Testing for Ripple

Discussion in 'General Electronics Chat' started by celingwalker, Dec 11, 2012.

  1. celingwalker

    Thread Starter New Member

    Mar 5, 2011
    2
    0
    Good day all. I have been working in electronics for quite some time however, my experience using an O-scope is very limited. I am trying to test a power supply for ripple and have tried a number of times but I don't think Im doing it correctly. For years I have just resolved to using my multimeter. I have searched the web but I still don't get it. Anyone that would be able to explain it and how to interpret results to a simpleton, I would greatly appreciate it.
     
  2. #12

    Expert

    Nov 30, 2010
    16,316
    6,818
    First, make sure the power supply does not share a ground that the scope could use to cause a short on the power line. Set the scope for AC response, 1 volt per division, and 5 milliseconds per division. Then attach the scope probe from ground of the power supply to the first filter capacitor. You will be expecting less than 3 volts peak to peak at 60 or 120 Hz. Sometimes it is as low as a few tenths of a volt.

    If this doesn't answer your question, try asking again while including more information.
     
  3. JMac3108

    Active Member

    Aug 16, 2010
    349
    66
    #12, those numbers that you quoted will vary widely depending on the supply. What if its a small buck regulator with a 400KHz switching frequency? The ripple could be as small as 20mVpp, triangle wave at 400KHz.

    (1) Always measure right at the output capacitor on the power supply.

    (2) Take the ground clip off the scope probe and wind a small piece of bus wire around the metal part to make a super short ground connection. This will make a HUGE difference inthe amount of ripple that you see.

    (3) Know the switching frequency of the supply and set the time base on the scope appropriately to see that frequency.

    (4) Test with the supply loaded appropriately for the application. The ripple can go up significantly at light load depending on the type of power supply.

    Be aware that depending on the topology of the supply, you may see more than one thing on your scope. The ripple is usually a triangle wave at the switching frequency of the supply. But you may also see spikes at the top and bottom of the triangle wave that you will need to zoom in and look at on the scope.

    Hope this helps :)
     
  4. celingwalker

    Thread Starter New Member

    Mar 5, 2011
    2
    0
    Thank you for the information. I have only the +, -, and 0 volts binding posts to attach test leads too. Everything else is underneath and inaccessible.
     
  5. #12

    Expert

    Nov 30, 2010
    16,316
    6,818
    Then ceiling walker can tell us what he's actually trying to measure.
     
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