Problem with LM324

Discussion in 'The Projects Forum' started by onlyvinod56, Oct 29, 2010.

  1. onlyvinod56

    Thread Starter Active Member

    Oct 14, 2008
    362
    1
    Hello,

    I planned to design a single phase bridge inverter with 4 mosfets.
    Pulse sequence will be a PWM obtained by comparing a sine wave with a triangular.

    1. I took an LM324 and the pins 4 & 11 connected to the supply (12v).
    2. Pins 12 & 13 connected to sine and triangular signals from two different generators.
    3. The pin 14 is connected to an oscilloscope with respect to the supply ground.

    Here goes my problem.
    I have checked the signal shape in oscilloscope before connecting to 324. The signals are pretty good. But when the two signals (sine and tri) connected to the opamp, the inputs are displaying only half wave and the output is not a PWM.

    I have made some trials with some resistors too to avoid floating terminals. Are there any special things to follow for getting this?
     
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Hey welcome.
    First try to read the tutorials that are here on AAC, see the top row..
     
  3. Wendy

    Moderator

    Mar 24, 2008
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    The LM324 is a decent quad, but it is slow. I would never recommend one for digital use. If you are switching a MOSFET (which assumes digital) then you probably should be using a comparator, such as the LM339.

    The is also the issue of rail to rail performance, at which most op amps suck.

    Care to show us a schematic?
     
  4. onlyvinod56

    Thread Starter Active Member

    Oct 14, 2008
    362
    1
    ok Bill.

    From next time i'll attach the schema.
    Here iam attaching the results.

    Can i use LM741.
    If so, which supply i have to go for? is it single or dual?
     
  5. BillB3857

    Senior Member

    Feb 28, 2009
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    The last picture (PWM signal) looks like it may have been taken with the scope set for a different time-base and synchronizing on the square wave. Try doing the dual trace with the sine wave as the trigger and see what your PWM signal looks like.
     
  6. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    If you want to compare signals, better use a real comparator.
    A LM339 (quad comparator) will do the job.
    (see datasheet).

    Bertus
     
  7. onlyvinod56

    Thread Starter Active Member

    Oct 14, 2008
    362
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    Ok Mr Bill.

    I'll check it tomorrow. Wat about 741? Which supply i have to select?
     
  8. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    The 741 is also a slow opamp and not a comparator.

    Bertus
     
  9. onlyvinod56

    Thread Starter Active Member

    Oct 14, 2008
    362
    1
    ThanQ bertus.

    I'll do with 339 and post the results tomorrow.
     
  10. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Don't forget the pull-up resistors at the output.
    The comparators have an open collector output.
    See the datasheet for more info on that.

    Bertus
     
  11. SgtWookie

    Expert

    Jul 17, 2007
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    Select a pull-up resistor that will result in the output of the LM339 to sink about 3.5mA current. In any event, keep it below 5mA, or you will start to see a high saturation voltage on the output.

    Example: if your Vcc = 12v, then
    Rpullup = 12v/3.5mA = 3429 Ohms. You could use 3.3k or 3.6k.

    Then use a voltage follower/buffer to give you decent current gain.
    A pair of NPN & PNP transistors in a common emitter configuration will work pretty well as a voltage follower, and is cheap.

    Note that if you have any unused comparators in the IC, you should ground the inputs or you will have strange problems.

    Make certain to use a 0.1uF cap across the power/ground pins of all of your ICs.
     
  12. onlyvinod56

    Thread Starter Active Member

    Oct 14, 2008
    362
    1
    ThanQ sgtWookie. Thanks for the info.

    We can design a voltage follower / buffer with an opamp right. There are two unused opamps in the IC. Can i use it? Why are you preferring transistor configuration?

    I found CC configured voltage followers in google and those are not NPN & PNP based. Can u attach schematic? One more thing...what is Darlington pair? Is it a buffer?
     
  13. SgtWookie

    Expert

    Jul 17, 2007
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    You're welcome.
    An LM339 is not an opamp; it is a comparator. Opamps in general have very slow rise/fall times in their outputs compared to a comparator.
    Did you try a search on "Voltage follower" and "Voltage buffer?"

    It's a pair of transistors, connected in a way that the gain of one multiplies the other. Google it.
     
  14. onlyvinod56

    Thread Starter Active Member

    Oct 14, 2008
    362
    1
    OK. its cleared sgt. thanQ.

    yes i tried. I didnt get anything as u said.
     
  15. Wendy

    Moderator

    Mar 24, 2008
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  16. onlyvinod56

    Thread Starter Active Member

    Oct 14, 2008
    362
    1
    Thankyou Mr Bill....... i too got one.see the attachment.
    And one more thing.... i got the spwm pulses with LM339. Thanks for Bill, Sgt, and bertus for the guidance.

    I want to continue my discussion for completion of my single phase sine wave inverter.

    Is it possible to change the post title? or shall i continue with this post? or shall i start a new one?
     
  17. SgtWookie

    Expert

    Jul 17, 2007
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    I suggest that you start a new thread with your new title, and provide link(s) back to this thread and any other threads you have going which are directly relevant to your new thread.
     
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