Strange behavior on LPF

Discussion in 'General Electronics Chat' started by André Ferrato, Jul 14, 2015.

  1. André Ferrato

    Thread Starter Member

    Apr 5, 2015
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    Can someone explain this phenomenon for me ? It seens something related with the Parallel Resistance of the Inductor... The default for Proteus is 1M ohm, when i lower it, the filter seens more like a LPF and not that weird bandpass.

    Sem Título-1.jpg
     
  2. Hypatia's Protege

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    In a perfect 'world' wherein the reactors exhibit pure reactance purely of the indicated sign and value, the function of the circuit is, indeed, that of a low-pass filter...

    Thus it seems that parasitic 'parallel' capacitance in L1 and/or parasitic 'series' inductance in C1 'come into play' above ~5MHz

    Please note that the theoretic F(R) of the attached circuit ~159Hz

    As a side note -- Inasmuch as parasitic 'quantities' are unspecified and largely unpredictable ('component-to-component') and, hence, poorly modeled -- You are advised to regard simulated results of such behavior with caution...

    Best regards
    HP
     
    Last edited: Jul 15, 2015
  3. #12

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    Same answer, different way of saying it:
    This circuit analysis is an exquisite example of the fact that inductors have parasitic capacitance (in parallel) and capacitors have parasitic inductance (in series). Of particular note is the failure of the electrolytic capacitor in the range between 1MHz and 10 MHz because this is typical, and in the correct frequency range, of real components you can buy today.
     
  4. MikeML

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    Quite well behaved in LTSpice...

    247.gif
     
  5. #12

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    Another excellent example of the difference between simulation and reality. Simulators use, "ideal" components. Be aware of this if you intend to build actual circuits with physical parts.
     
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  6. André Ferrato

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    Apr 5, 2015
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    I see.. it was my guess also. The assumptions in the formulas are to be taken into an ideal world, they lack the component and material difference. So i should continue to think this is a perfect LPF and adapt to the material and components that i have? I didnt knew that electrolytics would fail when these frequencies were sensed. This is the first step of a filter for an Full Wave Rectified 50Hz AC that is on the process of being built, any tips?
     
  7. Hypatia's Protege

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    Why do you feel you need to 'filter off' RF?
     
  8. MikeML

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    So why bother simulating it from DC to daylight?
     
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  9. MikeML

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    On the tips department. For a power supply filter, it is much easier, cheaper, lighter to forego the inductor, and just use an electrolytic followed by an electronic regulator....
     
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  10. MikeML

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    Knowing how to use a simulator does not obviate the need to know when an ideal component is good enough vs knowing when to use a more complex model which accounts for parasitics and other non-ideal behaviors.
     
  11. #12

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    A 50 Hz power line has almost perfectly zero amplitude of frequency components in the MHz range, however, a large percentage of active voltage regulators will oscillate if not damped with a small ceramic capacitor which is effective in frequency ranges higher than a large aluminum electrolytic capacitor. In today's world, inductors are not cost effective in this position.
     
  12. #12

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    That's cute! :)
     
  13. André Ferrato

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    Well, if i am not mistaken, this filter, if used as a CM will supress EM emission and interference right?
     
  14. Hypatia's Protege

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    "Alphabet soup' aside the filter should function well-nigh as predicted ELF through AF --- That said you are advised to consider the comments in posts #9 and #11

    Best regards
    HP
     
  15. #12

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    Ai Chihuahua! You've been reading books, I suspect. :D
    Those component values are nowhere near the right size for common mode filtering, you don't do CM filtering after the power transformer, a full wave rectifier doesn't cause that kind of interference, and thus, you don't need to do CM filtering unless your circuit is actually going to generate frequencies that must not be allowed to radiate from the power cord. You have not suggested anything which needs CM filtering. Please continue if there is some high frequency component generated in your project.

    Edit: Common mode filtering is usually done at the power cord entrance with a pre-fab connector which contains some small inductors and capacitors optimized for the purpose. If you need one, it is probably available in a small, effective, inexpensive package.
     
    Last edited: Jul 15, 2015
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  16. André Ferrato

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    Apr 5, 2015
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    Indeed, all i know of electronics has been learned in the past three weeks ahahaha. I had some practical experiencies but know i am trying to really learn theory and everything. The only thing my circuit has is 555 generating a square wave of 998 Hz at his output. I designed the circuit without any filtering like this one, it had only a beefy cap with a 7815, so lets say a nice ripple rejection. Then i started reading about the AC line filters and i thought it was needed to not radiate EM waves, but i feel like this post is making me realize it is an overkill and a confusion of concepts.
     
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  17. Hypatia's Protege

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    Thanks #12! I thought I was loosing my mind!:eek:

    With marked relief
    HP:D
     
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  18. Hypatia's Protege

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    A low-power 1 KHz square wave won't produce significant EMI --- FWIW the principal purpose of a line filter is protection of connected equipment from line noise...:)
     
  19. #12

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    Attaching a 10 uf electrolytic cap in parallel with a 0.1 uf ceramic cap right across the 555 chip power terminals will probably be sufficient.
     
  20. #12

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    There are also legal requirements to suppress radiation created inside your project. This usually becomes a concern at RF frequencies. You will see such labels on video and TV equipment.
     
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