Wall-wart mania!

Discussion in 'General Electronics Chat' started by iONic, Nov 3, 2013.

  1. iONic

    Thread Starter AAC Fanatic!

    Nov 16, 2007
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    OK, I'm stumped
    I have a wall-wart rated: 120V AC input with an output of 9V @ 500mA.

    For some reason my multimeter (which I have checked to be accurate) is reading nearly 23V DC. Unplugged after nearly a week it was reading 3.xV.

    What's up with that!

    essentially it has primary and secondary turns, 4 diodes, and a capacitor. Since it is unreliable I opened it. Nothing looks toasted.

    Any possible explanation for this?
     
    Last edited: Nov 3, 2013
  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Some wall warts need a load (50 tp 100 mA) before they start regulating. Evan a big computer atx power supply needs a minimum load of a half amp or so.
     
  3. iONic

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    Nov 16, 2007
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    Regulating? I'm not sure this is the case with this thing. The diodes exist to convert to DC and the capacitor is for smoothing and/or noise. Other than that the turns ratio for the transformer is fixed, unless someone stabs it with a knife, creating a short/different turn ratio.
     
  4. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    From post #1 it's clear there is no regulator. The secondary may be wound such that the alleged 9V output occurs only at the max 500mA load. A load current less than that may well result in an output voltage much higher than 9V.
    As for the 3.xV after a week, capacitors can hold charge for a surprisingly long time if their internal leakage is low.
     
  5. iONic

    Thread Starter AAC Fanatic!

    Nov 16, 2007
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    Your right about the cap and 3.xV after a week, did not think of that initially.

    Not sure I ever had a wall adapter, especially a wall-wart, measure that far from it's rated voltage....always within a few volts and usually higher.
     
  6. MaxHeadRoom

    Expert

    Jul 18, 2013
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    That was typical of the older simple linear types, They usually relied on the rated load to bring them down to the rated voltage.
    All the ones I have seen and those I picked up in the last while have been well regulated switching types.
    Max.
     
  7. Mike33

    AAC Fanatic!

    Feb 4, 2005
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    If you hooked it up to a 20 ohm/5W or greater resistor, you'd likely see about 9V....that's the nominal voltage with the specified load, and the wart is not regulated. The actual load would be 18 ohms, not a standard value....
     
  8. iONic

    Thread Starter AAC Fanatic!

    Nov 16, 2007
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    Well, I am concerned as I connected this "wart" to a LED device that was supposed to accept between 7.4V and 14.xV. I used the "wart" one or two nights while taking photographs. The following day the device did not turn on either with the "wart" or with the Lithium battery rated for 7.4V. It was the second LED device I purchased (Polaroid), the first one did not function at all from the get-go.

    Love the light when it did work. It could vary the brightness and color balance of the light. It has 140 LED's 70 warm, 70 cool. 3300K - 5500K

    Not sure I want to give it a 3rd try! They cost $90 from Amazon.
    http://www.amazon.com/gp/product/B007CM4TOM/ref=oh_details_o00_s00_i00?ie=UTF8&psc=1
     
  9. tubeguy

    Well-Known Member

    Nov 3, 2012
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    Yes, simple wall warts usually supply a considerably higher voltage unloaded than when loaded. But 23 volts unloaded from a 9 volt rated supply seems unusually high. I would expect voltage maybe 30 to 50 % higher than rated with no load.

    If you can, test it with the suggested load of around 20 ohm's. The resistor will dissipate over 4 watts. A 5 watter will get VERY (burn your fingers) hot.
     
  10. iONic

    Thread Starter AAC Fanatic!

    Nov 16, 2007
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    Too late, when I removed the casing, I busted one of the 120V leeds...not going to fool around with that and the wall outlet at this point.

    Bottom line, I wanted to assure myself that my wall adapter did not bur out the regulator of the LED unit I just returned. It doesn't possible for it to have done so.
     
  11. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    My guess is the wall-wart is the culprit and did burn out the light unit :(. Perhaps it's wrongly labelled as 9V, or if it's a switcher then its internal feeback is incorrect?
     
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