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Zener Diode Dilemma

Discussion in 'Electrical Engineering' started by jjk, Oct 18, 2013.

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  1. jjk

    jjk Guest

    Thanks again Andrew. this is making more sense. I never used 3.3v zeners before.
    The 3.3v protection I was talking about is to protect output pins of 3.3v devices. The gpio pins on a Broadcom processor on the Raspberry Pi is one example. If an external source that is attached to the pin fails and applies a voltage greater than 3.3v, the Broadcom device can be damaged. There are articles on the net describing the use of a zener for protection and that'swhat I attempted to do. I guess the safest thing to do is add buffers in between the processor and external devices.
  2. You can do that. When I've driven pi inputs from 5V outputs, I've used
    a potential divider (1k8/3k3) to drop the input voltage.
    If there's a significant risk of the pi's 3.3V supply failing whilst
    the 5V output is still active, you could add a regular diode from the
    potential divider's mid-point to the 3.3V supply, to limit the extent
    the input can exceed the Vcc pin of the pi during such a failure, but
    the pi isn't particularly expensive, and you could spend more trying
    to protect one than it costs to replace it if you do damage it.
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