RC Time

Discussion in 'General Electronics Chat' started by Lundwall_Paul, Apr 1, 2012.

  1. Lundwall_Paul

    Thread Starter Member

    Oct 18, 2011
    220
    19
    What are valid RC time constant are there general rules or capacitance to resistance ratios that are recommended?

    Like a 1 Farad capacitor (which if existed would be huge) and a three ohm resistor makes a real bad choice for a three second delay.

     
  2. #12

    Expert

    Nov 30, 2010
    16,248
    6,745
    You always try to keep the capacitor small and the resistance large because all (small) resistors are the same price but big capacitors cost big money.

    The other people may now point out the exceptions.
     
  3. Blofeld

    Active Member

    Feb 21, 2010
    82
    18
    I agree with #12.

    An upper limit for the value of the resistor is given by the fact that in a real world circuit there are always parasitic resistances that are in parallel to the nominal resistance, their exact value is often not known.

    So you want to keep the resistor much smaller than the parasitics, so that they don't have much influence.
     
  4. Wendy

    Moderator

    Mar 24, 2008
    20,764
    2,534
    It mostly depends on the impedance of the circuit you are tapping into the RC circuit with. Modern JFet op amps are extremely good.

    There is also the issue of really large electrolytic caps being extremely leaky. This makes them unusable for many applications.
     
  5. paulktreg

    Distinguished Member

    Jun 2, 2008
    609
    120
    One Farad capacitors do exist and they really aren't huge. ;)
     
  6. atferrari

    AAC Fanatic!

    Jan 6, 2004
    2,644
    759
    Yes, but, they cannot be used anywhere. Like the more common ones.
     
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