Low Pass Filter Design...

Discussion in 'The Projects Forum' started by czwick, Jul 5, 2012.

  1. czwick

    Thread Starter New Member

    Jul 3, 2012
    8
    0
    Hi,

    I have a pretty simple circuit, but I need to send an output PWM signal through the same place I'm getting my voltage. The voltage is DC, so I figure since my PWM is an AC current, I can use a low pass filter to send the signal through, while getting DC current from the supply. I'd prefer to use a capacitor/resistor filter rather than an inductor, but I'm unsure how I would go about creating one for this particular circuit.


    [​IMG]

    For an inductor, I'm guessing I'd put it on the path to VCC of my microcontroller? Where would I do it with a capacitor?

    Thanks!!
     
  2. afternath

    New Member

    May 7, 2012
    17
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    I have no idea of what you want to do, explain more if you want to be helped
     
  3. czwick

    Thread Starter New Member

    Jul 3, 2012
    8
    0
    My MCU is receiving a DC voltage from my power supply.

    However, I need to send a signal up the power supply as well that is AC. So I figure I need to somehow prevent AC from going to the MCU so that it will go up the power supply instead. Does that clear it up?
     
  4. WBahn

    Moderator

    Mar 31, 2012
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    Is the power supply expecting to see a signal driven onto the node it is driving (and trying to keep at a constant voltage)?

    My first thought would be to AC isolate the microcontroller's power pin by putting a choke (inductor) before it and a cap between the inductor and the MCU pin. Then, put a cap between the incoming power and your MCU output to AC couple the PWM signal onto the power signal. Be forewarned, you now have a couple of LC circuits that are going to have resonant frequencies, so you will need to choose your values with some care.
     
  5. Engr

    Member

    Mar 17, 2010
    114
    5
    you can try an RC filter so that AC signal cannot reach the Vcc pin of your MCU. place a resistor on the supply - to - Vcc path then from Vcc pin place a capacitor going to ground. you can compute for the cut off frequency using the frequency of your signal.

    try the attached circuit
     
  6. WBahn

    Moderator

    Mar 31, 2012
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    You need to be careful putting resistors in series with the supply pins (which is why I recommended a choke). You can't get very large at all before you start having problems. If the MCU is always drawing very low currents, then you can get away with it. The capacitor can can handle real short transient draws, but the resistor still has to be small enough to let it recharge between transients.
     
  7. Engr

    Member

    Mar 17, 2010
    114
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    I agree with WBahn. If the MCU is drawing a significant amount of current and the resistor value is high enough to create a significant drop across the resistor then the MCU might not work properly but if you can keep the resistor value very small that the voltage drop across it wouldnt be significant then it will be okay and will not create any problem. Can you tell us the supply current of the MCU?
     
  8. czwick

    Thread Starter New Member

    Jul 3, 2012
    8
    0
    Thanks for the replies!

    @WBahn, Yes, the power supply is expecting a signal.

    So would you recommend something like this?(and maybe with a capacitor between the inductor and vcc pin of the mcu)

    My PWM cycle is at 1kHz so I'd like to filter that out of the MCU vcc line.

    I'm new to this, what other info do I need to calculate the values for the choke and capacitors? Also, how do I calculate it? Do you guys have any recommendations for values?
     
  9. WBahn

    Moderator

    Mar 31, 2012
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    Connect the inductor directly to the Vcc pin, and connect the capacitor between the Vcc pin and ground.

    I'm guess (and keep in mind that I am doing just that, guessing as I sit here typing) that you want to set the resonant frequency of the frequency (1/sqrt(LC)) to be well below the 1kHz frequency of the PWM signal. I'm pretty sure you do NOT what to set them equal, because that would result in a large reactive voltage being seen across the capacitor, which the MCU can't tolerate.

    I'm not sure about how best to size the coupling capacitor. My concern is that this is essentially a differentiator, so it will tend to want to just give you transient spikes at the PWM signal edges. But perhaps that is all that is needed and the circuit on the receiver at the supply can deal with it.

    I would think that a supply that wanted this kind of feedback information would document fairly well how to go about producing the necessary signal.
     
  10. czwick

    Thread Starter New Member

    Jul 3, 2012
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    0
    @Engr, the MCU requires ~.75mA

    @Wbahn I guess i will try to look up formulas and try different capacitors and inductors.
     
  11. Engr

    Member

    Mar 17, 2010
    114
    5
    if the supply requires ~0.75mA I think it is safe to use the RC filter. @500ohms resistor the maximum voltage drop across the resistor will only be 0.375V, I dont think this will create a very significant problem to the supply of the MCU. With 500ohms resistor you can use a ~50pF capacitor to have a cutoff frequency of ~1KHz.
     
  12. MrChips

    Moderator

    Oct 2, 2009
    12,415
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    50pF seems rather low. You're sure you didn't mean 50μF ?
     
  13. czwick

    Thread Starter New Member

    Jul 3, 2012
    8
    0
    My one concern with the RC filter method is that won't the high frequency just go to ground? I want it to go back up the supply. If that is true, it seems like I need an inductor then?

    I'm completely inexperienced with inductors and have no clue how many henry's I need or where to get inductors (it seems like the only method I found online was how to roll your own out of many feet of wire). Could someone help me in terms of acquiring an inductor that is right sized that would not allow any frequency above 100-500ish Hz through? I'm assuming then my signal would go a different route in the circuit (i.e. the power supply?) My voltage of my signal will be minimal (mV).

    Thanks so much!
     
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