I am still pretty noob in electronics, but I am in the process of completing a homemade power supply project and I would love to hear some advises.
The supply itself is fairly straight forward - it takes 20-ish volts DC from a laptop power supply, has a bunch of switching regulators (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 and is cool and useful, but when I attached my homemade (arduino based) oscilloscope to its outputs I noticed there is some ripple on the 3.3V and 5V channels. It was much better with a 100uF cap, but still present, and practically gone with 1000uF cap at the output (to be precise 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 circuits was simple enough, so I didn't really need to make one before implementing it. Lets assume the question can be simplified down to just a DC power source and a step-down switching voltage regulator. I do have some power mosfets switching the channels on and off, but I don't think it changes anything significantly.
Again, the question is - is there a reason not to put a 1000uF capacitor at the output of a switching voltage regulator used as a general purpose power supply?

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

  • \$\begingroup\$ They didn't want to mess it up any more than it already is? \$\endgroup\$ – Tim Spriggs Apr 3 '16 at 1:30
  • 2
    \$\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

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.


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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