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What type diode for battery reversal protection?

Discussion in 'Electronic Basics' started by royalmp2001, Feb 26, 2005.

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  1. royalmp2001

    royalmp2001 Guest

    Do I need a signal diode (like IN4148) or a rectifier diode (like
    IN4001) to add to the +ve line to protect against polarity reversal?
    Would the available supply after the diode be 0.6V less? Is there any
    way of avoiding this?
    Thanks, everyone
  2. At rated current, it may be more like 1 volt less. If you use a
    schottky diode it will be about half as much loss. If you use a
    mosfet hooked up backwards (turned on channel in parallel with the
    body diode when correctly connected), it can be considerably less than
    that, but you may have to complicate the circuit a bit to provide
    spike protection for the gate. What supply voltage and load current
    are we talking about?
  3. John Popelish wrote:

    How does one go about protecting the gate? I heard about this trick in
    passing from one of those "Bob Pease" online lectures, and was
    intrigued, but he just said 'fet', and didn't go into detail, so I never
    too the time to figure out what it was about.

    It seems like a useful technique. I guess just ensuring that the gate
    doesn't go outside it's max rating limits is enough.

    It also seems like somebody would offer one of these as a discrete
    protection device, with built in limiting for the gate, and datasheet
    characterization for max voltages, Ron, currents, etc. Or, do the normal
    specs work for this backward connection?

    Robert Monsen

    "Your Highness, I have no need of this hypothesis."
    - Pierre Laplace (1749-1827), to Napoleon,
    on why his works on celestial mechanics make no mention of God.
  4. Rich Grise

    Rich Grise Guest

    Yes - never discharge your pack to the point that any cell reaches
    zero volts. By the time the reverse voltage across the cell reaches
    a diode drop, the cell has already been destroyed.

    In other words, don't do it.

  5. Don Bruder

    Don Bruder Guest

    Sorry, Rich, but I don't think your answer is very useful...

    Unless I'm misunderstanding him horribly, he's looking for info on what
    to use as what I've always heard called "steering diodes", so that no
    matter which way a battery/battery pack gets hooked to the circuit, the
    polarity is correct, not info on keeping a dead cell from reversing a

    What I've always done (in non-weight-critical applications) is "over
    design" the power supply (Add an extra cell, ferinstance) appropriately
    to compensate for the diode drop that will be involved, and then plug
    the power (Either AC or DC...) into a bridge rectifier's inputs. Put DC
    on the inputs of a bridge, and it's going to put out DC of the right
    polarity on the output pins regardless of which way the supply voltage
    is hooked up. Free bonus: If your circuit doesn't have a problem
    handling AC ripple, you're also set to just hook up the correct voltage
    AC (from an ultra-cheap wall-wart, for instance) to the same pins you'd
    hook the battery to. If it does have a problem with ripple, a couple of
    caps and coils "downstream" from the power input can easily clean things
    up to tolerable when used with AC, yet sit there quietly doing nothing
    if the device is being fed DC.
  6. Rich Grise

    Rich Grise Guest

    OOpps! Yes, I've reread the original post again, and you're absolutely
    right. Please disregard what I said.

  7. I just had to design one of these for a device that runs on 4 AA
    cells. I added 100k in series with the gate to the normally positive
    battery clip, drain normally negative clip and source to circuit
    negative rail and a 6V8 zener, anode to source, cathode to to gate.
    Since people may be touching the cells as they touch the battery clip,
    I had to allow for body capacitance discharge into the clips without
    zapping the anti reverse fet switch. Put the cells in backward and
    you have only 100k in series with a forward biased zener across the
  8. royalmp2001

    royalmp2001 Guest

    9V Battery supply and about 7mA current
  9. royalmp2001

    royalmp2001 Guest

    Er...Rich...its a rectangular 9V Battery actually
  10. I would probably use a 1N5817 schottky in series. At this current,
    the drop is only about 0.2 volts.
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