I have recently read in older book (I don’t have it on me, it was from at least 90’s) about LC filter with resonance in chapter about filters in power supplies. It looks like regular pi LC filter but with additional capacitor parallel to inductor. Resonance frequency of additional capacitor and inductor was equal to frequency of rectified signal which give this filter good suppression of ripple voltage. It sounds great, but I never seen it used in any device, why?

Example of this filter:


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ draw it here falstad.com/circuit/e-filt-hipass-af.html \$\endgroup\$ Mar 18 '20 at 18:01
  • \$\begingroup\$ You haven't specified the sine wave frequency... \$\endgroup\$ Mar 18 '20 at 18:19
  • \$\begingroup\$ It's an old, but not perfect solution, as the zero introduced by the parallel capacitor will be felt. Or, at least, might be. \$\endgroup\$ Mar 18 '20 at 18:21

It sounds great, but I never seen it used in any device, why?

It sounds great until you start to realize that the inductance needed to give you a blocking resonance (on regular AC supplies with bridge rectifiers) is probably physically larger in size than a modern wall wart and then some again. I remember being taught this technique at college in the 1970s but nobody used them back then either. Semiconductor regulators have marched so far forwards that any ripple that might be suppressed by a chunky inductor resonated with a capacitor is easily dealt with by them. Then along came switch mode power supplies and changed the game again.

I'm not saying that they aren't used in places but I am saying the need for them is the tiniest of a fraction of a percent compared to what might be called state of the art in the 1950s.

  • 2
    \$\begingroup\$ It may make more sense in a SMPS than it does at 100 or 120Hz :-) \$\endgroup\$ Mar 18 '20 at 18:18
  • 1
    \$\begingroup\$ My first experience in seeing one of these power supply filter inductors (and they sometimes used a second one to precede what's shown in the schematic [inrush]) as a kid was to think I'd found a transformer. But upon seeing only two wires, I realized they had to be an inductor. Which meant I had to learn why, then. Anyway, yes they were big. \$\endgroup\$
    – jonk
    Mar 18 '20 at 18:19
  • \$\begingroup\$ @urhen are you done with this question now? If so then please formally accept the answer or raise a new comment asking for clarification. \$\endgroup\$
    – Andy aka
    Oct 11 '20 at 8:57

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