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Question about power supplies

Discussion in 'Electronic Basics' started by Abbie, Nov 20, 2003.

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  1. Abbie

    Abbie Guest

    Hello all,
    I have some question about a 15V 3A variable voltage/current
    power supply circuit. Firstly, the op-amps in the circuit have
    a negative feedback capacitor. Does this mean, since power
    supplies are dc, that these op-amps will actually be comparators
    and not amplifiers. The other question is this: There are two
    different power sources, one is the main one, which is to be
    regulated, and the other is from another set of windings on the
    same transformer, which are regulated with zener diodes and
    a fixed voltage regulator, and are used to power up the op
    amps and the controlling circuit. The funny thing is that the GND
    of this other source seems to be connected to the plus of the
    regulated output of the supply, does this sound o.k. ?
  2. No. A capacitor in negative feedback makes the opamp act as an
    integrator. That is, an amplifier that has lower gain at higher
    frequencies (and very high gain at DC. This is a very common circuit
    block in feedback controls (search PID controllers). If this opamp is
    being used as the integrator function in a negative feedback loop, it
    provides more accuracy if you are willing to wait longer (amplifies
    the error voltage more and more as time passes, forcing the error to
    eventually approach zero. Eventually, here, may be measured in
    microseconds, milliseconds or seconds, depending on the size of the
    This is not at all unusual. It all depends on which side of the
    supply the regulating device in series with, and which side has the
    current sense resistor. The output of the supply is not a positive
    voltage or a negative voltage as far as the control circuit is
    concerned, but a pair of terminals with a voltage between them.
  3. Abbie

    Abbie Guest

  4. It makes some sense, but I think there are errors in the schematic.
    R11,12,13,14,15 do not make sense as part of a current limit function
    and I suspect a ground connection (the common of the +-12 volt
    supplies) is missing on R1,2. There has to be a return path to the
    +-12 supplies for the base current they deliver to Q1 and 2.
  5. Abbie

    Abbie Guest

    I have made a few mistakes which I hope I have corrected,
    please find the diagram at

    Again, all I want to know is if it looks o.k. , and if so how
    exactly is the voltage or current controlled. I have omitted
    several capacitors because they just clutter the whole thing
    and I don't think they affect any analysis.

    Please answer even if you are not sure.
  6. This is quite different that the first version. How sure are you that
    this is now correct? I hate to waste a lot of time trying to
    understand how a mistaken schematic functions. Did you discover a
    connection between OPRED and the common of the +- internal supplies,
    or did you add this because I said it ought to be there, somewhere?

    By the way, any feedback capacitors around the opamps may not bee
    needed to figure out the control concept, but they are essential in
    understanding the stability of the control loop.
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