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Discussion in 'Electronic Design' started by kell, Jan 16, 2006.

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  1. kell

    kell Guest

    Looking for a little help with a simple voltage regulator I'm building
    for an antique vehicle with a DC generator.
    The armature puts out a current that goes through a blocking diode in
    the regulator, into the positive terminal of the battery. I need to
    limit the current to 10 amps.
    If I just string the b/e junction of a bjt across a sense resistor,
    then the resistor would have to dissipate 6 or 7 watts during current
    limiting. I don't want to use a bulky sense resistor and develop that
    much extra heat so I'm looking for a scheme that will use a sense
    resistor with a lower voltage drop across it. Any ideas?

    P.S. I have to put the sense resistor between the armature and the
    battery positive because the ground connection of the voltage regulator
    will carry only the field current, not the current from the armature.
    The charging current from the armature goes through the chassis and the
    ground strap attached to the battery's negative terminal.

    Thanks in advance, Kell
  2. Rich Grise

    Rich Grise Guest

    Go to your local auto recycler (you may have to shop around a bit) and try
    to find a voltage regulator from the same vintage and model of car, clean
    its contacts, and slap it in. :)

    Good Luck!
  3. scada

    scada Guest

    Have a look at: They have three terminal current
    sensing devices that will output to 5V! Nice package...
  4. kell

    kell Guest

    I just thought of something very simple.
    With a pnp transistor's b/e junction set across a sense resistor placed
    in the current path into the battery's positive terminal, I can bring
    the pnp transistor's base down a few tenths of a volt by connecting it
    to a voltage divider across the battery, effectively reducing the
    voltage needed across Rs to turn the transistor on.
    Anybody see any problems with this?
  5. Guest


    The LM105/205/305 are all obsolete, but they used this technique.The
    problems are the tolerance on the base-emitter voltage (around 200mV)
    and the temperature dependence (-2mV/C).
  6. Guest

    use an opamp, or comparator.

    A linear discrete amp is another possibility.

  7. David

    David Guest

    Or feed the current carrying wire through a few turns of wire to develop
    the control voltage (ie. CT)


    Kind Regards

    David Huisman
    General Manager
    ORBIT COMMUNICATIONS Pty Ltd - Wireless Solutions that Work
    (Telemetry, Control, Monitoring, Security, HVAC ...)
    A.C.N. 107 441 869

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  8. ehsjr

    ehsjr Guest

    It doesn't reduce the rsense wattage, and the divider
    isn't a divider - the top resistor is shorted.

    Draw it - you'll see the problems:

    + -----+---[Rsense]---+---+---> To Batt (+)
    | | |
    | | [Rdivider]
    \e | |
    PNP |------------+---+
    /c |
    | [Rdivider]
    [Rload] |
    | |
    - -----+------------------+---> To Batt (-)

    You could use a .01 ohm 3W as the sense resistor,
    with a comparator circuit if you must avoid a high
    wattage resistor.

  9. ehsjr wrote...
    A comparator or opamp that works at the +Ve rail is good,
    but it's possible to work with common discrete transistors
    and low-value sense resistors, if two transistors are used...

    .. + -----+--+-- Rsense --+----> To Batt (+)
    .. | '-----, |
    .. Rc | |
    .. | c\| |
    .. \e |----'
    .. PNP |---+--e/| NPN
    .. /c |
    .. | R
    .. Rload |
    .. | |
    .. - -----+-----+---------------> To Batt (-)
  10. kell

    kell Guest

    Ed, clip that conductor that shorts the upper resistor and you'll see
    what I was talking about.
    Anyway, I just thought of something a little better and just as simple.
  11. kell

    kell Guest

    it's possible to work with common discrete transistors
    I guess I'll have to get your book, look up the circuit you just posted
    and read about it so I can get my naches (Yiddish for brains) around it.
  12. Rick

    Rick Guest

    I'm wondering why you want to limit the generator output to 10 amps....
  13. Jasen Betts

    Jasen Betts Guest

    doesn't seem substantially differnt to this:

    . + -----+----- Rsense --+----> To Batt (+)
    . | |
    . Rc |
    . | |
    . \e '
    . PNP |---+---|<----'
    . /c |
    . | R
    . Rload |
    . | |
    . - -----+-----+---------------> To Batt (-)

    which was what I was about to propose.
  14. kell

    kell Guest

    Look into antique generators and you'll find out why.
    It's not an alternator.
  15. kell

    kell Guest

    Sorry I yelled Rick.

    Win: It looks like the tempcos of the transistors cancel!
  16. I vaguely recall a low-drop high-side current sensing
    circuit along the lines of the ascii sketch below.

    V+ -------+-----/\/\----+---> Iout
    | |
    R1 R2
    I1| |I2
    e\| Vbase |/e
    /| |\
    | |
    +------+ +----> Vswitched
    | | |
    \| | |/
    e/| |\e
    | |

    Iout*Rs = I1*R1 - I2*R2

    The relative values of R1 and R2 were adjusted so
    that Vswitched changed logic state at a defined Iout.

    I haven't done any sums yet to find out the optimum
    value for Vbase (probably with respect to V+), etc.
  17. Derek Potter

    Derek Potter Guest

    Stability. The threshold depends on VBE which has a temperature
    coefficient of -2mV/C. Also the bias voltage is derived from the 12V
    which is probably not too stable. A better solution is to use a PNP
    long-tailed pair. This gives a good CMRR without needing presicion
    resistors round an op-amp
  18. Fred Bloggs

    Fred Bloggs Guest

    What kind of generator is that- it looks like the field coils and
    armature are fed in parallel from Batt(+) and the current limit opens
    the high side armature/field feed while placing resistance in series
    with the field coil low side return to GND. The low side of the armature
    is permanently connected to GND. It looks like the old time current
    regulators routed a portion of the field coil current through the
    armature current sensor magnet for hysteresis. Your circuit proposals
    are shunting the armature current away from the battery and doing
    nothing to limit generator output. Part of the current limit is to
    protect the generator against overcurrent and your scheme does nothing
    about that. It would be a shame to burn up that relic. You can use an
    LM185 to make a precision high side current sensor if you want to stay
    away from GMR or Hall Effect. But this isn't going to do you any good
    unless you use it attenuate that field coil current.
  19. Rick

    Rick Guest

    Gee, maybe if you would have said it was for a BIKE in the first place I wouldn't have
    hurt your little feelings by asking a simple question.

    I can read and know the difference between an alternator and a generator.

    Got a real nice book on automotive generators (from 1925), by the way...
  20. Fred Bloggs

    Fred Bloggs Guest

    Go monolithic reference, then even a cheap pot works with this:
    View in a fixed-width font such as Courier.

    View in a fixed-width font such as Courier.

    .. .---------------------. R1
    .. | | 1.24V - (Vled-Vbe)*--
    .. | | R2
    .. ----+-----[Rs]------+-------> I= ----------------------
    .. | | | RS
    .. | | |
    .. | | |
    .. | [R1] |
    .. | LM185 | |
    .. --- | |
    .. / \ -------------+ ---
    .. --- | \ / led
    .. | | ---
    .. [R4] [R2] |
    .. | | |
    .. | | |
    .. +---------------' |
    .. | |
    .. | |
    .. | |
    .. >| |
    .. |-------------------+
    .. /| |
    .. | |
    .. +---> to comp [R3]
    .. | |
    .. [Rc] |
    .. | |
    .. --- ---
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