. give the credit to Hewlett.

  18. Mikey says:

    Interesting final point she leaves us with. You could potentially use a lightbulb oscillator to detect physical movements… like a piezo..

    • morganism says:

      Jeri is the Bill Nye of the circuit geeks !

      another great optical factoid:
      you can dissolve the filament of a bulb with peroxide to get a salt of tungsten. 30% minimum.

  19. n0lkk says:

    when we where studying bridges at tech school I thought I’d build projects to measure the value of unknown capacitors & inductors. Have yet to do so. I guess I didn’t that as much as I thought I would :) Icould say I have been waiting for inexpensive uproccesor dev. boards to include a digital readout. :)

  20. Drone says:

    There was Way too much distortion in her circuit even with the light bulb inserted.

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