bad power supply? bad multimeter? user error?

Discussion in 'The Projects Forum' started by slipstick, Jul 21, 2006.

  1. slipstick

    Thread Starter New Member

    Jul 6, 2006
    6
    0
    I'm confused by the multimeter readings I'm getting from a wall wart power supply.

    Manufacturer says:
    OUTPUT
    12 Vdc

    Multimeter readings:
    OUTPUT
    5.67 Vdc <--measured dc voltage
    11.7 Vac <--expected voltage but it's AC

    Would a bad diode leave the AC unrectified (outputting 12 Vac rather than 12 Vdc)?

    PS I have a cheap multimeter, with instructions in Chinese. I could be using it wrong.

    thanks.

    willis
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    Some of the problem is the cheapo meter. Most of it is in the wall wart. All you get is a diode for DC - the filter cap is up to you in the equipment. That's why your meter reads mostly AC out of it. Put a capacitor across the output and it will look more like 12 VDC.
     
  3. windoze killa

    AAC Fanatic!

    Feb 23, 2006
    605
    24
    Try putting the output under load. Sometimes you won't be able to measure the correct voltage unless the output is supplying enough current.
     
  4. Gadget

    Distinguished Member

    Jan 10, 2006
    613
    0
    Is it a Switch Mode unit..? (can often tell by weight as no big transformer, i.e. lighter than normal, and also often multi voltage input).
    The high frequency components that may still be present on the output of one of these can cause some really Bizzare readings on many digital meters.

    All other comments just as valid as well.
     
  5. slipstick

    Thread Starter New Member

    Jul 6, 2006
    6
    0
    Yes. It's a switch mode. Good point.

    I tried starting it under load (1 M Ohm resistor). Same readings, adding resistors did give me a voltage drop.

    I also tried the biggest capacitor I have (4700 u F). No change.

    Do you think I need a better multimeter?
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    Try a smaller load resistor - something that will draw at least 10% of the rated currrent. Any cap over 10 microfarad will do.

    Do you have another DC supply anywhere you can use to see if your meter is working properly? That's cheaper than buying a new one.
     
  7. Gadget

    Distinguished Member

    Jan 10, 2006
    613
    0
    As a lot of the garbage on Switch Modes is fairly high frequency, try a 100nF disc ceramic and possibly also a 10uF tant....??. The pack may also be radiating a little which may also upset things.
    There is always the possibility it may be faulty as well.
     
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