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I'm a beginner in electronics, and in the process of completing a homemade power supply would love to get some advice.

The power supply itself is fairly straightforward - I'm using a laptop power supply with an output of about 20V and it powers my circuit which consists of:

  • 2 fixed at 3.3V and 5.0V
  • 2 adjustable with voltage and current limit
  • and another 5.0V to power an arduino nano
  • 4 INA219 breakouts to measure each channel voltage and current
  • an arduino nano to collect the data
  • and an LCD to display voltages, currents, current limits, etc.

It all works, however, when I attach my homemade (arduino based) oscilloscope to the outputs, I'm noticing some ripple on the 3.3V and 5V channels, adding a 100uF cap improved it but didn't get rid of it, so I upped it to 1000uF which got the job done, (or at least within +-0.03V which is the margin of error of my crappy home-made oscilloscope).

So, the question is - is there any reason not to put a 1000uF cap at the outputs of these channels?

I don't have a schematic because the circuit was simple enough, so I didn't make one before implementing it.

To simplify my question further, imagine just a DC power source and a step-down switching voltage regulator, would there be any reason not to put a 1000uF capacitor on the output of the regulator?

EDIT: Added line breaks, I couldn't figure out how to do it last night. Sorry for the mess.

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  • \$\begingroup\$ They didn't want to mess it up any more than it already is? \$\endgroup\$ – Tim Spriggs Apr 3 '16 at 1:30
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    \$\begingroup\$ Wall of text...!! Don't you know how to create paragraphs? \$\endgroup\$ – soosai steven Apr 3 '16 at 2:14
  • \$\begingroup\$ Good for including full information about your question. But PLEASE learn to format it so that we can read it. Few people an read that much detail in such a huge "wall of text"! \$\endgroup\$ – Richard Crowley Apr 3 '16 at 3:56
  • \$\begingroup\$ There can be downsides - see electronics.stackexchange.com/questions/224421/… for pros and cons. \$\endgroup\$ – Peter Smith Apr 3 '16 at 13:58
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The main reasons against are

  1. inrush current

    During startup, the capacitor effectively looks like a short to the power supply, so it will need to limit the current here.

  2. "higher-than" requirements

    The +5V rail might need to be kept above the +3.3V rail all the time, even during startup and shutdown. If you add large output capacitors and the +5V rail drops faster during shutdown, then the +3.3V capacitors need to be discharged into the +5V rail to protect any dual-supply ICs. If you have a large capacitor, this needs to be a fairly beefy diode.

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If your DC/DC converters are current-limited, then inrush current isn't an issue.

Be sure they are stable with a large capacitor.

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