Low-pass filter help please.

Discussion in 'The Projects Forum' started by HurricaneJesus, Nov 1, 2007.

  1. HurricaneJesus

    Thread Starter New Member

    Nov 1, 2007
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    I am designing a low pass filter to be used in conjunction with some metal oxide varistor's in order to create a sort of power conditioner.

    It is to be passive in nature, and run off of standard 120V 60Hz source. I am having trouble figuring out how to determine the values of the parts required.

    I would like to use either a 'pi' or 'T' type filter.

    If anyone could provide some help on how to go about this, it would be greatly appreciated.

    Now here is what I know:

    I am going to use 3 220V rated MOV's for preventing voltage spikes.
    I would like this filter to be passive in nature.
    This is for the purpose of attaching electronics like TV's, computers, etc.
    It should also be able to handle a 3/4 HP motor.

    The point of the filter is to eliminate 'noise' on the lines when inductive or capacitive loads are attached.

    I really just need help getting steered in the right direction. I am also confused about what my cut-off Freq should be. I was thinking around 200Hz?

    *sorry for posting this on two boards, I was not sure where it belonged or where it would get noticed.*
     
  2. niftydog

    Active Member

    Jun 13, 2007
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  3. HurricaneJesus

    Thread Starter New Member

    Nov 1, 2007
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    I am supposed to order the parts myself and build.

    Can you tell me how to calculate the values for the resistor and capacitor in a circuit like this? I would like to filter out freq over 200 Hz i think. I understand that would be my 'cut-off' freq.

    [​IMG]
    *ignore the values already listed. I need a filter for 120V 60Hz

    I know it is a simple filter, but I would plan to cascade a few together.
     
  4. niftydog

    Active Member

    Jun 13, 2007
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    The trouble with using a topology like that is that R1 must pass ALL the current supplied to the load. Such filters are normally reserved for small voltage signals such as audio. This is why mains filters like the one I posted use inductors and capacitors only. How much current does the filter need to pass?

    Simple RC low pass filter equation:
    f = 1 / (2.pi.R.C)
     
  5. HurricaneJesus

    Thread Starter New Member

    Nov 1, 2007
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    It would be attached to a standard residential breaker, so 15A at the very most, but most likely less than that. I would say half.

    If you have some reading material or a link to some information that I could use to design such a thing, I would greatly appreciate it. I am kinda lost on this project I am working on, and not getting any help from the person that should be helping me.

    Essentially I just need to make a simple power strip, I was thinking with two standard outlets to attach an inductive load, and a capacitive one. I figured to throw the MOV's in there to protect against surges, and a filter so I can display the nice wave on an oscilloscope when it is finished and see that it is working.

    Is there something else I need? How do I go about dealing with harmonics, or is that covered by the low pass filter?

    Thanks again for any responses.
     
  6. thingmaker3

    Retired Moderator

    May 16, 2005
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  7. niftydog

    Active Member

    Jun 13, 2007
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    Beats me why anyone would want to design something that's so easy to just go out and buy off the shelf of virtually any hardware store. But, if I were you, that's where I'd start. Go buy a mains filter and check it out - reverse engineer it. Up the ratings for the current you require, add the MOVs and you're done.
     
  8. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
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    Assuming 82% efficiency, a 3/4 HP motor will draw roughly 6 Amps at 120V while running. It will draw 4x to 6x more while starting.
    Unless the teacher says "order the parts and build it from scratch." Teachers do stuff like that from time to time.:rolleyes:
     
  9. HurricaneJesus

    Thread Starter New Member

    Nov 1, 2007
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    Yeah I am supposed to be building it myself. I think it seems a bit too simple myself.

    I read up on those filters, but I had a question regarding harmonics. Should I set my cut off freq to be 100 Hz to prevent any harmonics?

    I was also wondering how to factor in the change in loads. I understand the motor will have different impedances whether it is running with no load or full load, so how can I determine the correct filter hardware in order to handle this?

    Should I just design it under full load conditions?

    Thank you for all of the help so far.
     
  10. niftydog

    Active Member

    Jun 13, 2007
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    They sure do! I had to design a 5W class A amplifier. Wee... such an appropriate use of my time and so relevant to modern electronics! NOT!

    What concerns me most, if this IS a student project, is the fact that it's a mains powered project!!! My teachers had enough sense to keep us away from deadly sources of electricity!!!!
     
  11. HurricaneJesus

    Thread Starter New Member

    Nov 1, 2007
    6
    0
    Ok so I am trying to get a circuit into multisim, but I have no idea how to simulate the suppression choke inductor. Here is the circuit:

    [​IMG]

    I just want to get this in Multisim or Orcad and simulate it working so I can order the parts.

    Any help?
     
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