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What is this for

Discussion in 'Electronic Basics' started by Wong, Nov 19, 2003.

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  1. Wong

    Wong Guest

    Hi,
    I am learning electronics and I found something like this:

    Vcc
    -----
    |
    |
    ---
    / \ Diode
    ---
    |
    signal_in --------|----------------------- output
    |
    ---
    / \ Diode
    ---
    |
    |
    |
    -----
    ---
    -

    What is the purpose of the circuit ? As what I can understand, both
    diodes would not conduct if the signal_in range from 0 to 5v, but once
    the voltage of signal_in below 0v or more than 5v, either one will be
    conducted, am I right ? So what's purpose of the circuit ?
     
  2. It is a clamping, or limiter circuit to prevent damage to an input by
    out of range voltages.
    --
    I say, the boy is so stupid that he tried to make a back up copy of his
    hard drive on the Xerox machine!

    Michael A. Terrell
    Central Florida
     
  3. Its intended to clip the input. If the input gets higher than Vcc+0.6, the
    top diode conducts, and protects whatever output is feeding. If the input
    goes less than -0.6V, the bottom one conducts, and again protects the next
    device.

    Its often used to protect sensitive transistors from abuse. For example, a
    PIC microcontroller has this circuit on its inputs to protect from abuse.

    Regards,
    Bob Monsen
     
  4. Wong

    Wong Guest

    If either one is conducting, are we going to say that it is a shorted
    circuit ? Since there is no current limiting resistor.
     
  5. Baphomet

    Baphomet Guest

    Although not specified, I guess it can be assumed that the current is
    limited by the source impedance.
     
  6. Jim Large

    Jim Large Guest

    That would depend on what the input is connected to.
    Either the clamping diodes are going to conduct as
    much current as it takes to keep the voltage from
    going more than 0.6V outside the rails, or the Magic
    Smoke is going to come pouring out.

    You may wonder why the designer would give you such
    an easy way to let the smoke out of a circuit. Well,
    if the circuit is a CMOS logic gate, and the input
    voltage goes as much as 1V above Vcc or 1V below
    ground, the smoke is going to come out anyway, and it
    may even damage circuit board traces and power
    supplies in the process. Search Google for a fun
    phenomenon called SCR Latchup, and you will see why
    most CMOS ICs has those diodes on every pin.

    -- Jim L.
     
  7. Its not intended to handle current forever, simply to clip transient spikes.
    Thus, the diodes will survive the abuse, because they are not getting
    whacked too often.

    If you connect the input to a voltage source outside the range {-0.6,
    Vcc+0.6} you'll fry the diodes.

    Regards,
    Bob Monsen
     
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