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Detecting down to 25mA-150mA on 0-20A line?

Discussion in 'General Electronics' started by John, Nov 15, 2005.

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  1. Fred Bloggs

    Fred Bloggs Guest

    You're welcome to post a Tech-chat schematic of your brilliant idea.
    Anyone can shoot their mouth off with a vague description of "just do
    this" or "just do that", and when we get down to it, most of the time
    they don't know their butt from a hole in the ground. So let's see a
    schematic of your brilliant simplification the rest of us incompetents
    overlooked.
     
  2. Fred Bloggs

    Fred Bloggs Guest

    I used the same LM324 model and I don't see it, the whole thing is way
    overdamped with a 250-350us charge-up of the '3704 gate no matter what
    time constant used for local feedback compensation. I did solve the
    problem of abusive battery plug-in with high surge current into the
    circuit, and that is to simply turn the MOSFET backwards, it makes a
    heck of a good clamp diode, and circuit dynamics are essentially
    unchanged. This thing will be finished with some interlock stuff to
    prevent discharge on power loss and reverse battery protection:
    View in a fixed-width font such as Courier.
     
  3. Fred Bloggs

    Fred Bloggs Guest

    Actually if power supply abuse is a concern, leaving the MOSFET forward
    connected and reducing the gate current drive will give it milliseconds
    to respond to the connection and pull the current limit into regulation,
    but the MOSFET needs to be heatsinked for something like 20W:
    View in a fixed-width font such as Courier.
     
  4. Jim Thompson

    Jim Thompson Guest

    Did you zoom in on where the waveform reaches closure? I saw some
    fine "oscillation"/slew-hunting.
    Good idea, Fred!
    [snip]

    ...Jim Thompson
     
  5. Jamie

    Jamie Guest

    Hey, your ok Fred! :))
     
  6. Zak

    Zak Guest

    Implementation depends on regulator, input voltage, etc.

    If the regulator is switching the current could be detected by looking
    at duty cycle, frequency, or whatever. That would be teh simplest solution.

    For a linear regulator: bypass the pass device with a current regulator
    set to the desired end point. Note: current source should have a disable
    input.

    In the linear regulator, detect when the error amp output hits its
    limit. It may well be that this ouput goes to 0 v or negative. Detect
    with comparator, spare opamp, discrete transistor with base resistor, or
    even tie to logic with a resistor.

    This output going 0 or negative has to shut off both the linear
    regulator and the current source. Shutting off the current source and
    reducing the output set voltage of the power supply is enough: the
    supply will stay out of regulation as the setpoint is lower, but
    charging has stopped.

    Do not make the voltage too low: the supply has to come back into
    regulation when an empty battery is connected.

    The reduced voltage will keep the supply out of regulation, giving a
    schmitt trigger effect.

    Kill the current source by pulling a node to ground.
    Change the reference or feedback, depending which way you feel, to
    reduce teh voltage of the main regulator.

    TO do it quick and dirty, one needs:

    Three diodes, two resistors and a transistor for a (poor) current source.

    Tie its base to ground to shut it down: 2nd transistor. Mayeb a diode in
    the ouput for protection.

    3rd transistor + resistor is the required inverter that drives this from
    the op-amp output. Use the collector of this transistor as the ground of
    the feedback divider if you feel like doing things the dirty way
    (changing the supply output voltage).

    You need some capacitance on the output (probably via a resistor) to
    keep everything from oscillating of no battery is connected.

    I hope you enjoy your schematic drawing program, I'm looking forward to
    the picture.


    Thomas
     
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