op-amp help

Discussion in 'The Projects Forum' started by NeuroGrad, Apr 24, 2013.

  1. NeuroGrad

    Thread Starter New Member

    Jan 25, 2013
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    Hi Guys, I have built a voltage to current amplifier and need some help - I am having "power supply issues."

    The op-amp I am using is the high voltage, high current OPA549T: http://www.ti.com/lit/ds/symlink/opa549.pdf

    When I tested my amplifier with the Radio Shack 13.8 VDC/3 AMP power supply it worked just fine: http://www.radioshack.com/product/index.jsp?productId=2103959

    However, when I tested it with the meanwell SP-150-12 power supply my amplifier did not work (i.e. I did not measure any current across the load): http://www.meanwell.com/search/SP-150/SP-150-spec.pdf. I want to use this power supply to take advantage of the high output current of the op-amp (10A max).

    I based my circuit design on this website (see figure 3): http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/opampvar2.html. Just as the figure shows, R1 is my load and I have made R2 just the wire (so that output current is as high as possible).

    My qusetion is: what could cause my amplifier to not work when using the meanwell power supply? I really need help. :(
     
  2. #12

    Expert

    Nov 30, 2010
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    You have provided links to everybody else's work, and they all work just fine. How about showing YOUR work so we can inspect it for mistakes? Can you see where you've crippled us trying to help you?
     
  3. NeuroGrad

    Thread Starter New Member

    Jan 25, 2013
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    Ok. Do you mean a picture of my actual amplfiier circuit? Or, a schematic of exactly how I wired it? Just want to make sure I understand. :)
     
  4. #12

    Expert

    Nov 30, 2010
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    Yes. Both of those would help.
     
  5. NeuroGrad

    Thread Starter New Member

    Jan 25, 2013
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    Here is a drawing of how I set up my circuit:
    http://i1353.photobucket.com/albums/q666/vmpunal/circuitdiagram3_zps77e4f81e.jpg

    Here is a picture of the entire circuit (in this case, you will see the wires for V- and V+ power supply pins are hooked up to the radio shack power supply):
    http://i1353.photobucket.com/albums/q666/vmpunal/amplifiercircuit_zps156f651b.jpg

    Here is a picture that is a close up of the op-amp. You can see both the input and output connections:
    http://i1353.photobucket.com/albums/q666/vmpunal/opampcloserup_zpsafa1ed18.jpg

    And, here is picture showing you the load (which is a coil):
    http://i1353.photobucket.com/albums/q666/vmpunal/load_zps2a6e98c7.jpg

    Please let me know if more pictures are needed :)
     
  6. #12

    Expert

    Nov 30, 2010
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    Ok. First impression: I don't see a resistor on the amp chip so your current limit doesn't exist. The "wire" to ground sets the gain of the amplifier at (approaches) infinity. At this point, it's just a switch. I also fear a problem with the chip trying to protect itself from excess current and wattage, it does have internal safetys, or you might have killed the chip.

    Cures: install a current limit resistor to set the current limit, install a resistance to ground (after the coil) to set the current gain at less than infinity. A couple of capacitors, 10uf and .1uf right at the power terminals on the chip would guard against oscillation caused by those long wires you are using to connect everything.

    Other people are invited to comment.
     
  7. NeuroGrad

    Thread Starter New Member

    Jan 25, 2013
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    Thank you for your suggestions. Do you have any ideas for what level of resistance I can use between the coil and ground (I want to be able to output high current)? And, I will take your advice and order capacitors right now. I will let you know of the results once capacitors are installed. :)
     
  8. #12

    Expert

    Nov 30, 2010
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    This drawing shows how to get 10 amps per volt out of that chip, except I forgot to put the current limit resistor in. The important point is that without a tenth of an ohm in the feedback loop, you get infinity amps per picovolt. Completely uncontrollable.

    and the power supply capacitors get attached right at the amplifier power terminals.
     
