Simple diode circuit help

Discussion in 'General Electronics Chat' started by roybk, Aug 23, 2011.

  1. roybk

    Thread Starter New Member

    Aug 23, 2011
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    Excuse me, my circuit skills just got rusty.
    Can anyone please give me a clue what would the voltage rating be at the purple node? appreciated!

    R.
     
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  2. SgtWookie

    Expert

    Jul 17, 2007
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    Here's a datasheet for the BAV99; a dual diode:
    http://www.fairchildsemi.com/ds/BA/BAV99.pdf
    It's being used as a clamp to limit the voltage excursions above and below ground.

    The AUTO_IN - whatever that is - limits the current through either diode in the BAV99 in case the signal goes above 5v or below ground. The clamp at low current values (1mA) is the positive supply +~.7v to ground - ~0.7v
     
  3. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    Do you mean voltage rating or range?

    Presumably input AUTO_IN can have a somewhat unspecified range (within limits). This bit of circuitry seems to be designed to ensure the volatge at the node in question does not go outside the nominal range of 0 to 5V.

    The two diodes are orientated & biased to achieve that outcome.
     
  4. Adjuster

    Well-Known Member

    Dec 26, 2010
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  5. AdyCh

    New Member

    Aug 20, 2011
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    That voltage at that point, if we assume the fvd of diode 0.7V, will not be less than -0.7V and greater than +4.3V
     
  6. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    Greater than 5V+Vf ..... The node has to climb over 5V to get the top diode forward biased.
     
    Last edited: Aug 23, 2011
  7. Adjuster

    Well-Known Member

    Dec 26, 2010
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    An important point, that last. When adding diodes like this e.g. to protect an IC, it may be difficult to keep to safe voltages, which are often just a few hundred mV beyond the rails. Using Schottky devices may help, as may putting series elements between the big diodes and the device - the latter strictly restrain the current rather than the voltage.
     
  8. AdyCh

    New Member

    Aug 20, 2011
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    Correct. Yes it must be 5+Vd . Didn't notice at once
     
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