Can I use this configuration of diode to cut the voltage when above 5V?

Discussion in 'The Projects Forum' started by yaantey, Jan 26, 2012.

  1. yaantey

    Thread Starter Member

    Oct 7, 2011
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    Hey,
    Attached is an amplification circuit. I want to know whether I can use the configuration of the diodes to cut-off any voltage above 5V. The output from amplifier is connected to microp ADC. So can anyone let me know if I can use this configuration? If not can you direct me to a circuit I can use.
    Thanks.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    D1 clamps the output at 5v + the Vf of D1, or ~5.7v.

    If you want to clamp it at 5v, then you'll need to use something like a 4.7v or 5.1v Zener from ground to the output signal.
     
  3. yaantey

    Thread Starter Member

    Oct 7, 2011
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    @SgtWookie: can u provide a simple circuit to show what you mean? I couldn't understand. Thanks.
     
  4. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    He meant remove D1 and replace D2 with a zener diode, 5.1V or 4.7V.
     
  5. yaantey

    Thread Starter Member

    Oct 7, 2011
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    I tested with with a 6.2V zener diode to clip off the voltage. I connected the zener diode in the configuration as shown in the link (http://www.electronics-tutorials.ws/diode/diode_7.html) under "single zener diode clipping" and used a function generator to input a AC signal and an oscilloscope to see the ouput. The voltage was cut-off at 7.25V. So, my question is can I use a lower rated zener diode in the same configuration to cut-off voltage at 5V?

    Thanks
     
  6. yaantey

    Thread Starter Member

    Oct 7, 2011
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    One more question is, what does the power rating of zener diodes indicate? Which power rating zene diode would be good enough for me? Will 500mW do?
     
  7. PaulEE

    Member

    Dec 23, 2011
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    yaantey,

    If you only need to limit the output at +5v, you do not need two diodes. Two diodes establish limits at +/-V, where "V" is a difference of V volts across the diode terminals. As drawn, you'd limit your output to +5.7v (approx.) one way, and the bottom diode wouldn't do anything because your circuit cannot generate an output voltage less than common (ground). If, however, your circuit had split supplies on the op-amps, THEN the bottom diode would limit the output to -0.7v.....

    Basically, if you want to limit your output voltage to +5v (approx.), use a +5.1v zener diode as the bottom diode (cathode to output of amplifier, anode to ground), and remove the top one.
     
  8. yaantey

    Thread Starter Member

    Oct 7, 2011
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    The anode of the zener diode is the end without the band right and the cathode is the end with the band right?
     
  9. kubeek

    AAC Fanatic!

    Sep 20, 2005
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  10. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Typically microcontrollers give you a little "wiggle room" above Vdd and below Vss. The exact amount depends on the diodes they place on the inputs just as you are doing in your circuit. Do check the spec for the part you are using to determine this voltage and that there are really diodes inside the micro where I say they are. Some newer devices eliminate these diodes on some pins. They are the traditional ESD protection diodes.

    It is common to use the internal diodes for protection; you have a nice big 10K resistor to limit the current going thru these diodes. Should you wish to use external diodes use schottky types for their lower diode drop.

    Don't use a zener, it will affect the accuracy below 5V. They leak you know, even below the knee.
     
  11. mcasale

    Member

    Jul 18, 2011
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    Hey, here's a dumb idea: drive the op amps with a 5V supply!

    If you need the outputs to go all the way up to the rail, there are others that will do that.

    Just a thought.
     
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