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Favorite reverse bias protection for battery circuits

Discussion in 'Electronic Design' started by Charlie E., Mar 16, 2010.

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  1. Charlie E.

    Charlie E. Guest

    Hi All,
    Still working on my color reader, and getting close. Biggest problem
    is that now I have almost TOO much drive on my LEDs, but that I can
    deal with!

    Now, to the latest problem. My system is for the visually impaired. I
    just put the batteries in one of my prototypes backwards, and it
    didn't last very long. While I expect this device to have very long
    battery life, since it normally doesn't operate but for a second or
    two, it looks like the MC1253's don't like getting reversed biased
    very long. So, for my design question of the day - What is your
    favorite circuit for preventing reverse bias from the batteries? The
    old diode trick drops too much voltage, so wondered if there are any
    clever FET tricks to block backwards voltages.


  2. BillyGates

    BillyGates Guest

    Yes, there are fet "tricks" that when reverse biased bias the fet to turn it
    off. It is essentially an "active diode".
  3. Hammy

    Hammy Guest

    I usually use #2 on this page a shorting shottky. Polarity Protection for DC/DC Converters.html
  4. Rich Webb

    Rich Webb Guest

    This Maxim appnote
    describes the pros and cons of several standard approaches.
  5. Joerg

    Joerg Guest

    That renders the unit dead and a blind or almost blind person will have
    a hard time finding or buying the proper fuse and installing it.

    Charlie: Solution #3 is the ticket (look on Hammy's link). But make sure
    that the FET is guaranteed to be fully turned on at the lowest allowed
    battery voltage, IOW the point where an UVLO comes on.
  6. Joerg

    Joerg Guest

  7. D Yuniskis

    D Yuniskis Guest

    Hi Charlie,
    Solve this problem *mechanically*. You need to prevent the
    batteries from being *installed* wrong, in the first place.
    E.g., any sort of circuit that protects (the rest of) the
    circuit from reversed battery won't do anything to tell the
    VI user *why* your device isn't working. Are the batteries
    *dead*? Installed wrong? Corroded terminals? Or is the
    *device* broken??

    "Key" the battery holder so the user can feel the correct
    orientation for the batteries *and* so it prevents the
    cells from making electrical contact if not oriented

    Think of how the *user* is going to interact with your device.
    Close your eyes and "figure out" why it doesn't work :-/
  8. Hammy

    Hammy Guest

    Thats what they have seeing eye dogs for:) Jokeing.

    Of course your right for a blind person either #3 or a keyed battery
    connection. With a brail instruction manual
  9. Joerg

    Joerg Guest

    Swallowed by ON, just like AMI was :-(
  10. Joerg

    Joerg Guest

    Ok, I'll ask our new Labrador when she is back :)

    She went through the first three phases of guide dog training and then
    had to be discharged for a medical condition (soft trachea, happens a
    lot with guide dog Labs).

    It's actually even better these days. A nearly blind relative has a
    Casio watch that talks.
  11. Charlie E.

    Charlie E. Guest

    Would be the best result, but these are standard AA cells, no way to
    key them without using expensive custom battery packs. Not the way to
    go for inexpensive...

    Will be looking for a good PMOS FET that will still conduct with two
    low AA batteries, probably around 1.8VDC, if they make one! ;-)

    Thanks All!

  12. D Yuniskis

    D Yuniskis Guest

    Hi Charlie,
    I take it you aren't making a custom (molded) case?
    Why not purchase COTS battery holder that *is* "keyed"?
    If you go that route, consider how your design can tell the user
    "yes, I am working" vs. absence of that indication so the user
    can deduce "something is wrong with the batteries or their
    installation" (or the device)
  13. Hammy

    Hammy Guest

    Are you a foster dad for pups?
    You must dread having to give them up?

    I just took my Golden Retriever to the vet. He started to develop a
    sunken eye. The vet doesn't know the cause yet. I'm hoping its nothing
    to serious.
    That's nothing look at Hawkins chair that dude is wired for sound and
    then some. :)
  14. Joerg

    Joerg Guest

    No, we are puppy sitters. So we jump in when a family goes on vacation
    or visits a sick relative where they can't take the guide dog puppy.
    Even then it's tough when they have to go back to San Rafael, especially
    for my wife since she sees them every week during trainings. But when
    you see them paired up with a blind person you know it's all worth it.

    We used to also sit the one that came home now. She instantly remembered
    our other dogs, where everything is in the house, etc.

    Hopefully not. Our Rottweiler is now at an age where it's just a matter
    of time :-(

    No idea what a Hawkins chair is.


  15. Charlie E.

    Charlie E. Guest

    He's talking Stephen Hawkins (sp?) the physicist with MS.

    Well, my wife's computer talks to her, she has two watches that talk
    (one red, one blue) a talking calculator that includes a calendar and
    timer function, and another talking calculator that just does math...

  16. Hammy

    Hammy Guest

    Mine just turned 9. My last one was 11 when he died.
    He's that British theoretical physicist who came up with a theory for
    the origin of the universe among other things.

    He has amyotrophic lateral sclerosis. So he uses a chair with all
    sorts of gadgets on it.
  17. Joerg

    Joerg Guest

    Ah yes, Stephen Hawking:

    Once a disabled person masters technology it can add so much to their

    Same with non-tech skills. A neighbor is deaf and he and his brothers
    can "talk" from vehicle to vehicle without any technology, by signing.
    It's amazing. But he also uses video phones etc.
  18. Joerg

    Joerg Guest

    Anything other than AA cells is expensive. And as Charlie wrote AA cells
    are difficult to key without going to precision molding or milling.

    Very easy: Oscillator that is connected via a diode so it works only
    when batteries are reversed, drives piezo. Battery inserted the wrong
    way -> beep ... beep ... beep ...

    If you want to also cover the case where only one of the batteries is
    reversed use two of those, can drive the same piezo.
  19. Joerg

    Joerg Guest

    Why PMOS?
  20. D Yuniskis

    D Yuniskis Guest

    Hi Joerg,
    Yes. Find a battery holder that someone *else* has incurred
    the cost of that molding! Preferably, something that lets
    the user access the "battery compartment" without having
    to disassemble the device itself.

    OTOH, if he is making "one off", a bit of plastic glued on either
    side of the '+' holder terminal can do the trick.

    I'd also look for those (camera?) batteries (two cells side
    by side... 3V?) as I think that is keyed (at the very
    least, it eliminates the problem of putting one cell in
    correct and one backwards)
    You don't need to be that extravagant. Just something in the
    *normal* usage of the device that people can count on for
    feedback/confirmation. E.g., if the device uses audio
    output (synthetic speech or just "tones"), then design
    the device so that immediately on turning on, it "chirps"

    [I don't recall what he has chosen for his output medium;
    personally, I would opt for limited vocabulary speech as
    it can be *very* small]

    This gives a positive indication to the user in *normal*
    operation (i.e., those times when you HAVEN'T just
    changed batteries) that the device is, in fact, "on".
    And, knowing to expect this, it can be used when replacing
    batteries to act as a confirmation that the batteries
    are, in fact, installed correctly, aren't "dead", etc.
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