op amp, capacitive loading

Discussion in 'General Electronics Chat' started by bug13, Apr 29, 2013.

  1. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    Hi all

    I still working on an electronic load, here is the problem I have, I can't seen to get the circuit into a stable state.

    So my question, how can I stabilize the circuit. the op amp in question is lm358p (I know it's old, but those are the only one I have at the moment).

    I am hoping to get some pointers, or a better design.

    here are what I have tried (app note from here):
    - Snubber Network at the output of U1, it only work without U2 amplify stage (
    - series resistor at the output of U1, it makes it worse.

    datasheet of IRF540N, and LM358P

    Thanks

    [​IMG]
     
  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    1) The maximum output LM358 can only get to Vcc - 1.5 volts. This will not be enough to turn on the IRF540 since your load is connected to the source ( move the LOAD between the DRAIN and the MOSFET). Normally a MOSFET gate needs to be 10 volts above your SOURCE pin to turn on completely.

    With that, your (+) input on U2 must be moved. I hope that is possible.

    What is the goal of your circuit?
     
  3. crutschow

    Expert

    Mar 14, 2008
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    You circuit is likely unstable because you have two op amps in one loop which can give enough phase-shift to cause oscillations. You don't need U2. Remove it, along with R2 and R3, and just scale Vin accordingly.
     
  4. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    can I still achieve 1mA minimum resolution if I remove U2?
     
  5. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    the current to the load (Vcc in this) is controlled by Vin
     
  6. crutschow

    Expert

    Mar 14, 2008
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    You will have 1mA resolution, but the LM358 can have as much as 7mV of input offset so your current accuracy can also be off by 7mA, with or without U2. If you want better accuracy, then you need an op amp with lower offset.
     
  7. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    Do you know an op amp that is under 1mV offset but it's very common and low price in dip package?
     
  8. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    This is an analog circuit, not a switching circuit. The MOSFET doesn't have to turn on completely. For example, if the maximum current is 1A, then Vgs might only have to be 3 or 4 volts, although it could be a couple of volts higher.
    Also, I didn't see any mention of the value of vcc, so I don't understand how you could have come to your conclusion.:confused:
     
  9. Ron H

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    How much is low cost?
     
  10. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    It is an analog circuit, so you will have all the resolution in the world. Other things are offset like crutshow pointed out, and noise but that should not be a problem.
     
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  11. #12

    Expert

    Nov 30, 2010
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    This drawing shows 5 ways to stabilize this circuit. Most of the networks are 10k ohms and a few dozen pf.

    ps, equation for this, in DC, is: I through the 1 ohm resistor is Vin/ (1+ (R2/1k))
     
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  12. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    Low cost means under $10 for 10 chips in my personal opinion. :)
     
  13. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    Vin is fed from a 10bit DAC, I was hoping to divide it into two range, 1mA to 200mA with a resolution of 1mA, 200mA to 5A has a resolution of 10mA or something.

    (these are not the exact figures, as I haven't get to that part yet)

    at what condition should I start worrying about noise? (I properly should read into that a bit more)
     
  14. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    The lowest cost through-hole device with Vos<100μv that I could find on short notice is OPA277PA, @ $1.95 for 10 pcs.
     
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  15. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    For the case 2 you need to add a resistor between the op amp output and non inverting input.

    Also in February I have a similar problem with a voltage control current source.
    And I was force to add 22K and 1nF capacitor to prevent circuit from oscillations.

    Without 22K and 1nF
    [​IMG]

    And after I add 22K and 1nF.
    [​IMG]
     
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    bug13 and #12 like this.
  16. crutschow

    Expert

    Mar 14, 2008
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    As I previously noted, you don't need A2. If you want some gain in the feedback loop (the apparent purpose of U2), just remove A2, connect R3 between ground and the (-) input of A1, and connect R2 between R1 and the junction of R3 and (-). The transconductance gain will then be (1 + R2/R3) x 1/R1 A/V. That circuit should have no stability issues.

    Edit: Of course R2 and R3 will draw off a small amount of current that will add to the MOSFET current so you want to make the resistance sum of R2 and R3 high enough so that this current has negligible effect on your desired accuracy.
     
    Last edited: Apr 30, 2013
  17. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    Hi #12, I have tried all the methods but with different values, I will try them again this weekend (if it's not earlier) with the values you suggested, thanks
     
    Last edited: Apr 30, 2013
  18. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    Hi Jony130,

    Thanks for you idea, I will try your suggestion this weekend, if it's not earlier.

    How do you come up with the value of 22K and 1nF? my circuit is oscillating at around 343kHz.
     
  19. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    Thanks crutschow,

    Why didn't I think of that, it looks like a good idea, I will try it and report back
     
  20. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    Same chip in NZ(New Zealand) costs NZD$3.48/US$2.92 each, not fair :mad:, but thanks anyway Ron H, your help is appreciated.:)
     
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