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Mains adapter for a well known LED light company

Discussion in 'Troubleshooting and Repair' started by Noobster, Nov 22, 2013.

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  1. Noobster

    Noobster

    18
    1
    Oct 28, 2013
    Hello

    I'm trying to repair a mains adapter for a well known LED light company which has stopped working 2 years from purchase. It's a switching power supply, input 100-240v 47-63Hz AC, 0.1A. Output 4v 100mA DC.

    Opening up the device it was quickly apparent that two of the electrolytic capacitors had burst (E-Cap +-20% 3.3uF 8x12mm 400V).

    Having replaced the obviously leaking caps I still cannot get a voltage reading from the output plug.

    There are two more E Caps that I can see, but they don't look like they've burst. Are they to be suspected more then any other part of the circuit?

    Also why do computer circuit boards last decades yet PSU conk out after two years? Do they use some special E Caps that have superior reliability? The PSU does look like it's been thrown together with all the bare wires topside.
     

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  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,491
    2,833
    Jan 21, 2010
    The 400V capacitor is generally under lower stress then the lower voltage capacitors.

    A common cause for it failing is a short circuit of one or more diodes in the input circuit where the mains is rectified. So checking them would be my first test.

    If you find a failure there, then there is also (almost certainly) some fuseable component that has died, otherwise the same fate would have rapidly befallen the new capacitors.

    If this was not the mode of failure (do you get a deadly -- not kidding -- 350V or thereabouts on these capacitors?) then their demise may have been cause by simply being close to something quite hot.

    Many failure modes of SMPS regulators end up with the circuit trying to pass impossibly high currents. However certain design standards require things to fail safely, and this can be done by making some components go open circuit. It may be that something hard to replace (I'm thinking the transformer) has done so.

    Why do these things die quickly? The basic answer is heat. Heat causes most electronic components to deteriorate or age faster. SMPSs are generally made as small as possible, and sometimes smaller! The output capacitors are often subject to very high ripple currents. In addition to that the input of an SMPS is often subject to all the rubbish that the mains has on it (surges, spikes, etc.) and because it has been made to a size and cost constraint, the input filtering may have been reduced to the bare minimum (or less!).
     
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