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Schmitt Oscillator

Discussion in 'General Electronics Discussion' started by duke37, Oct 14, 2012.

  1. duke37

    duke37

    5,211
    718
    Jan 9, 2011
    Schmitt Oscillator

    I have managed to repair a Copel electric fencer even though the makers would not supply me with a diagram or information on the integrated circuit. I deduced that this is a 40106 hex Schmitt inverter.

    I have used previously a simple oscillator as in A in the diagram but Copel used the B version. I have noticed that the B version is used in chips with a built in oscillator. What is the advantage of the extra complication?

    A. Frequency = 2.7/(RC)
    B The frequency with the components shown is 0.7Hz. Is there a general equation to determine the frequency?

    Thank you.
    Duke
     

    Attached Files:

  2. Laplace

    Laplace

    1,252
    184
    Apr 4, 2010
    The two-gate oscillator is discussed in Fairchild AppNote AN-118 "CMOS Oscillators" (page 3).
    http://www.fairchildsemi.com/an/AN/AN-118.pdf
    It may be difficult to obtain a general formula for frequency due to the dependency on the threshold voltages which have a wide tolerance, see attachment. It should be possible to derive a formula based on exact measurement of VT+ & VT- for the particular Schmitt input connected to the capacitor. Note that the purpose of the gate input resistor R2 is to limit current flow when the capacitor voltage is clamped by the input protection diodes. It will also complicate the task of deriving a general formula.
     

    Attached Files:

  3. duke37

    duke37

    5,211
    718
    Jan 9, 2011
    Laplace

    Thank you for the reply. The reference gave an interesting read.

    I knew that the oscillator would not have an accurate frequency but I did not realise how wide the spreads in the thresholds are, it seems best to set up each case individually.
     
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