How to correctly power op-amp?

Discussion in 'The Projects Forum' started by Aries01, Aug 11, 2014.

  1. Aries01

    Thread Starter New Member

    Aug 11, 2014
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    I want to make this pwm for controlling motors speed without sacrificing torque. I followed the schematic and put it together on a bread board. But I have never messed with op-amps before, so I don't exactly know how to power it.
    here is the schematic.
    Here is the link to the site: http://www.michaels-electronics-lessons.com/pwm.html

    I really want to make this work. But I don't know where the positive or the negative wire of the power supply should go. Also in the diagram it show a ground symbol with a 0 next to it, what does that indicate?
    thank you in advance.
    please forgive my ignorance as I'm new to making circuits, but I really want to understand.

    here is the data sheet: http://www.taydaelectronics.com/datasheets/A-927.pdf
     
    Last edited: Aug 11, 2014
  2. alfacliff

    Well-Known Member

    Dec 13, 2013
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    7 is ground, 14 is +12 volts.
     
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  3. Aries01

    Thread Starter New Member

    Aug 11, 2014
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    so that means 7 is negative, right?
    so to power it, all i do is hook my positive wire to 14 and the negative to 7.
    thank you for the quick response.
     
    Last edited: Aug 11, 2014
  4. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    Link the datasheet of the part, tell us if you want single supply or dual supply for the part, and we will tell you which pin receives what.
     
  5. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    It means that:
    * in single supply setup Pin 7 is connected to 0 volts (ground)
    * in dual supply setup Pin 7 is connected to negative voltage supply
    So.
    Are you using single supply or dual supply?
    (There are two answers to that question and you only need to choose one of them.)
     
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  6. Aries01

    Thread Starter New Member

    Aug 11, 2014
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    I think I am using a single supply, it is an ac to dc wallwart style supply with different voltage options and a polarity switch.
     
  7. Aries01

    Thread Starter New Member

    Aug 11, 2014
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    posted the datasheet at the top, and am fairly certain that i am using single supply.
     
  8. crutschow

    Expert

    Mar 14, 2008
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    Your schematic shows a single supply so Pin 7 would be connected to ground. 0 next to the ground symbol just means that it's the 0V reference point for all the other voltages.

    I don't see how that circuit can work with diode D2 in series with the input to U1B. There's no path for the discharge current of capacitor C3.

    I don't think that circuit was ever built or even simulated. You should always beware of circuits on the Web. Many of them have serious design errors. You should at least simulate them first before trying to build them.

    Below is a simulation of a modified U1B sawtooth generator that works. V2 and R2 simulate the output of U1A. I didn't simulate the rest of the circuit, so don't know if that part works.

    PWM.gif
     
    Last edited: Aug 11, 2014
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  9. RichardO

    Well-Known Member

    May 4, 2013
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    crutschow:
    You missed that the op-amp in the circuit is a Norton amp. Since both inputs are low impedance The diode is not necessarily a problem.

    Aries01:
    I think using the Norton amp is a bad way to start a new project. I would use the LM324 circuit posted by crutschow.
     
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  10. crutschow

    Expert

    Mar 14, 2008
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    Yup, I did miss that. :p I knew in the back of my mind that there was something different about the LM3900 but I didn't follow through with that thought.
     
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  11. Aries01

    Thread Starter New Member

    Aug 11, 2014
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    So do you think this circuit is valid, and if so where exactly would my two power wires go. and if not could you recommend an alternative, I don't know anything about the lm324, could i just put it in place of the lm3900?
    Thanks for sticking with me.
     
  12. crutschow

    Expert

    Mar 14, 2008
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    No you can't replace the LM3900 with an LM324 since they operate differently. I didn't initially realize that so that's why I made the error in my previous post. The LM3900 is a Norton amp (input current operated) whereas an LM324 is a common voltage input operated op amp. If you changed to an LM324 then you would have to modify other parts of the circuit also so that it would work.
     
  13. crutschow

    Expert

    Mar 14, 2008
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    Update: I did simulate the sawtooth generator in your schematic using a simple model of an LM3900 I found and it does indeed generate a 10V sawtooth as indicated.
     
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  14. Aries01

    Thread Starter New Member

    Aug 11, 2014
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    I did it! I was able to dim an led. Thank you so much for your help. Now I just have to find a Tip33 transistor, so I can hook it to a motor.
    crutschow, thank you for going to the trouble of simulating the circuit, I greatly appreciate it. I would still be confused had I not posted.
     
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