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Why when I put even a light load on the bridge rectifier output shows 100Hz ripple?

Poor man's oscilloscope

schematic

simulate this circuit – Schematic created using CircuitLab

SR240 datasheet

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  • \$\begingroup\$ 16 mV peak? Is that correct? Where do you probe? \$\endgroup\$
    – winny
    Feb 8, 2020 at 7:55
  • \$\begingroup\$ @winny The cheap scope shows that, so... I'm probing right on the output terminal and it only oscillates under load. \$\endgroup\$ Feb 8, 2020 at 7:58
  • \$\begingroup\$ Oh! It's AC coupled. Then it makes perfect sense. \$\endgroup\$
    – winny
    Feb 8, 2020 at 18:05

1 Answer 1

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That's not "oscillation", that's "ripple". The capacitor discharges between the peaks of the mains, which occur 10ms apart with a 50Hz mains and a full-wave bridge rectifier.

The peak-to-peak ripple will be approximately proportional to the load current for small ripple.

See Andy's answer here

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  • \$\begingroup\$ But there's 4400uF capacitors on the output, I think that should stand for example a 10mA load without any ripple, right? \$\endgroup\$ Feb 8, 2020 at 8:09
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    \$\begingroup\$ Any load will result in some ripple. For a 10mA load at 50Hz full wave and 4400uF I would expect around 23mVp-p. \$\endgroup\$ Feb 8, 2020 at 8:12
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    \$\begingroup\$ And the scope says 22 mV. That's about as good a confirmation as you get. \$\endgroup\$ Feb 8, 2020 at 17:09

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