Forming Capacitor

Discussion in 'General Electronics Chat' started by Paulo540, Dec 31, 2009.

  1. Paulo540

    Thread Starter Member

    Nov 23, 2009

    In my current workbook for school, it notes that I need to 'form' one of the capacitors before building the FET amplifier circuit. They are 50v 10uF, just curious exactly how I do this properly. The book didn't explain it :(

  2. SgtWookie


    Jul 17, 2007
    Polarized electrolytic and tantalum capacitors may need re-forming after being idle for a period of time.

    The basic method for reforming the dielectric is to charge the capacitor via a current limited voltage source to its' rated voltage until the leakage rate is acceptable, or until you get tired of waiting.

    I use an adjustable voltage supply with a 10k resistor in series with the cap.

    Increase the voltage slowly until the cap is charged to its' rated voltage. You need to calculate the acceptable leakage rate and acceptable voltage drop across the resistor first.

    Leakage_Rate_in_mA= Capacitance_in_uF * Voltage_Rating * 0.00015
    So for your 10uF 50v caps, the maximum acceptable leakage rate is 0.075mA, or 75uA.
    Multiplying that by the value of resistance, you get 0.75v, or 750mV drop across the resistor or less.

    So, setting your adjustable voltage supply to 50.75v with a 10k Ohm 1/4W resistor in series and waiting until the voltage across the resistor was less than .75v will ensure that your capacitor is acceptable.
    Don't allow the voltage across the resistor to exceed 25v at any point in time.
    Last edited: Jan 1, 2010
  3. Paulo540

    Thread Starter Member

    Nov 23, 2009
    Great, thank you very much for the example and the PDFs too.