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Polarity reversal protection,how to??

Discussion in 'Electronic Design' started by Antonio Spedicato, Nov 12, 2004.

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  1. Hy, i would to build a polarity protection for my dive light (12 volts
    nimh batteries) but i don't known ho to do it!!
    Can someone help me?
    There are 10 batteries 1,2 volts 3000 mah serial installed.
  2. mike

    mike Guest

    Protection from what?
    Does the light care?
    Batteries in wrong? Use mechanical means.
    Charger backwards? Use dedicated cable.
    Other??? use fuse.
    It can obviously get much more sophisticated if you can
    describe exactly what you're trying to protect against.
    100% protection is MUCH harder than 90%.

    Return address is VALID.
    500MHz Tek DSOscilloscope TDS540 $2200
    Wanted, 12.1" LCD for Gateway Solo 5300. Samsung LT121SU-121
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  3. Tam/WB2TT

    Tam/WB2TT Guest

    Not sure why a lamp would care about polarity, but assuming you want no
    voltage drop, you could use a fuse, and a shunt diode to blow it if it is

  4. Fred Bloggs

    Fred Bloggs Guest

    I think we have posted this 10,000 times in the past:

    View in a fixed-width font such as Courier.

    | | |
    | | |
    | | +-+
    --- | |L|
    - BATT | |O|
    | | |A|
    | | |D|
    | | +-+
    | G |
    | ----- |
    | - - - |
    +------------D S-------+

  5. I forget a word......polarity reversal, this is the problem.
    For me is hard to do it..can you expplain me how i can protect batteries?
  6. mike

    mike Guest

    You need to remember MORE words. Polarity reversal where?
    When installing the batteries?
    When connecting a charger?
    Some other thing we don't know about?
    Might, depending on your objectives, be helpful to know the
    current drain and technology of the lamp, depending...maybe...who knows...

    Making some grand unsubstantiated assumptions, I'd say put the batteries
    in, put a screw in the cover so some idiot can't get them out and use
    a polarized charging connector.

    Now, there are a whole bunch of issues surrounding charging. But you
    haven't asked that...

    Then there's the issue about leaving the switch on and reverse charging
    some of the cells. Low voltage cutout is likely to be more of a
    necessity than putting the cells in backwards.

    I'm starting to rant, but what the heck, bits are almost free...
    Rechargeable batteries are unsuited to use with flashlights. And here's
    why. You need to turn the light off IMMEDIATELY at the first sign of
    dimming to prevent cell reversal. If you haven't charged it in a week,
    this may happen almost immediately when you try to use it.
    So, do you want the light to turn itself off to protect the
    batteries? Or do you want it to stay on as long as possible to help
    you through the emergency?
    "He was hit by a car in the dark...but the good news is that the
    batteries in his flashlight were protected".
    Return address is VALID.
    500MHz Tek DSOscilloscope TDS540 $2200
    Wanted, 12.1" LCD for Gateway Solo 5300. Samsung LT121SU-121
    Bunch of stuff For Sale and Wanted at the link below.
  7. Joop

    Joop Guest

    When one wants to connect multiple devices to the same battery and
    share a common and quality ground (HF equipment) it would be desirable
    to 'switch' in the + line.
    Do you perhaps have a simple solution for this as well? I was thinking
    along the lines of moving this mosfet to the + line and use an helper
    voltage generated by some inverters to lift the gate way above the
    battery level.
    But when things get too complex, a relay would become a simpler, but
    less energy effecient solution.

  8. Fred Bloggs

    Fred Bloggs Guest

    Just use a P-channel FET with SOURCE terminal on the LOAD, GATE to gnd,
    and DRAIN to Batt(+).
  9. Brian

    Brian Guest

    If you want to keep a battery (in a series string), from developing a
    reverse polarity, put a diode across each battery. Hook the cathode of the
    diode on the positive side of the battery and the anode on the negative side
    of the battery. I think this is what you are looking for.
  10. James Meyer

    James Meyer Guest

    By the time a cell has discharged to the point that a diode will turn on
    and "protect" it, it is likely to be already ruined.

  11. Brian

    Brian Guest

  12. Wich type of diode and wich value?
    Can you send me in pvt some picture about it?
  13. Brian

    Brian Guest

    You could use something like a 1N5820. Go to and
    type in 1N5820 (in the search box), it will take you to where you can get
    the data sheet.
  14. Brian

    Brian Guest

    While the diode protection method works, the best all around method is to
    match the batteries in series. This way, they all die at about the same
    time, and will give you the longest running time per charge.
  15. Fred Bloggs

    Fred Bloggs Guest

    Right- the deep discharge limit for NiMH is +1.0V and not -1.0V or
    thereabouts on a bypass diode at 3Amps. This type of information is
    available in the various handbooks from manufacturers like Panasonic.
    It would be better to use batteries with reverse battery and
    overdischarge protection built-in:
  16. Active8

    Active8 Guest

    +-+-- +
    A A
    + or - ---------+ |
    - + ---------(-+
    A A
    +-+-- -
    created by Andy´s ASCII-Circuit v1.22.310103 Beta
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