Diode questions

Discussion in 'General Electronics Chat' started by AllWires, May 24, 2010.

  1. AllWires

    Thread Starter New Member

    Oct 7, 2008
    I'm messing around with different diodes and I have a few questions.

    1. When I plot a diode for really high voltages, the i-v curve seems to become linear. Even when the resistance of the diode is set to zero. Can anyone explain why this happens?
    2. Do only Zener diodes go into the breakdown where the current quickly becomes negative? When I put silicon or schottky diodes past their breakdown voltage, nothing seems to happen.
  2. sage.radachowsky


    May 11, 2010
    I will answer my best understanding, and then I hope other people will have more better info.

    (1) --> There is *no* diode with zero resistance. after you pass Vf (forward voltage) then the resistance gets very low, but it is not zero. The resistance can vary based on the diode type and geometry, but it is always a bit of a bottleneck compared to copper, so you have to consider it a resistance.

    (2) --> The Zener diode is made for the specific breakdown voltage of its rating. That is the key feature of the diode, and it's sold to be used for that purpose. A regular Schottky or silicon diode is rated for a *minimum* reverse breakdown voltage. That means they guarantee it will not breakdown at less than this voltage. But if the breakdown voltage exceeds the rating, this is good -- more insurance against failure. I am sure that if you continue to increase the reverse voltage, you'll find the point where it fails, and it may either fail fused or open. I don't know which is more common failure mode. Maybe other will know?
  3. logicman112

    Active Member

    Dec 27, 2008
    When we get negative current with Zener diode, means the semiconductor is conductive(has started to conduct) and it is absolutely in breakdown region.

    For a regular diode there is a safe region of operation in terms of used active power. If you pass the breakdown voltage for a regular diode, as far as your average power spent is below PDmax, the device is safe. Violation from this maximum power may cause permanent damage to the device. But The device(as far as it is OK) is in breakdown region, if it is not OK, the atomic structure of the device will change and it is not a diode , it is an insulator.