Power Supply prob

Discussion in 'General Electronics Chat' started by lordratner, Jul 7, 2013.

  1. lordratner

    Thread Starter New Member

    Jul 30, 2012
    20
    0
    Hi all,

    I have an AC to 12v DC power supply that is passing AC voltage (anywhere from 20-120 volts depending on what it touches). I've used it with no issues in the US (120v AC) but now in the UK (240v AC) the PS makes a faint whining sound and produces the AC voltage. It is still producing the 12v DC with no issues, and the PS is rated for 110 - 240 v input.


    EDIT: Some more information from testing. It turns out the PS passes AC voltage with 120vAC input too. At 120vAC input it passes a ~40vAC current, and at 240vAC input it passes a ~80vAC current. This current is enough that I can feel it if I touch it (moreso with the 240v input, which makes sense). I measured it with a multimeter at about .6 amps. I thought maybe something else inside could have been creating the AC current, but when I disconnected the PS from the AC input, the 40/80vAC random current vanished.

    Any ideas?

    Thanks!
     
    Last edited: Jul 7, 2013
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,998
    745
    Is this a Switched mode psu , like for a mobile phone charger or computer psu?
     
  3. lordratner

    Thread Starter New Member

    Jul 30, 2012
    20
    0
    Yes, I believe I stole it out of the wall wart for a wireless router or something like that
     
  4. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,998
    745
    They do whistle and buzz depending on the quality of the pcb build, if its a plastic case its safe , if its a metal case make sure its earthed.
     
  5. lordratner

    Thread Starter New Member

    Jul 30, 2012
    20
    0
    Yeah, the whistle isn't a huge concern of mine, but the 80vAC thats coming from the PS is the issue. Is this normal, or should I just scrap the PS?

    If I have to replace it, what is a good source of 12v power supplies (1-2A)? I don't have many spare wall warts to rip apart
     
  6. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,998
    745
    So your saying that you have an 80v ac ripple on the DC output side? If thats the case there is a small capacitor on the mains negative line to the DC negative line inside, remove that and it will remove the ripple.

    Better still BIN IT and buy a descent psu.
     
  7. crutschow

    Expert

    Mar 14, 2008
    13,027
    3,236
    Measure the resistance between all input and all output leads. If it's in the megaohm region then the supply is probably sufficiently galvanically isolated but may have capacitive coupling between input and output that is causing the observed voltage. If you use the device, ground the output common to the wall safety ground. That will minimize any possible danger from the leakage voltage.
     
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