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high-side current sensing chips

Discussion in 'Electronic Components' started by gearhead, Apr 8, 2008.

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  1. gearhead

    gearhead Guest

    Hi all, I'd like to help out some friends on an antique motorcycle
    forum that have a 6 volt regulator project. One of them built a
    regulator on perfboard and posted the schematic. It has a couple of
    things I'd tweak, but it works.
    The main challenge: his regulator design doesn't incorporate current
    limiting. Even the original relay-style mechanical regulators
    incorporated current limiting, because the generators required it.
    Somebody on the forum suggested the zxct1009, but it only comes in
    surface mount which means we can't rejigger the existing perfboard
    project to include the new chip. It would mean having a custom-
    printed circuit board, and soldering techniques perhaps a little too
    demanding for somebody building his first circuit -- which probably
    describes a lot of the guys on the forum.
    pdf of the circuit:
    http://www.hydra-glide.com/phpBB2/download.php?id=1173
    I've tried to find a through-hole component that fits the bill, but
    can't seem to come up with anything just right. I saw the micrel
    MIC5021, but it supposedly operates on 12 volts and up.
    http://www.micrel.com/_PDF/mic5021.pdf
    Now, perhaps they make that statement on the datasheet because the
    chip actually also has a voltage doubler and the high side drive needs
    that kind of voltage to turn on a mosfet, and it's possible the chip
    itself will actually run on a much lower voltage. But the datasheet
    doesn't say anything to this effect. Has anybody used the MIC5021 and
    know if it might actually turn on at say 5 volts or perhaps know of
    any other through hole chips for current sensing?
     
  2. That Micrel chip is not a current sensor, it is a mosfet driver. Looking for
    a highside current sensor, try the MAX4374. A ZXCT1008 or ZXCT1009 may be
    even better. But I doubt you to need that special (and expensive) chips.

    Can't be sure what current you want to limit, but usually the field current
    is limited to limit the output voltage of the generator. Which in turn
    prevents the battery from being overloaded. This is exactly what the circuit
    of your schematic is supposed to do. Just use R1 to set the correct voltage.
    For a 6V lead-accid battery this will be about 6,9V. So even for fully
    loaded battery the voltage should never exceed that value. You may have to
    take some voltage loss accross D6 into account which is about 0.2V for this
    particular diode.

    petrus bitbyter
     
  3. gearhead

    gearhead Guest

    I've done plenty of scheming about how to do it with discretes. But
    like you said, I'd still want to use a chip in the end, to get matched
    transistors. Pairs and arrays, I see ony surface mount, including
    zetex.
    If I have to go that way, might as well use the ZXCT1009F in SOT-23
    with pins 1.9 mm apart according to the diagram. Almost a tenth of an
    inch, could probably solder it right onto the pads of a perfboard, eh?
     
  4. gearhead

    gearhead Guest

    Current limiting protects the generator, not the battery.
     
  5. gearhead

    gearhead Guest

    Don't say the micrel chip is not a current sensor. It does current
    sensing.
    And current limiting is for the generator, not the battery.
     
  6. Wimpie

    Wimpie Guest

    Hello,

    When you buy about 10 general purpose PNP transistors from one batch,
    you will probably find several transistors that are within some mV
    with same Ic. When you design the 2 transistor current sensor at low
    bias current, self heating can be neglected and it saves you from
    soldering SMD devices like NXP's BCM857. When matching is impossible,
    you might add a trimmer potentiometer for nulling.

    best regards,

    Wim
    PA3DJS
    www.tetech.nl
    please remove abc from the address
     
  7. gearhead

    gearhead Guest

    Doesn't gain determine the balance in a current mirror? If so, one
    would have to match for gain. I did match for gain last time I
    experimented with current mirrors. I'd like to hear from the experts
    about the need for this.
    About temperature matching. I calculate that one tenth of a degree
    centigrade difference between the transistors in a mirror would add
    about one percent error to the current mirror. I wouldn't want
    temperature vagaries causing any more error than that. I'm wondering
    if you could count on discrete transistors on a board to stay within a
    tenth of a degree to each other. More fodder for the experts.
     
  8. Wimpie

    Wimpie Guest

    Hello,

    For such circuits matching on Vbe with constant collector current is
    sufficient. I normally do it by connecting B to C, bias at required
    current and measure the voltage across the diode. Of course for high
    volume applications I use a matched pair.

    When 1% additional tolerance is unacceptable, you also should evaluate
    all other components. I think of current sensing resistor, other
    resistors, voltage variatons, offsets in control loop etc. Regarding
    temperature, put them close together, take some epoxy resin and some
    piece of coper/alu, and temperature difference due to environment will
    be negligible.

    I can hardly imagine that your original application requires that high
    accuracy. Last year I designed several current limiting circuits for
    hot swap applications and inrush current limiting and less then 5%
    accuracy without adjustment and cheap components will be very
    difficult.

    Best regards,

    Wim
    PA3DJS
    www.tetech.nl
    please remove abc from the address.
     
  9. "gearhead" <> schreef in bericht
    | Don't say the micrel chip is not a current sensor. It does current
    | sensing.
    | And current limiting is for the generator, not the battery.

    Hmm... Did you check the high-side current sensors I mentioned? As you know
    the datasheet of the mic5021 can you explain how you get a signal from it
    that is proportional to the measured current? I don't see it, but I like to
    learn.

