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Dummy Load II

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by jocanon, Jan 12, 2013.

  1. jocanon

    jocanon Member

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    got it! Thanks :)

    So, I am thinking I will just build this, then if I still have the oscillating problems I can think about getting the scope and scoping it out at that point.
     
  2. ronv

    ronv Well-Known Member Most Helpful Member

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    I can't remember the little meters. Were they ok to run on 12 volts?
     
  3. jocanon

    jocanon Member

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    Yes, they're good up to 100v.
     
  4. dave

    Dave New Member

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  5. ronv

    ronv Well-Known Member Most Helpful Member

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    Do you have a link?
     
  6. jocanon

    jocanon Member

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  7. ronv

    ronv Well-Known Member Most Helpful Member

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    Cool,
    The only other thing I can think of as a caution is to try to keep the wires on the output of the op amps away from the + input.
     
  8. jocanon

    jocanon Member

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    Will do
     
  9. jocanon

    jocanon Member

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    So it looks like all the op-amp outputs are tied together and connected to ground with 4.7k ohm resistors? I just want to make sure I am reading the schematic correctly.
     
  10. jocanon

    jocanon Member

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    One other thing I just thought of. In the original schematic I penciled those .1uf caps on each op-amp's Vcc and a TVS on the +24 load voltage to ground. Should I incorporate those into the new schematic as well and in the same spots?
     
  11. ronv

    ronv Well-Known Member Most Helpful Member

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    The output of the op amps aren't tied together, they just have a 4.7k resistor to ground.
    Yes, Keep the decoupling caps and the TVS just add the larger cap as well.
    There are a few component changes around the current meter amp and the pot as well as the .1 ohm jobs.
     
  12. jocanon

    jocanon Member

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    Okay, cool. So the 4.7k resistors, is that to slow down the gate response time?
     
  13. ronv

    ronv Well-Known Member Most Helpful Member

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    No, the 358 op amp has a tendency to have little oscilations when driving a capacitive load ( like the FET). The resistor keeps it from doing that. Slowing things down in the feedback loop will tend to make it unstable. The way it works is if the current goes up the op amp senses it and turns it down. If it is too slow it will oscillate.
     
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  14. jocanon

    jocanon Member

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    I see, thanks ronv for the explanation.
     
  15. ()blivion

    ()blivion Active Member

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    OH, so I had it somewhat backwards then? I figured things need to be be slowed down to give the Op-Amps time to see that it's output has created the desired effect, so it doesn't over correct. When you start fishtailing in a car, you are supposed to slow down your response more in order to get things back under control. That's how I understand it anyway.

    Meh, whichever the case, most likely the Op-Amp control loop is what's oscillating.
     
  16. ronv

    ronv Well-Known Member Most Helpful Member

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    Margin

    Hi (),

    It is kinda counter intuitive.

    Here are a couple of sims of the circuit.

    One has a pulse input at the adjust input and traces for each of the FET outputs - one with and one without a 1k gate resistor. You can see the ringing from the step in the input for the one with the resistor while the other is pretty clean.

    The second is the frequency response of the same "loop". If you look at the gain and the phase the one with the resistor almost has gain at 180 degrees of phase shift so it is close to oscillation.
     

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  17. schmitt trigger

    schmitt trigger Active Member

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    ronv;
    you are absolutely correct about certain aspects of feedback theory being counter intuitive.

    All; there is another analogy on how a too-slow system could oscillate. The analogy is far from perfect, but provides an idea.

    Imagine you are taking a shower, on which the faucet changes the water from icy cold to scalding hot with a minimal movement, and once that you make an adjustment, it takes a loooong time for the water mix to reach the shower head.
    It can be easily seen that it will be extremely difficult to adjust the water comfortably, and you'll end up jumping in and out of the shower all the time.

    Such a sensitive faucet can be describe as having high gain (too much temperature change with minimal movement) plus it will have a large phase lag (your hand will respond faster that the time the mixed water reaches the shower head).

    Control theory states that, when your phase lag reaches 180 degrees and you still have a gain larger than unity (0 db), then you'll have guaranteed oscillation.

    Margin;
    you are much better than myself with LTSpice...how did you include the potentiometer model in the Sim?
     
    Last edited: Jan 14, 2013
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  18. jocanon

    jocanon Member

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    You know, even to a rookie like me, that kind of makes sense... thanks schmitt trigger. If I am getting it right, you are saying that if the response time is too slow it will oscillate because the op-amp will not see the change as taking effect until it is too late, then it will try to over-correct and again, not see the correction until it is too late again...hence oscillation.
     
  19. Mr RB

    Mr RB Well-Known Member

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    For testing DC PSUs I just run a passive dummy load, it's basically a heap of transistors, driven by 2 darlington stages and a pot.

    Because there are no active components (no opamps) it does not need a battery or PSU to power it, and that also makes it pretty bulletproof;

    [​IMG]

    It will drift a few percent in current as it gets hot, but that is not usually a problem as in normal use I adjust it to different test currents anyway, or if testing one fixed current it might need a "tweak" of the knob once it is hot.
     
  20. schmitt trigger

    schmitt trigger Active Member

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    "it will oscillate because the op-amp will not see the change as taking effect until it is too late, then it will try to over-correct and again, not see the correction until it is too late again...hence oscillation"

    You got the basic idea.
     
  21. jocanon

    jocanon Member

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    So, I am pretty content right now with the schematic ronv designed as it seems to accomplish my purposes and it did not cost me too much to build...but, I should keep an open mind, so just curious, how high a wattage do you usually go up to on your set up. Is it reasonable to get up to 1500+ watts. Also, is it variable, can you adjust it to any current level? Lastly, could you ball park the estimate cost to build one within a range of say $100, like would it be 2, 3 hundredish or more like 5, 6?

    Edit:
    I didn't see the picture at first, it showed up later. Now after looking at the picture it definately looks pretty inexpensive, probably I would guess around $100 to build, is that close?

    Thanks,
    Jeremy
     
    Last edited: Jan 14, 2013

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