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Diode and Capacitor question

  1. Jan 21, 2007 #1

    In lab the reverse leakage current of 4 different diodes was measured. Then ranked from smallest to biggest. Then we are asked to decide which diode would be most suitable for charging up a capacitor and allowing the capacitor to keep its charge for the longest period of time?

    I think it would be the one with the smallest current because it would allow the capacitor to discharge slower. However, it would take longer to charge. Am I right?:confused:
  2. jcsd
  3. Jan 21, 2007 #2
    Yes, the one with the smallest leakage current will discharge the capacitor the slowest. As for the charging time, it has nothing to do with the leakage current, because when you charge the capacitor, the diode operates in the forward bias region.

    Now it's true that the diode with the biggest leakage current might happen to be a Schottky diode (because they have bigger leakage current than pN junction diodes), and the Schottky diode has a lower forward voltage drop at same forward currents: ~0.2V compared to ~0.7V for a pN junction diode, which means that the capacitor will be charged to a higher voltage and thus take a longer time to discharge.

    But given that you only know the leakage currents, you assume the diodes to be of the same type and the correct answer is the diode with the smallest leakage current.
  4. Jan 21, 2007 #3
    Thanks for your help :smile:
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