    I am not aware of an automotive generator with a current limiter other then
    the fields. Normaly the generator is big enough to provide the required
    power for the vehicles appliances with a wide margin but yes, you can damage
    it by overloading it. Most of the times however you will blow one or more
    fuses when you try.

    petrus bitbyter
     
  10. gearhead

    gearhead Guest

    The MIC5021 does not put out a proportional signal, it trips at 50mV.
    Yes, maybe it would make the lights flicker if it were to toggle on
    and off too slowly. And I looked at the sensors you mentioned, okay?
    You need to know, this isn't an alernator. Are you familiar with the
    old-fashioned generators with the long, skinny armatures with
    segmented commutators? They are different from modern alternators in
    that they will burn themselves up if you overload them.
    Is English a second language for you?
     
  11. gearhead

    gearhead Guest

    My first dog was named Wimpie.
    And there was a Wimpie character in the old Popeye comic that used to
    say,
    "I will gladly pay you Thursday for a hamburger today!"
    Ah, Saturday mornings in front of the TV.

    Thanks for the tip about testing Vbe, it makes sense. I guess putting
    a bunch of diode-connected transistors in series would really be the
    best way, you could go down the line testing voltage and sort them out
    right quick.

    73 de KF2HI
     
  12. I'd pick up a surfboard and give up on the thruhole idea.
     
  13. "gearhead" <> schreef in bericht
    |
    | The MIC5021 does not put out a proportional signal, it trips at 50mV.
    | Yes, maybe it would make the lights flicker if it were to toggle on
    | and off too slowly. And I looked at the sensors you mentioned, okay?
    |
    | You need to know, this isn't an alernator. Are you familiar with the
    | old-fashioned generators with the long, skinny armatures with
    | segmented commutators? They are different from modern alternators in
    | that they will burn themselves up if you overload them.
    | Is English a second language for you?

    I remember those old things and the electromecanical circuits that
    controlled them, though I never got into their inner workings. So I do not
    know about current control and how it was done.

    English is not my native tongue.

    Consider a ZDX1009. It is SMD but large enough to be mounted on perf board.
    The datasheet shows the internal schematic which is very simple so you can
    also build it using discretes. You have to use matched transistors as others
    stated already.

    petrus bitbyter
     
  14. gearhead

    gearhead Guest

    You mean the ZDS1009, petrus.
    Check out that circuit in the datasheet -- if it doesn't leak, it
    won't turn on.
    The circuit below shouldn't have any problem starting up.
    Below Ilimit, the collector output will not source any current. Just
    as load current passes Ilimit the output collector should start
    sourcing a small current. I can use this collector current in a
    feedback loop with the voltage regulator and make the load current
    settle at Ilimit.



    V+----+-----Rs-------+-----+---load
    | | |
    Re | |
    | | |
    | | |
    \ PN2907 / |
    |---+----| |
    /| | |\ |
    / | \ |
    | | | |
    +------' | /
    | | |<
    | ,-------+--|
    | | | |
    \ | / \
    \| | |/ \
    |---+----| '---out
    /| |\
    < PN2222x2 >
    | |
    '------+-------'
    |
    current sink
    I = 2 Ilimit Rs / Re
    |
    |
    gnd

    For example, Isink = 200 uA, Re = 1k, Rs = .01, Ilimit = 10 A
    The current mirrors reach an equilibrium at Ilimit, with 100 uA on
    each side.
    Below Ilimit, the right side of the current mirror pulls up and the
    left side pulls down. The output transistor stays turned off.
    Above Ilimit, the left side pulls up and the ride side pulls down,
    drawing some current from the base of the output transistor.

    Whaddya think, it might work?
     
  15. gearhead

    gearhead Guest

    Correction: instead of a mirror it needs separate current sinks on
    the ground in order to work.
    And another transistor on the output for more gain, if necessary.
     
  16. "gearhead" <> schreef in bericht
    Correction: instead of a mirror it needs separate current sinks on
    the ground in order to work.
    And another transistor on the output for more gain, if necessary.

    Yes, I meant the ZDS1009.

    I gave a quick glance and I doubt this circuit will work. The way the
    outputtransistor is connected will ruin whatever the others might do. For
    instance, most of the current through the right current source will come
    through the BE-junction of that transistor. So the balancing for the PN2907
    will be gone. Besides, you will have to match both current sources which
    brings you directly to the original schematic. Sure, it will not start
    without leakage. As a matter of fact, that leakage is specified in the
    datasheet. All bipolar transistors I met so far, had some leakage. It's
    inherent to to physics of the transistor. Once amplified, it will become
    serious current which brings the circuit to live. I ever build a current
    sensor like that and it wordked though not very accurate. I used no matched
    or twins transistors and did not bother to make them thermal connected. At
    the time, it was good enough for my application.

    petrus bitbyter
     
  17. gearhead

    gearhead Guest

    Fred Bloggs turned me on to this circuit patented by Analog Devices:
     
  18. neon

    neon

    1,325
    0
    Oct 21, 2006
    use LM117 as a regulator and as a limiter for current just 3 resistorsno SPICE required just pencil and paper
     
  19. JosephKK

    JosephKK Guest

    It is not 1 percent, it is 1 percent per degree C. if you have a 10
    degree C temperature difference change you get a 10 percent unbalance.
     
  20. gearhead

    gearhead Guest


    At 300K and Vf=.6, a change of 0.1 degree would result in delta Vf =
    200 uV.
    At a current decade for every 60 mV, that introduces an error (added
    to existing offset) of 10^.003333 = 1.0077 or about eight tenths of
    one percent.
    At Vf = .7, it comes out closer to 0.9 percent.
    So I get a round figure of about one percent error in the current
    mirror for each tenth of a degree temp mismatch centigrade in the
    transistors.
     
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