12v car chargers output voltage

Discussion in 'General Electronics Chat' started by decworld, Jul 31, 2011.

  1. decworld

    Thread Starter New Member

    Jul 31, 2011
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    Hi,
    I have a genuine motorola car charger to charge an xoom that requires 12v.

    The charger , model ASMMZ601CARCHR-TRI9A, is stated as 12v and 24v input compatible.

    However, I put my meter across the output of the charger with 19.5v in 19.5v comes out. The xoom requires 12v in.

    Anyone understand why this is possible and its safe to use.

    Thanks,
    Dec
     
  2. tom66

    Senior Member

    May 9, 2009
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    Did you properly connect all grounds while testing?
     
  3. decworld

    Thread Starter New Member

    Jul 31, 2011
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    Hi,

    I am connecting the meter across the output only, is this incorrect ?

    Thanks,
    Dec
     
  4. tom66

    Senior Member

    May 9, 2009
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    Are you connecting the ground of the meter to the outer sheath of the connector (or otherwise, to the minus terminal) or to the battery/car ground?
     
  5. decworld

    Thread Starter New Member

    Jul 31, 2011
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    To the outer sheath of the connector.
     
  6. tom66

    Senior Member

    May 9, 2009
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    In which case I would return the charger as it sounds like it is faulty.

    One thing - some cheap (really cheap) SMPS buck/boost converters tend to have a higher output voltage under no load. Can you find, say a 100 ohm resistor, and connect it to the output, then measure the output. The resistor will dissipate 1.44W, so use a 3W wirewound (not a small 1/4W resistor - it will melt) - you can find them at Radioshack / Maplins for cheap.
     
  7. decworld

    Thread Starter New Member

    Jul 31, 2011
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    I wonder if Motorola figured that as it will be connected to around 12v, and the device requires 12v so that no modification to voltage was required.
     
  8. decworld

    Thread Starter New Member

    Jul 31, 2011
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  9. tom66

    Senior Member

    May 9, 2009
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    Is your car 12V? 19.5V is too high for a 12V car electrical system and it's too low for a 24V one(!)
     
  10. decworld

    Thread Starter New Member

    Jul 31, 2011
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    My car is 12v.

    But, I wanted to use my 15v laptop charger to power the xoom instead of carrying an extra charger. So using the car charger to get 12 from 15v.

    This is why I tested higher voltages, a 15v and 19.5v supply.
     
  11. iONic

    AAC Fanatic!

    Nov 16, 2007
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    Forgive my ignorance, but what is a xoom? Is it a battery or a device you want to provide 12V to? A 12V battery charger that provides only 12V will not charge the battery as the voltage must be higher than the battery voltage to provide current flow. It is typical for a battery charger voltage to be around 15V during the bulk charging phase.

    On the other hand, if the "xoom" is a electronic device that requires 12V to operate, you absolutely do not want to try to operate it with a battery charger.
     
  12. tom66

    Senior Member

    May 9, 2009
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    Seems like all that might be in there is a few filtering components and a fuse, if that... pretty cheap of Motorola. If it works on 24V like it works on 12V, you should be able to get away with up to 24V. But no guarantees.

    Is the Xoom a phone or another device?
     
  13. decworld

    Thread Starter New Member

    Jul 31, 2011
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    The xoom is a tablet from motorola.

    It requires 12v input @ 1.5a, as per the mains charger. The battery inside is 7.2v.
     
  14. iONic

    AAC Fanatic!

    Nov 16, 2007
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    The xoom charger that came with the xoom, what output voltage is measured here?
     
  15. tom66

    Senior Member

    May 9, 2009
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    I guess the Xoom has a wide range input but I think it's quite sneaky of Motorola to sell that adapter at £20 when it probably costs 50p to make!
     
  16. decworld

    Thread Starter New Member

    Jul 31, 2011
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    The 12v original mains charger reads 12.15v on my meter.
     
  17. tom66

    Senior Member

    May 9, 2009
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    That's because it must be regulated. 230V is 325Vp-p, which would probably cause the release of a lot of magic smoke from the Xoom, so it does actually have to have some circuitry inside it.
     
  18. vicky1212mark

    New Member

    Jul 31, 2011
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    thank you for post keep posting stuff like that...............
     
  19. iONic

    AAC Fanatic!

    Nov 16, 2007
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    Then I wouldn't put 19.xxV into it.
     
  20. kanoji_cb

    New Member

    Jul 25, 2011
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    As per my knowledge, the battery chargers are giving output of rectified A.C. and it is unregulated D.C. The unregulated voltage will be reduced on putting load. You might have checked the output voltage without putting load. As tom66, Senior member suggested you can test the voltage by putting a load resistor of 100 ohm/3watt wire wound resistor accross the terminal. Please note that this is not a modification to your motorola charger. If you check the output voltage after putting load, then it may show correct reading.
     
    Last edited: Aug 1, 2011
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