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Diode for reverse polarity protection

Discussion in 'Electronic Repair' started by Ramset, Jun 13, 2007.

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  1. Ramset

    Ramset Guest

    I fried a diode used for protection in a Bearcat scanner. It was fried
    by connecting the wrong power cord (reverse polarity) into it.
    The AC adapter puts out 13.8VDC and 700ma. I tried replacing it with a
    IN4001 and it powered up the display and started scanning. However
    when I plugged the speaker and display light back in it didn't seem to
    have enough power to start it all up. The display flashes once and I
    hear a pop in the speaker but that's it. The original diode had
    markings printed which I think was RA above VB.
    Any suggestions on another diode to try?

    Thanks
    Kevin
    Minneapolis
     
  2. mike

    mike Guest

    If it's a reverse protection diode across the power supply,
    it ain't your problem. More likely, you fried the supply
    when you shorted it out with the diode.
    mike
     
  3. Or just the power supply is inadequate to the task, assuming it is a
    replacement.

    Mark Z.
     
  4. Meat Plow

    Meat Plow Guest

    The diode is ok.
    --
    #1 Offishul Ruiner of Usenet, March 2007
    #1 Usenet Asshole, March 2007
    #1 Bartlo Pset, March 13-24 2007
    #10 Most hated Usenetizen of all time
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    Pierre Salinger Memorial Hook, Line & Sinker, June 2004
    COOSN-266-06-25794
     
  5. UCLAN

    UCLAN Guest

    You might want to give us the model number of the scanner. Many Uniden
    scanners use REGULATED power. It could be that when loaded by the
    speaker and display light, the AC adapter no longer provides the needed
    voltage.

    My BCD396T uses a 6vdc 800mA REGULATED adapter. I don't think the use
    of the 1N4001 is your problem. I think that a) the AC adapter was
    damaged by the incident, or b) the scanner was damaged by the incident.
     
  6. Ramset

    Ramset Guest

    The scanner is a BC780XLT. Today ,after work, tried turning the
    scanner on and got the same response, the backlight blinked and I
    heard a pop in the speaker. I unplugged the speaker and the turned the
    scanner on. The display and its backlight came on and it started
    scanning. I plugged a remote speaker into the jack on the back panel
    and got audio. I disconnected the remote speaker and plugged in the
    chassis speaker and it produced audio. All the scanners functions are
    now working normally. I figure the new diode is right and I know the
    power supplies, both the 12V and the AC adapter, are good because they
    work on an identical scanner. For now I'm going to leave it alone and
    if I have more problems I'll check my soldering work. Thanks for all
    the responses.
     
  7. m kinsler

    m kinsler Guest

    There are two kinds of reverse-polarity-protection diode. Or, rather,
    two arrangements for them. One has the diode in series with the
    device, so if you connect it wrong the device simply won't energize.
    The problem with this is that the diode drops about half a volt, and
    that's a big chunk out of a 12v source. So for car stuff what they do
    is to jump the diode across the power line. That way, if you connect
    the power backward, the diode will conduct and blow the fuse. If
    there is a fuse. Otherwise, the diode burns up, smelling like you've
    just burned up the Lord's original microprocessor. If you remove the
    diode completely, the device will work once again, but there will be
    no further protection.

    Ergo, it's easy to get confused with these things. Back in the days
    of Kinsler Hi Fi Service I'd connect a diode in series with the power
    supplies of CB radios that I thought would be receiving abuse. The
    performance would suffer only a bit, and the device would keep
    working.

    M Kinsler

    delighted to be out of that business
     
  8. JANA

    JANA Guest

    This sounds like the power supply is not adequate, or has a failure, thus
    causing too low a supply current.

    If you put your volt meter across the power supply output while running the
    scanner, and if you see a dip in the voltage, you will know for sure that
    the power supply is not proper. You can also do a current test to see how
    much the scanner is pulling. Refer to the service specs to see what the
    maximum current pull is for your scanner, and then compare to know if there
    is a fault.

    --

    JANA
    _____


    The scanner is a BC780XLT. Today ,after work, tried turning the
    scanner on and got the same response, the backlight blinked and I
    heard a pop in the speaker. I unplugged the speaker and the turned the
    scanner on. The display and its backlight came on and it started
    scanning. I plugged a remote speaker into the jack on the back panel
    and got audio. I disconnected the remote speaker and plugged in the
    chassis speaker and it produced audio. All the scanners functions are
    now working normally. I figure the new diode is right and I know the
    power supplies, both the 12V and the AC adapter, are good because they
    work on an identical scanner. For now I'm going to leave it alone and
    if I have more problems I'll check my soldering work. Thanks for all
    the responses.
     
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