Capacitors and Oscilloscope?

Discussion in 'General Electronics Chat' started by dds737, Feb 27, 2008.

  1. dds737

    Thread Starter New Member

    Feb 27, 2008
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    0
    Hi,

    How can I check with oscilloscope condition of capacitors in live circuit?

    Thanks.
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    How do you define "condition of capacitors"? Filter capacitors, tuning capacitors, coupling capacitors, timing capacitors?
     
  3. dds737

    Thread Starter New Member

    Feb 27, 2008
    3
    0

    Ive found this on net:
    -------------------------------------------------------------------------
    A third method is to look at the signal over a capacitor in a live
    circuit with an oscilloscope. If the capacitor is good the AC component
    should be small. The advantage of this method is that you see how the
    capacitor behaves under real live conditions. The disadvantage is that
    for circuits like the primary side of a SMPS this method can be
    dangerous to your health.
    -------------------------------------------------------------------------
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
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    That is valid for filter capacitors. The same degree of measurement may be made with a voltmeter set to AC volts.

    The warning about poking at the primary side of a SMPS is very serious.
     
  5. dds737

    Thread Starter New Member

    Feb 27, 2008
    3
    0
    How can I do that?
    How do I figure out which SMT capacitor is good/bad in a circuit?

    Thanks.
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    That depends on the type of capacitor. The monolithic ceramic kinds generally don't go bad. Tantalum capacitors almost always go short, get hot, and turn dark. Electrolytics usually get hot inside. The can gets taller and so the plastic sleeve doesn't cover the can properly. If they open up, then ripple will be excessive.

    Checking in circuit is difficult. The ones that fail may affect the circuit so as to prevent operation. Just pull any suspicious cap and see if it checks shorted with an ohmmeter. Placing an ohmmeter across the terminals of an electrolytic should show charging as an initial low resistance that increases over time to infinity. If it has no charge time or shows 0 ohms, it's bad.

    There are many other things that can cause problems in a SMPS. I had a problem with one in a projector. The main filter on the primary side was obviously bad - the can was swollen and had vented. I checked all the semiconductors before replacing it with a similar low ESR capacitor. The projector worked for 4 more hours and blew that same cap. It was buried in the projector case, so I could not observe operation. I salvaged some interesting parts and gave up.
     
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