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Discussion in 'Electronic Basics' started by kell, Feb 7, 2006.

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  1. kell

    kell Guest

    I need a blocking diode and charging current sensing into the positive
    terminal of a twelve volt battery. It occured to me that I could use a
    mosfet as the blocking diode and bias it on during charging, with the
    Rds serving as sense resistor... and the side benefit of eliminating a
    wasteful power diode.
    How accurate is the Rds listed in mosfet datasheets? They all seem to
    have a number with one or two sig figs and no tolerance spec.
    And important is the Rds temperature coefficient; supposed positive but
    I have no idea of the magnitude.
    Max charging current is ten amps in the application. If it works, I
    may use the same circuit on a 35 amp charger.
  2. Jim Thompson

    Jim Thompson Guest


    But I don't think RDS(ON) is going to be a very reliable value.

    ...Jim Thompson
    | James E.Thompson, P.E. | mens |
    | Analog Innovations, Inc. | et |
    | Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
    | Phoenix, Arizona Voice:(480)460-2350 | |
    | E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
    | | 1962 |
  3. notme

    notme Guest

    Rds is a function of a variety of things including process variations,
    current and temperature. There is not a single temperature coefficient on
    a FET but many which could result in significant variations. I can't see
    why you'd want to use one over a resistor thats explicitly designed to do
    a good job at this application.

    I'm also not clear how you would use a MOSFET as a "blocking diode". If
    you are using a diode connected MOSFET with the gate and drain tied
    together you are still taking a Vgs drop in the FET which may be more than
    a Schottky diode drop.
  4. Noway2

    Noway2 Guest

    My project has the same requirement, monitoring the battery charger
    current and isolating it from the charger to verify that the battery is
    still present. I did use a mosfet to isolate the battery from the
    charger and I chose one with a small Rds to limit power disipation as
    the charger current can be as high as 10A.

    To monitor the current, I used a four terminal resistor of .005R. The
    one I used costs a couple of dollars as I chose a high precision
    device. The net result is that I can read the current with my 12bit
    ADC (0 to 10A) both positive and negative direction with better than
    ..1Amp accuracy.

    I agree that Rds is too variable of a parameter if you wish a
    measurement with accuracy. If you are looking for a crude measuremetn
    of wheter the charger is charging, it may be sufficient for your needs.

    I would also recommend a snubber circuit across the mosfet!.

    If you are interested in implementation details, contact me at the
    email address listed in the post header and we can discuss.
  5. kell

    kell Guest

    I'd use a p-channel mosfet with the source connected to the positive
    terminal of the battery. With the mosfet turned off, it would act like
    a diode, blocking the battery from discharging. Connecting the gate to
    ground (battery negative) during charging would turn the mosfet fully
    on and reduce the power dissipated in the mosfet, assuming
    I(charge) x Rds
    is less than
    Vf of the body diode
    The mosfet would be conducting in the opposite direction of usual.
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