Guard Ring

Discussion in 'General Electronics Chat' started by shreyas_bhat, May 2, 2005.

  1. shreyas_bhat

    Thread Starter Active Member

    Jul 26, 2004
    47
    0
    I have a parallel plate capacitor arrangement, and need to reduce the fringe field effect at the corners. For this purpose, I have incorporated a guard ring around one of the plates. So, what I have now is two plates, and around the top plate, I have a metallic ring. If the bottom plate is at a potential V1, and the top plate is at a potential V2, at what potential should the guard ring be maintained to minimize fringe field effects and why ?
     
  2. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Shreyas_bhat,

    I posted a link to a website that I stumbled onto in your guard-ring topic post in the "FEEDBACK AND SUGGESTIONS" forum .

    hgmjr
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    You'll want to hold the guard ring at circuit ground. Tying it to either capacitor plate voltage will have mysterious effects on the measuring circuit. Ground is supposed to be a conduit to remove extraneous noise from the circuit. If you do it right, the impedance will stay low enough that most noise will actually go away.
     
  4. alienator

    New Member

    Feb 20, 2009
    3
    0
    i have connected the guard ring to ground ,
    one plate is at +'ve potential , other at -'ve , i have guard ring for both the plates , capacitance is not varying much cause of external disturbances.

    now what happens to the capacitance ? will it increase or decrease
    as compared to the capacitance when i didn't have any guard ring


    :cool: i observed that the capacitance is less when i have the guard ring surrounding capacitance plates .

    :confused: which i think is counter intuitive as the -ve charge will accumulate on guard ring surrounding +ve plate , and will make the plate more positive for the same supplied potential C=Q/V

    some body please help me out as soon as possible
     
  5. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
    2,039
    287

    "Mysterious effects."" I love it! But, of course, you are absolutely correct.

    eric
     
  6. isobel9

    New Member

    Jul 17, 2009
    1
    0
    Sorry, Beenthere, but unless we are understanding different problems then I disagree with your answer. To minimize the fringing effects you must hold the guard ring at the same potential as the plate it is guarding, V2. Then measure the capacitance between the bottom and top plate.

    To understand why this works think about the way the current is traveling though the dielectric: the electric field lines will be perpendicular to the plates, except for the edges where there will be significant rounding (fringing). Now, if the plate that is surrounded by the guard ring has the same voltage as the guard ring then the current traveling through the dielectric will travel perpendicularly from the bottom plate to the top plate and the fringing will occur around the guard ring with minimal fringing around the inner plate. This method was discovered by Kelvin. Cool, huh? Now, if you want even more accuracy then look at ASTM D257-07 to determine the amount of fringing with a guard ring.
     
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