Needing help determining why this circuit did not work

Discussion in 'The Projects Forum' started by gte, Sep 8, 2014.

  1. gte

    Thread Starter Active Member

    Sep 18, 2009
    347
    4
    I am trying to build an analog voltage-controlled pulse width modulator.

    I'm trying to get the first half working with the saw blade portion that is outlined by a red square but I cannot get that working. IC1 is also not outputting a square wave signal, so I believe my problem lies there. I have double checked my connections on my breadboard and they match up to the circuit.

    Does anyone have any idea why this circuit does not work? My scope just sees a flatline VCC voltage value.

    I'm modifying a circuit I found here:
    http://pcbheaven.com/circuitpages/Triangle_Wave_Generator/
    http://www.pcbheaven.com/circuitpages/Voltage_Controlled_PWM_Generator/

    Thanks.

    [​IMG]
     
    Last edited: Sep 8, 2014
  2. #12

    Expert

    Nov 30, 2010
    16,250
    6,746
    Wrong chip. The 741 is not designed to work with 5 volts.

    I also have some doubts about whether the 2N2222 will turn your wave back into a square shape and wreck the results. I think the sawtooth input and the DC level should work well enough without a buffer.
     
    gte likes this.
  3. gte

    Thread Starter Active Member

    Sep 18, 2009
    347
    4
    I'm feeding it 12 to 14v, I just modified the picture, sorry for the confusion.
     
  4. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,669
    804
    Why are the inputs of IC1 and IC2 tied to VCC?
     
  5. bertus

    Administrator

    Apr 5, 2008
    15,641
    2,344
    Hello,

    The problem most likely is the use of the 741.
    You could try an 324 (quad opamp) in stead.

    Bertus
     
    gte likes this.
  6. #12

    Expert

    Nov 30, 2010
    16,250
    6,746
    Darn! Missed again! :mad:
     
  7. MrChips

    Moderator

    Oct 2, 2009
    12,418
    3,355
    741 is still wrong chip. 741 is intended to be used with bipolar +/- 15V supply.
     
    gte likes this.
  8. wayneh

    Expert

    Sep 9, 2010
    12,089
    3,027
    (responding to Kubeek's post) Yeah, I just built an op-amp square wave generator and it didn't look like that circuit.
     
  9. wayneh

    Expert

    Sep 9, 2010
    12,089
    3,027
    You can rarely go wrong by making that statement!
     
    Sensacell and ErnieM like this.
  10. gte

    Thread Starter Active Member

    Sep 18, 2009
    347
    4
    I don't know, that was what was confusing me. This is an excerpt from the first link I posted

     
  11. MrChips

    Moderator

    Oct 2, 2009
    12,418
    3,355
    The 741 op-amp is over 40 years old and is past its best use date.
    It is featured extensively for teaching in textbooks, classrooms and lab instruction, and there it should remain.
    Move on to more modern opamps.
     
    gte likes this.
  12. gte

    Thread Starter Active Member

    Sep 18, 2009
    347
    4
    Is the rest of the circuit solid or should I search for another circuit that does use the 324? I think I have a few 324's in my parts bin.


     
  13. crutschow

    Expert

    Mar 14, 2008
    12,981
    3,221
    It's not necessarily the 741 that's the problem, it's the error in the bias circuit. The op amps' bottom inputs are biased to the supply voltage through the 10k ohm resistor which saturates the op amps. You need an additional 10k ohm resistor to ground at that point to bias the amps at 1/2 the supply voltage (creating a virtual ground).

    Also the comment about the transistor not amplifying the triangle-wave correctly (if that's the purpose of the transistor) are valid. It needs a resistor in the emitter to ground to stabilize the bias point and define the circuit gain. But I don't see a real need for that transistor circuit. Just drive the comparator directly from the triangle-wave at the op amp output.

    Note that a 741 output, as a comparator, goes between about 2V less than the supply voltage to about 2V above ground. If you want the PWM output to go between the supply and ground, use a dedicated comparator chip such as the LM339.
     
    gte and #12 like this.
  14. wayneh

    Expert

    Sep 9, 2010
    12,089
    3,027
    For what it's worth, here is the square wave generator I'm using with an LM358 op-amp (old, but not as ancient as the 741). It's not as simple as some circuits you can find but it does a better job of making a near-50% duty cycle, which matters for my application.

    Untitled.png
    ƒ = 1/(2*LN(2)*R*C) ƒ in Hz, R in Ω and C in Farads
     
    gte likes this.
  15. gte

    Thread Starter Active Member

    Sep 18, 2009
    347
    4
    Since I already had this circuit on the breadboard, I tried adding the 10k as you suggested (even if the lm741 is not ideal for me) but I'm still getting an analog voltage signal, no square wave and no triangle wave ... it is now just half of Vcc like you said. Why isn't it generating any square or triangle wave signals?


     
  16. #12

    Expert

    Nov 30, 2010
    16,250
    6,746
    You might lower that 47k resistor to get a higher gain from IC1.
     
  17. gte

    Thread Starter Active Member

    Sep 18, 2009
    347
    4
    I just build this with the lm339 and got no square wave from output 1 and no triangle wave from output 2.

    Here are the IC's I have

    LM339
    LM741
    LM386
    LM324

    If someone could point me to a circuit that is known to work, I would really appreciate it. I created a 555 timer circuit yesterday that gave the correct frequency, but I believe to manipulate the duty cycle I need a triangle wave?

    Thanks


    [​IMG]
     
  18. gte

    Thread Starter Active Member

    Sep 18, 2009
    347
    4
    I just build this circuit with the LM339, no square or sine waves were formed at all

    [​IMG]
     
  19. MrChips

    Moderator

    Oct 2, 2009
    12,418
    3,355
    Wouldn't work with LM339 since that is a comparator. Try LM324 quad opamp.

    Here is a circuit straight out of the LM324 datasheet:

    [​IMG]
     
  20. MrChips

    Moderator

    Oct 2, 2009
    12,418
    3,355
    Here is a circuit straight out of the LM339 datasheet:

    [​IMG]
     
    gte likes this.
Loading...