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Help with relaxation oscillator

  1. Feb 22, 2015 #1
    All, I am confused as to exactly how the capacitive relaxation oscillator works.

    From the diagram above: Because the lamp is in parallel, why is it not immediately receiving the voltage necessary to light instantly? Thanks.
  2. jcsd
  3. Feb 22, 2015 #2


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    hi there

    The neon globe has a minimum strike voltage .... lets say ~ 80V ( 75 - 90V common)
    This is the neon's threshold voltage. That voltage will only be reached when the capacitor is charged to that threshold voltage
    The neon will then conduct with a flash of light and the voltage across the capacitor will drop to well below the threshold voltage.
    The capacitor will then start charging again till that voltage is reached and the neon then discharges again.
    this cycle continues on and on ...

    The cap doesn't charge instantly ( or near) because of the series resistor limiting current to the capacitor causing it to take time to charge
    but when threshold is reached it discharges almost instantly .... ie. it oscillates with a sawtooth pattern

  4. Feb 22, 2015 #3
    Ahhh ok...so a capacitor acts like a voltage sink of sorts...instead of this being like a regular parallel circuit, the voltage across both legs is initially zero because the capacitor must first charge? Or maybe to reword, the battery voltage does not initially reach the bulb because it is being taken by the capacitor? Thanks-
  5. Feb 22, 2015 #4


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    Yes, initially, at time T=0, the capacitor "looks" like a short circuit, charge is flowing onto one plate and out of the other plate
    as the charge builds up, the voltage potential difference across the capacitor also does. Only when that voltage potential equals the
    discharge ( threshold ) voltage of the neon globe will current flow across the gap in the neon

    The same voltage that is across the capacitor is also across the neon globe, from the time the capacitor starts charging,
    that voltage slowly rises as the capacitor charges up
    have a look at this .....

    relaxation oscillation.jpg

  6. Feb 22, 2015 #5


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    The Neon bulb (gas discharge tube) relaxation circuit is a negative resistance oscillator.

    The operational voltage and currents are normally designed to be in the unstable portion of the bulbs VI curve.
  7. Feb 23, 2015 #6
    Thank you.
  8. Feb 23, 2015 #7
    Curiously, there is a semiconductor component that behaves like this, the DIAC.

    Unfortunately, it lakes the charming flash-flash-flash of the neon bulb.
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