I used a 9V power adapter on a 12V power device

Discussion in 'General Electronics Chat' started by MechRocket, Aug 20, 2005.

  1. MechRocket

    Thread Starter New Member

    Aug 20, 2005
    Hi, I did something really stupid. On my portable LCD screen that is powered by a 12V power adapter, I plugged in a 9V power adapter and now it won't work anymore. Is there any way I can fix it? My dad said something about a fuse probably being blown or something and we might be able to fix it by replacing it with another fuse from Radio Shack.

    Is he on the right track? If not, is there anything I can do?

    Thanks so much.
  2. pebe

    AAC Fanatic!

    Oct 11, 2004
    Plugging in an adapter with a lower voltage would not generally harm the unit. But if the polarity is reversed then it probably would cause damage and that may be what has happened.

    It may just have blown a fuse.
  3. MechRocket

    Thread Starter New Member

    Aug 20, 2005
    If I take apart my LCD screen, where should I look to see if the polarity has been reversed? Thanks.
  4. thingmaker3

    Retired Moderator

    May 16, 2005
    To check polarity, you will need to use a voltmeter to measure both the 9Vdc and 12Vdc adapters. If the polarity of the 9Vdc supply is opposite of the 12Vdc supply, then you will probably need to replace the LCD screen. :(
  5. CATV


    Apr 17, 2007
    I have made this mistake and generally the device has a fuse to avoid damage when a reverse current is applied. If you are handy with PC boards, look for this open fuse (sometimes looks like a tiny resistor) and test to see if it is open. If so, replace it.
  6. Salgat

    Active Member

    Dec 23, 2006
    Just a quick question, why wouldn't a diode be used inside the LCD Screen to prevent a problem like this from happening? (from the engineering standpoint)
  7. pfofit


    Nov 29, 2006
    Sometimes a diode is used.

    Some wallwarts will show the polarity on the wallwart in a little pictorial.
  8. goodbyegti

    Well-Known Member

    Apr 28, 2004
    Maybe you used an unregulated PSU? These often devlop voltages far higher than the specified output voltage under low load. When the monitor is in standby it presents little load on the PSU. Monitors usually use switch mode PSUs which are regulated.