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Thread: How to use a scope to test for ripple?

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    Default How to use a scope to test for ripple?

    I'll admit, I'm not the most electrically gifted, but as a reviewer for thebestcasescenario.com, I feel if I don't know something, learn something. In this case, I have a B+K Precision 2120 oscilloscope here, recent acquisition in the quest to find the best in tools to do my job with.

    I have a prerelease PSU here that I really want to know how to verify that my software (OCCT latest version) is right about on ripple, how would I go about doing that? I'm not completely daft, but I had to ask questions about my DMM too. I'd rather ask than fry.

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    Quote Originally Posted by Kayin View Post
    I have a prerelease PSU here that I really want to know how to verify that my software (OCCT latest version) is right about on ripple, how would I go about doing that? I'm not completely daft, but I had to ask questions about my DMM too. I'd rather ask than fry.
    I can't help you with setting up your oscilloscope, but I can tell you that OCCT cannot measure ripple at all, so you can just ignore that. Stick to your DMM for voltage readings, and forget about using software altogether.
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    Here's the basics of how to set up your scope:
    http://www.physics.udel.edu/~jqx/Phys245/lab/scope.html

    Since noise & ripple is in the 10s of mV, the voltage should be set to 10 mV / Div. I would set the scope to AC coupling and you could start by setting the trigger to Auto. Adjust the time base so the wave for can be clearly seen.

    That's the easy part. The critical part is having the PS supply a known amount of power for each of the voltage rails. If you just probe the various rails while the computer is running CPU and GPU stress tests, you still have no idea what the actual current draws are. Noise and ripple values aren't very useful unless that measured around the rated limit of the PS. There's a reason why there aren't many sites on the web that have good PS reviews: it's not that easy to do.
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    Don't forget to use the proper 10uF and 0.1uF capacitors in parallel with the measured load.

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    There are several ways to attach your probe to the test points. Since power supplies generate both CM and DM noise, and CM noise can convert into DM noise in some circumstance, you might easily get excessive "noise" on your scope. Be careful about that.
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