Zener Diode Dilemma

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

  1. jjk

    jjk Guest

    On Friday, November 1, 2013 4:25:59 PM UTC-4, Andrew Gabriel wrote:
    > In article <>,
    >
    > jjk writes:
    >
    > > On Thursday, October 31, 2013 4:42:08 PM UTC-4, Andrew Gabriel wrote:

    >
    > >>

    >
    > >> This exactly matches the datasheet, which says it needs at least 76mA

    >
    > >> in order to act as a zener voltage reference.

    >
    > >>

    >
    > >> This is a common issue with low voltage zeners - the current they

    >
    > >> need to operate is high (compared with higher voltage zeners).

    >
    > >>

    >
    > >> If you want a low voltage reference at low current (e.g. for battery

    >
    > >> operated equipment), an LED can often be used (forward biased) at a

    >
    > >> much lower operating current (1mA or less), although not quite as

    >
    > >> stable a reference.

    >
    > >

    >
    > > Thanks for your comments Andrew.

    >
    > > I'm wondering why the zener would conduct at all at voltages as low as 1.4v?

    >
    >
    >
    > Zener voltage regulation happens in reverse breakdown mode.
    >
    > All zeners leak reverse current at voltages below the breakdown voltage.
    >
    > With low voltage zeners, this leakage current is very high, up to 76mA
    >
    > for this part number. This makes low voltage zeners unsuitable for
    >
    > many applications, where you might expect a theoretically perfect
    >
    > zener to work.
    >
    >
    >
    > > My goal was not to use the zener as a regulator, but to protect 3.3v devices from potentially external over voltage events (microcontroller driving 5v circuits).

    >
    >
    >
    > If the 50mA or so leakage at 3V is an unacceptable issue for you,
    >
    > you might instead consider a potential divider across the 3V supply,
    >
    > with the centerpoint connected to an SCR gate to crowbar the supply.
    >
    > Adjust the potential divider ratio to give the right tripping voltage.
    >
    > A red led (forward biased) added at the top of the potential divider
    >
    > might make it more accurate/sensitive.
    >
    >
    >
    > I haven't actually tried this at such a low voltage, but I suspect it
    >
    > may work better than a zener.
    >
    >
    >
    > --
    >
    > Andrew Gabriel
    >
    > [email address is not usable -- followup in the newsgroup]


    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.
     
    jjk, Nov 1, 2013
    #21
    1. Advertisements

  2. In article <>,
    jjk <> writes:
    > 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's what I attempted to do. I guess the safest thing to do is add buffers in between the processor and external devices.


    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.

    --
    Andrew Gabriel
    [email address is not usable -- followup in the newsgroup]
     
    Andrew Gabriel, Nov 2, 2013
    #22
    1. Advertisements

Want to reply to this thread or ask your own question?

It takes just 2 minutes to sign up (and it's free!). Just click the sign up button to choose a username and then you can ask your own questions on the forum.
Similar Threads
  1. Joonhwan Lee
    Replies:
    1
    Views:
    1,240
    Soeren
    May 27, 2004
  2. Ronak Shah

    Zener Diode ID

    Ronak Shah, Nov 30, 2004, in forum: Electronic Basics
    Replies:
    13
    Views:
    1,565
    Rich Grise
    Dec 2, 2004
  3. Replies:
    10
    Views:
    1,965
    Yannick
    Mar 7, 2006
  4. Ken

    BZT03D240 zener diode/over volts suppressor

    Ken, Jan 23, 2004, in forum: Electronic Components
    Replies:
    3
    Views:
    420
  5. john

    High power Zener diode ?

    john, Oct 19, 2004, in forum: Electronic Components
    Replies:
    3
    Views:
    1,856
    petrus bitbyter
    Oct 27, 2004
  6. Diode Dilemma!

    , Aug 15, 2005, in forum: Electronic Components
    Replies:
    9
    Views:
    817
    Tom LeMense
    Aug 19, 2005
  7. Bicio

    Diode + Diode Zener and Inductive load

    Bicio, May 21, 2008, in forum: Electronic Design
    Replies:
    8
    Views:
    2,034
    Spehro Pefhany
    May 22, 2008
  8. Replies:
    18
    Views:
    1,016
    Phil Hobbs
    Aug 20, 2013
Loading...