Protecting op amp input using Diodes

Discussion in 'The Projects Forum' started by The Engineer, Nov 4, 2014.

  1. The Engineer

    Thread Starter Member

    Sep 3, 2014
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    I am currently building a metal detector, In one part of the circuit there are 6 diodes that are used in order to "protect" the op amp. Can somebody please explain to me how they are protecting the op amp ? and why they are arranged in the order that they are?

    Here is a link to the PDF describing the metal detector circuit:-
    http://www.geotech1.com/pages/metdet/projects/twinloop/twinloop.pdf

    The area in question is labelled "Fig 4. Circuit diagram of reciever"

    Any help is greatly appreciated !
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    During normal operation, none of the diodes conduct. At most, they add a little capacitance to the opamp inputs. However, if you touch the coil assembly to something that is highly statically charged, the diodes will conduct, clamping the voltage at the opamp inputs at ~+-1V. The 470 Ohm resistors are there to limit the peak current that can flow from the coil toward the circuit board.

    Read this
     
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  3. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Diodes D5/D6 clamp pin 2 to within 1 diode drop (0.7V) of ground. The other diodes clamp pin 3 to within 2 diode drops of ground.
     
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  4. The Engineer

    Thread Starter Member

    Sep 3, 2014
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    Thankyou I am starting to understand, So would I be correct in saying D5 will protect the input two of the op amp from any surges during the positive half cycle, and D6 will protect from surges during the negative half cycle ? Also can you please explain why it clamps the inputs at +- 1V as opposed to another figure ? Thankyou for the link this looks very informative and I am in the process of reading it :) It looks as if these products may work as a substitute for the diodes.
     
  5. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Using Silicon Diodes:

    1.gif 1d.gif
     
    Last edited: Nov 4, 2014
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  6. The Engineer

    Thread Starter Member

    Sep 3, 2014
    35
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    thankyou very much for the diagram this has cleared things up for me :)!
     
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