Sizing Zener to a PTC fuse for reverse- and over-voltage protection

Discussion in 'The Projects Forum' started by kender, Jun 11, 2012.

  1. kender

    Thread Starter Senior Member

    Jan 17, 2007

    There is a common problem: somebody will plug a wrong power supply (wall wart) with too high a voltage or wrong polarity. The agressor power supply may or may not have it’s own crowbar protection. This protection circuit looks like a simple reasonable idea:


    The victim circuit by itself can survive up to 24V. During normal operation, it will draw up to 5A. Instructions for use prescribe a 12VDC supply.
    PTC fuse: RGEF500, 5A hold current, 8.3A trip current, max time to trip is 2 seconds.
    TVS diode: 13V to 15V. But... To be useful, the diode would have to survive at least 8.3A for 2 seconds. What’s the right way to select this TVS diode? For instance, would something like SMCJ13A have sufficient ratings (at least in principle)?

    Any suggestion, insight or reference is really appreciated!

    - Nick
  2. praondevou

    AAC Fanatic!

    Jul 9, 2011
    I personnally would prefer to put an overvoltage protection circuit in there.

    This can be easily done with 2x TL431 and a p-FET in series with the input of the victim circuit. Polarity protection could be a simple diode in series with the input.

    I posted it somewhere here, if you don't find it I can post it again.
  3. kender

    Thread Starter Senior Member

    Jan 17, 2007
    Part of the problem, which I didn't mention earlier, is that I'm constrained on board space. This requirement for protection is a last minute feature creep, so I want something quick & dirty.

    Diode drop (even if it's a Schottky) is undesirable.