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AC adapter input resistor keeps blowing.

Discussion in 'Electronic Basics' started by Hobbyist, Dec 15, 2012.

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  1. Hobbyist

    Hobbyist Guest

    My son bought a cheap Chinese light display unit which worked
    fine at first but refused to turn on after it had been turned off
    for some time. I did notice that the AC plug was a little loose
    in the socket. I opened the separate 5V 1A switched-mode power
    adapter and sure enough, the inrush limiting resistor was open.
    It's a 1-ohm TH unit and looks like a 0.25W carbon type. I had a
    0.5W piece handy and used that. Before putting the PCB back into
    the adapter housing, I connected it to an AC outlet (not the same
    one) and turned it on, and it worked. However, after assembling
    the adapter, I carelessly plugged it into a live socket, i.e.,
    one that was alreasy switched on (without connecting it to the
    display unit. The same resistor went phffft! and the adapter went
    dead again.

    My question to the experts is: Is it normal for the adapter to be
    so intolerant of the contact bounces it must have encountered? Is
    it likely that there's some other defect? The resistor goes to a
    bridge rectifier and the main DC filter is a 2.2uF 400V electro.
    I thought that it wouldn't have enough inrush current to keep
    blowing the resistor even though I *was* rather careless with the
    plugging-in. Please note that I'm asking this to gain a better
    understanding of the technical reasons rather than a desire to
    save a few bucks by repairing the adapter.
  2. Phil Allison

    Phil Allison Guest

    "Hobbyist" ..

    ** That resistor is bound to fail.

    In a 230V country, the peak surge current is about 150 amps.

    A 22 ohm, 1W fusible type is required - or a small slow blow fuse.

    .... Phil
  3. Hobbyist

    Hobbyist Guest

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