Need Help Sizing a Capacitor

Discussion in 'The Projects Forum' started by mattd860, Apr 26, 2016.

  1. mattd860

    Thread Starter Member

    Jan 9, 2016
    I have a 5v ~50mA circuit shown below that has a capacitor that I need help with. I would like the capacitor to hold a 1-5 volt DC charge for about 120 milliseconds. The goal is to keep the voltage steady for 120ms after the variable resistor has changed.

    What size capacitor will be able to handle this and get me close to the 120ms range? I've tried a 0.1UF, 0.47UF, and a 10UF capcitor so far to no avail. Each one holds it's charge for only a fraction of a millisecond which isn't good enough.


    Capacitor Sizing.JPG
  2. wayneh


    Sep 9, 2010
    You haven't specified the current draw on the capacitor following a change. That's what determines the required capacity. Is it only the shown resistors? The time will change with the variable resistor setting.
  3. AnalogKid

    Distinguished Member

    Aug 1, 2013
    Not sure what you mean by "hold a charge"? If it is connected to 5 V through a 1 K resistor continuously, then the voltage across the capacitor will start to change instantly when the pot is adjusted.

    If you want the cap voltage to stay constant for two seconds after the pot resistance has changed, and then start to change to its new terminal voltage, that will take a much more complex circuit.

    If you want the cap to take 2 seconds to stabilize at its new voltage after a pot change, then we need to know the current load on the cap other than the pot, the value of the pot, etc.

    Please clarify your question.