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I am thinking about building a capacitor bank between the PSU and load to smooth out current surges from electric motors. However, if the capacitors are discharging rapidly during a current surge, does that mean the PSU also experiences a current surge in order to recharge the caps, thus negating the point of a capacitor bank?

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In the end, all of the current must come from the power supply-- regardless of how many caps you have in your cap bank. What the cap bank will do is average (a.k.a. smooth) out the current over a longer and longer period of time. Ideally the cap bank would average out the current over a time period that is much larger than the length of a surge.

If the surge is longer than the averaging period then the cap bank is not really doing anything useful.

One huge downside to using a really large cap bank is that the inrush current can get very large. When the power supply is first turned on, all the caps are discharged. The current required to charge the caps can be huge and your power supply needs to be able to deal with that current. Some SMPS chips have a "soft start" mode, or will current limit themselves.

Sometimes you can also put an inductor between the power supply and the cap bank. This works great for short surges, but can cause problems with longer ones.

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  • \$\begingroup\$ Great. The inrush is kinda what I was worried about. My supply can give 5V DC @1A sustained safely. What do I need to put between the (rather large) cap bank and the PSU? \$\endgroup\$ – learnvst Aug 11 '12 at 18:44
  • \$\begingroup\$ The safe thing would be some sort of current regulator. It is like a linear voltage regulator but regulates current instead. These are not super common and are difficult to make yourself. They are probably not a practical solution for your purposes and ability. The best solution is to simply get a bigger power supply and a smaller capacitor bank. \$\endgroup\$ – user3624 Aug 11 '12 at 19:18
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If the current from the PSU to the caps is not limited at all, and the PSU is capable of supplying the current, then yes, it will experience a surge of current (if there is zero resistance then the caps will do nothing - the PSU will supply the surge current)
As long as the current is safely limited in some way though (from the PSU) it should be fine. So it depends on the PSU, or you can add some external limiting in series (e.g. inductor, regulator) if necessary.

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