DC polarity switching, Rectifying, and Filtering Help Needed

Discussion in 'General Electronics Chat' started by driller, Nov 24, 2009.

  1. driller

    Thread Starter New Member

    Nov 24, 2009
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    I am trying to create a switching circuit for a brushless DC motor that will detect when I change the polarity of the source (via a switch) and provide a Input High/Low to the CW/CCW input of the motor controller IC.
    The source DC will pass through a Full-Bridge Rectifier and PI Filter so that I will always have a consistant voltage reference for the remainder of the circuitry.
    Please view the attached circuit drawing for details.
    I am having difficulty determining the PI Filter component values. The design is something I lifted from a similar circuit so I'm not even sure if all of the components will be necessary.
    Also, I am not sure how to make the switch for the High/Low input to the IC. I'm thinking that it would be of a Transistor type but not sure how to do it.
    Any help would be appreciated. Thank you.
     
  2. driller

    Thread Starter New Member

    Nov 24, 2009
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    I have made some progress on this circuit. Attached is an updated circuit drawing.
    Hopefully this will clarify my original request. I'm still working out the values of the components.
    The source voltage will be max 65Vdc and as I mentioned will be polarity swapped via a switch. The current will be max 6A as it will be driving a motor which will require a lot.
    Remember that there is a lot more circuitry to this drawing that is not shown that is required to drive the motor. That is where the "+Vdc" and ground go off to.
    I have denoted the Bridge Rectifier as U1 as it is a single component. U2 is an IC that controlls the direction of the motor, among other things.
    I hope this helps clarify the circuit to anyone who may be looking into it. So far, I can see it has been looked at but not replied too.
    Again, thank you for your assistance.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    Well, how much filtering do you really need?

    Pi filters are low-pass. Just for starters, try 100uH for the inductor, and 33uF in parallel with 0.1uF for input and output caps. That's probably far more than you'll need.
     
  4. driller

    Thread Starter New Member

    Nov 24, 2009
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    I was told that it wouldn't hurt to have more than enough due to the possible spikes I will be encountering. Without going into too much detail, it will be suseptible to large spikes. Do mean to make the left side (input caps), C1/C2, both 33uF and the right side (output caps), C3/C4, both 0.1uF? The circuit I'm taking this portion from has all four, input and output caps, 100u 63V. That 63V won't be enough for my normal operation.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    You can go as large as you like.
    I was suggesting to put a large cap in parallel with a small cap on both the input and the output; eg: C1 small, C2 large, C3 large, C4 small.

    The large cap tends to suppress the low frequency transients.
    The small cap tends to suppress the high frequency transients.

    If your current requirements for powering your circuit are going to be pretty low, you could put a resistor between the bridge and the caps, and use Zeners back-to-back or use a tranzorb to take care of the big spikes.
     
  6. driller

    Thread Starter New Member

    Nov 24, 2009
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    You mean like this?

    I need the 6A to get to the motor. I'll be cranking the power supply way up to keep the motor going the same speed with a big load on it.

    High voltage zenors could help but I don't think I will need the resistor here. It is taken care of before going into the Regulator IC.

    Thanks for your help!
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    Wait a minute - is your 6A motor being powered by this same supply?

    I haven't seen a regulator IC yet - and if the motor is after the regulator, you're bound to have some problems; like lots of heat dissipation.

    You've chopped off too much of the schematic, so I can't see what's above or below the supply filter.
     
  8. driller

    Thread Starter New Member

    Nov 24, 2009
    13
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    Yes. The 65Vdc, 6A branches off after the choke/filter. One branch (labeled MOTOR in my second post attachment) goes directly to gates which drive the motor. The other branch (labeled +Vdc in the same attachement) goes to a Voltage Regulator which regulates the voltages required to operate the remaining ICs. The two share the return.
     
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