Difference between inverter and Schmitt inverter?

Discussion in 'The Projects Forum' started by wayneh, Jun 4, 2012.

  1. wayneh

    Thread Starter Expert

    Sep 9, 2010
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    Does the 4069 inverter have more hysteresis than a Schmitt trigger?

    I'm thinking about building a ring tester for inductors, in particular a TV flyback transformer. There's a nicely documented schematic here that uses a 4069 hex inverter IC to make an oscillator and perform other functions. (That schematic is an updated version of the original.)

    I happen to have a pile of MC14584B hex Schmitt triggers and I think I can use one to replace the inverter 4069 in that schematic. Is there any reason to think it won't work?

    I understand the hysteresis provided by a Schmitt trigger but the 4069 datasheet is confusing me: The "on" and "off" voltages seem farther apart than for the Schmitt trigger. Does the 4069 really have more hysteresis than the Schmitt trigger? I'm guessing I'm not reading the datasheet correctly.
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    The 4069 has no hysteresis. You are reading the datasheet incorrectly.
    The hysteresis for MC14584B is 0.6V
     
  3. wayneh

    Thread Starter Expert

    Sep 9, 2010
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    No surprise there. Do I get credit for trying? ;)

    So, I think a little hysteresis in that circuit could be a good thing, maybe an improvement even. It should at least work?

    Here's the circuit. The IC cut off on the upper right is a 4015 shift register to light up LEDs like a bar graph.
    [​IMG]
     
  4. MrChips

    Moderator

    Oct 2, 2009
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    Try it. What is there to lose?
     
  5. crutschow

    Expert

    Mar 14, 2008
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    The ON and OFF voltages are manufacturing tolerance limits for different devices, not the value for a single device. Any particular device will have a switch point somewhere between the tolerance limits with the ON and OFF transition voltages being very close (probably less than a tenth of a volt difference).
     
  6. wayneh

    Thread Starter Expert

    Sep 9, 2010
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    Got it. So when it says the on and off are, say, >2 and <8 for a 10V supply, it doesn't mean those would ever apply to the same piece.
     
  7. WBahn

    Moderator

    Mar 31, 2012
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    Correct. In essence, the data sheet is saying that if you pick a random part and operate it within the specified conditions, that any voltage less than 2V will be seen and acted upon as a LO input while any voltage greater than 8V will be seen as a HI input. For voltages inbetween, it will be one or the other but different parts may make different decisions.
     
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