Current filter , Can u help ?

Discussion in 'General Electronics Chat' started by Zara Engineer, Feb 6, 2015.

  1. Zara Engineer

    Thread Starter Member

    Feb 6, 2015
    53
    0
    Hi everyone

    I have a problem of current filtering with my controller of brushless dc motor (BLDC).

    [​IMG]

    the problem that the current is PWM and i want to read it and show a fixed value like the multimeter process.

    so what's the filter that can be used to prevent this problem , or specially what's is the filter inside the multimeter ( i know that a Low Pass Filter!!).

    thanx in advance
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,646
    2,345
    Hello,

    The posted image does not show.
    You can not link images from edaboard directly.
    Please use the Upload a File button, below the replybox.

    Bertus
     
  3. Zara Engineer

    Thread Starter Member

    Feb 6, 2015
    53
    0
    Hi

    Sorry

    [​IMG]
     
  4. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    why not just measure the supply current to the pwm output stage? should be dc.
     
  5. crutschow

    Expert

    Mar 14, 2008
    12,993
    3,229
    What, exactly are you trying to measure?
     
  6. Zara Engineer

    Thread Starter Member

    Feb 6, 2015
    53
    0
    Hi Mr Crutschow

    if you see the image there's 2 resistor to measure the current ( pwm command) my goal is to measure it like a dc ammeter . so my question what's the type of this filter ??

    thanx in advance
     
  7. Zara Engineer

    Thread Starter Member

    Feb 6, 2015
    53
    0
    Sorry Mr alfacliff

    no i want pwm current on the source pin of mosfet .

    the supply current isn't practical
     
  8. crutschow

    Expert

    Mar 14, 2008
    12,993
    3,229
    A single-pole RC filter should work to smooth the PWM signal into the average value. The RC value should be at least 5 times the period of the PWM frequency.
    The trade-0ff is between ripple voltage and the time to respond to any change in the current level.
     
  9. Zara Engineer

    Thread Starter Member

    Feb 6, 2015
    53
    0
    Hi

    in my circuit this filter is implemented

    [​IMG]

    [​IMG]

    don't you think this filter is good for this (mission) ? ?

    thanx
     
  10. crutschow

    Expert

    Mar 14, 2008
    12,993
    3,229
    That is a differential circuit followed by a 1st-order low-pass RC filter with a corner frequency of 159Hz.
    If the bottom of R32 goes to ground then you don't necessarily need a differential circuit.
    What's VREF?
    What is the PWM frequency?
    Are you having a problem with that circuit?
     
  11. jjw

    Member

    Dec 24, 2013
    173
    31
    R=1k, C=1n -> corner frequency is 159kHz
     
  12. crutschow

    Expert

    Mar 14, 2008
    12,993
    3,229
    Correct. The characters are very small and I mistook the n for a μ. :oops:
     
  13. Zara Engineer

    Thread Starter Member

    Feb 6, 2015
    53
    0
    Hi

    thanx Mr jjw for your help.

    Mr crutschow , no i don't have any problem with this circuit but i told about the filter and you're right this is very little .

    but is there any other digital filter without changing the pcb board ?
     
  14. crutschow

    Expert

    Mar 14, 2008
    12,993
    3,229
    Why do you want to change the filter?
     
  15. Zara Engineer

    Thread Starter Member

    Feb 6, 2015
    53
    0
    hi

    to change the cutoff frequency , because pwm freq = 20 khz
    for 159 khz , this is not a good LPF

    so my question can i add a digital filter in the mcu instead of analog ?
     
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