CMOS Logic Probe

Discussion in 'The Completed Projects Collection' started by MCU88, Mar 22, 2015.

  1. MCU88

    Thread Starter Member

    Mar 12, 2015
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    CMOS Logic Probe ...

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  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    I query the 'Probe max 50VDC' spec.
    With 50V on the probe there will be ~49.4V on pin 8 of the LM393 (via D1), whose max supply voltage is only 36V :(. Also, the max cathode voltage for the TL431 is only 37V.
     
  3. MCU88

    Thread Starter Member

    Mar 12, 2015
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    D1 & D2 clamp / clip high voltages to 0.6V above and 0.6V below supply rails.
     
  4. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    I don't see how D1 can do so for +50V on the probe. D3 prevents that.
     
  5. MCU88

    Thread Starter Member

    Mar 12, 2015
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    Sorry that you cannot see it. Reading circuits can be tricky sometimes.
     
  6. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    My bad. The two 1k resistors share the 50V between them, so the LM393 doesn't see more than 25V.
     
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