  9. NeuroGrad

    Thread Starter New Member

    Jan 25, 2013
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    Also, I have a question about the capacitors. Texas Instruments recommends a tantalum type for the 10uF and a ceramic type for the .1uF. I found a 10uF capacitor that I think would work (what do you think?):

    10uF: http://www.kemet.com/kemet/web/homepage/kechome.nsf/vapubfiles/F3100_TaHermSeal.pdf/$file/F3100_TaHermSeal.pdf

    However, I can't seem to find a .1uF ceramic capacitor that would work for how my circuit is wired (since I am not using a board). Any suggestions?
     
  10. NeuroGrad

    Thread Starter New Member

    Jan 25, 2013
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    Another question. :) What kind of resistors should I purchase that can handle up to 10A of current? And, is the current limit resistor the one that goes between pins 6 and 8? If so, I actually don't need this - without that resistor, the current limit is automatically set to 10A per the data sheet.
     
  11. #12

    Expert

    Nov 30, 2010
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    Right. The current limit is set by the chip, as per the datasheet. Is that what you want? Any input causes 10 amps? You don't need an op amp to do that. You can just use a switch.

    The way to buy parts is: choose a vendor and look them up at the vendors site.
    http://www.mouser.com/Kemet/Passive...0znw3Z1z0wrkrZ1z0x1hvZ1z0x6d8Z1z0z819Z1z0z7l5

    http://www.mouser.com/Passive-Compo...0x3xjZ1z0x2vhZ1z0x83kZ1z0wv5kZ1z0x6d8Z1z0z7l5

    http://www.mouser.com/Passive-Compo...0wn9hZ1z0vu8tZ1z0wncfZ1z0wo9pZ1z0wjjdZ1z0wn1h
     
  12. NeuroGrad

    Thread Starter New Member

    Jan 25, 2013
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    What I want is to be able to output any current between 0A and 10A depending on input voltage and without needing to change resistor values. In other words, I need an amplifier that easily adjusts between various output currents as needed, which is why I chose this design. My next question was about the circuit schematic you sent me. With your design, can I only output 10A, or can I output anything between 0A and 10A depending on input voltage?
     
  13. #12

    Expert

    Nov 30, 2010
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    The design I gave you adjusts the input voltage from zero to 1 volt. The resulting current is anything from zero to 10 amps.
     
  14. NeuroGrad

    Thread Starter New Member

    Jan 25, 2013
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    Awesome! Ok, this is exactly what I want. :) After I choose the parts I think I need to buy, can I post them on this forum in order to get your opinion on them?
     
  15. #12

    Expert

    Nov 30, 2010
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    Certainly.

    (I would have said, "sure" but this site requires at least 10 characters in a post.)
     
  16. NeuroGrad

    Thread Starter New Member

    Jan 25, 2013
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  17. #12

    Expert

    Nov 30, 2010
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    Oh no. Only the .1 ohm needs to have some watts about it. 10 watts actual, buy 20 watts for a safety margin. The 1k resistor is a potentiometer, (like a volume control only with a linear response) and the 11k resistor only needs to be a quarter watt for about 6 cents.

    I already led you to the capacitors.

    Right now, I'm thinking, "No wonder you're about to smoke a tiny coil, you don't know what a potentiometer is."
    Good thing you found this site!
     
  18. NeuroGrad

    Thread Starter New Member

    Jan 25, 2013
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  19. #12

    Expert

    Nov 30, 2010
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  20. NeuroGrad

    Thread Starter New Member

    Jan 25, 2013
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    Got it. But, why do I want the 1 KOhm resistor to be adjustable if I am never going to adjust it? Sorry if I am asking too many questions, I just want to make sure I understand. :) In the meantime, I am going to look online to see if I can find a 1 KOhm potentiometer and then I will post it on here to see what you think.
    Another question: as you saw in the pictures of my amplifier, I am not using a board. So, when I hook up the resistors they will technically be dangling. What can I do to avoid this dangling? Can I mount them onto the heat sink? And, if so, is there a proper way to do this?
     
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