Square Wave Generator Construction

Discussion in 'The Projects Forum' started by ifiaxxy, Jul 21, 2014.

  1. ifiaxxy

    Thread Starter New Member

    Jun 5, 2014
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    hi, I am building a circuit with op-amp to generate a square wave. The circuit diagram is shown here: http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/square.html#c4

    The op-amp I use is TL074 and when I run it, there is a problem: I get the square wave in the first few minutes (like 20min or so) and then the output becomes a constant voltage. All my resistors are 10k and my capacitor is 4.7uF.

    Can anyone help me fix this problem? I want the circuit to generate the square wave constantly for up to 12hr.

    Thank you!
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    What are the supply voltage(s)?
     
  3. to3metalcan

    Member

    Jul 20, 2014
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    If you restart it, does it start running again?
     
  4. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Seems like there should be a R3 from + supply to + input. Difficult to discharge C to below ground when discharge path is ground or near ground.
     
  5. ifiaxxy

    Thread Starter New Member

    Jun 5, 2014
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    the supply voltage is 12V
     
  6. ifiaxxy

    Thread Starter New Member

    Jun 5, 2014
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    Yes, it runs again, but the same thing happens
     
  7. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    +/- 12VDC? Or just 12VDC? Note that the circuit you linked to requires a +/- power supply.
     
  8. Veracohr

    Well-Known Member

    Jan 3, 2011
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    The oscillation runs fine for about 20 minutes, then stops? Maybe it's the JFET-input opamp latching up. Do you have a different opamp model you can try?
     
  9. ifiaxxy

    Thread Starter New Member

    Jun 5, 2014
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    It's + and - 12V DC
     
  10. ifiaxxy

    Thread Starter New Member

    Jun 5, 2014
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    This is the only op-amp I've got... I think the op-amp is still working though, because when I unplug the power supply and put it back on, it works again
     
  11. wayneh

    Expert

    Sep 9, 2010
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    And "ground" is at 0V, right? In other words, you haven't confused V- and ground?

    Is there a load on Vout that might be causing the op-amp to overheat and shut down?
     
  12. ifiaxxy

    Thread Starter New Member

    Jun 5, 2014
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    GND is 0V and when I run the circuit, the op-amp doesn't get hot, so I guess there's no overload
     
  13. wayneh

    Expert

    Sep 9, 2010
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    OK, just wanted to check the "obvious" stuff.

    How about putting a 0.1µF decoupling cap on the op-amp's power pins? Maybe it's falling into an oscillation of some kind.
     
  14. ifiaxxy

    Thread Starter New Member

    Jun 5, 2014
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    for a decoupling capacitor, do you mean that I can simply connect Vcc+ and Vcc- with a capacitor?
     
  15. to3metalcan

    Member

    Jul 20, 2014
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    Run a cap from each power pin to ground...reduces the chances of hf oscillation swamping out your square wave. Also, tell us about your main timing cap. What value? What type? Have you tried swapping it for another?
     
  16. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    No. Use two caps. One from Vcc+ to ground and the second from Vcc- to ground.
     
  17. to3metalcan

    Member

    Jul 20, 2014
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    I'm guessing your 4.7uF is electrolytic...they're not always great for oscillators. If bypassing doesn't help, try increasing the value of R and using a smaller C...a value you can get as a film or ceramic.
     
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  18. to3metalcan

    Member

    Jul 20, 2014
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    Or just try a different cap! See if you can get oscillation at any frequency.
     
  19. Brownout

    Well-Known Member

    Jan 10, 2012
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    What voltage? Also, where are you connecting the ground?
     
    Last edited: Jul 21, 2014
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