low pass filter help

Discussion in 'Homework Help' started by hunterage2000, May 1, 2012.

  1. hunterage2000

    Thread Starter Active Member

    May 2, 2010
    400
    0
    Hi,

    Say you have a rectified AC voltage and wanted to smooth it out to convert to a current, for a low pass filter, would I make its frequency as low as possible to make the signal as straight as possible.
     
  2. WBahn

    Moderator

    Mar 31, 2012
    17,743
    4,795
    What do you mean by, "smooth it out to convert to a current"?

    How low you make the cutoff frequency is a balancing act between residual ripple in the waveform and how responsive it is. If you used a filter with a time constant of a week, you will not have much ripple, but it will take several weeks before your nice smooth output gets to the value you are expecting.
     
  3. hunterage2000

    Thread Starter Active Member

    May 2, 2010
    400
    0
    smooth it out so it resembles a DC voltage with as little ripple as possible but not having a high settling time, ideally. then use a voltage to current converter to provide a 4 to 20mA linear output.
     
  4. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    5,448
    782
    The answer is clear - you have to set down the specification first and then attempt to design to the specification. Until then it's entirely speculative.
     
  5. crutschow

    Expert

    Mar 14, 2008
    13,011
    3,233
    Normally a rectified voltage is smoothed and stabilized with a regulator such as the LM317.
     
  6. WBahn

    Moderator

    Mar 31, 2012
    17,743
    4,795
    It sounds like he is wanting to generate a signal that tells him what the magnitude of the AC voltage is, not to regulate it.

    If that is the case, hunterage2000, then you need to determine what constitutes "not having a high settling time", as well as how much ripple is too much ripple. If you have a better feeling for one than the other, then come up with a worst allowable spec for that one and then see if you can live with the other.
     
  7. crutschow

    Expert

    Mar 14, 2008
    13,011
    3,233
    Well, in Post #3 he also said he wants to use the smooth voltage to power a 4 to 20mA current output, so it seemed that a standard voltage regulator was appropriate.
     
  8. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    5,448
    782
    Using a basic diode rectifier + low pass filter arrangement may not produce a satisfactory accuracy in relating the AC input (RMS?) value to the equivalent 4-20mA DC value. A precision rectifier in the signal processing chain may be in order - particularly with scaling low level AC voltage values to an equivalent DC mA value.

    Again it all comes back to the device specification which would include matters such as input voltage range, % accuracy, allowable ripple, measurement settling time and so forth.
     
  9. WBahn

    Moderator

    Mar 31, 2012
    17,743
    4,795
    Well, he didn't say he wanted to "power" something, he said that he then wanted to use a voltage to current converter to provide a linear output. So I can see how it could reasonably be interpreted either way.

    The bottom line is still the point that has already been made repeatedly: he needs to tell us what the goal is. If it is a homework problem or similar assignment, he needs to provide the details of what is being asked, preferably along with what type and level of course it for. If it is for a real project, then we need some idea of the goals and requirements in order to offer meaningful assistance.
     
Loading...