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Theoretical Problem

Discussion in 'Electronics Homework Help' started by Engininja, Jul 18, 2012.

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  1. Engininja

    Engininja

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    0
    Jul 18, 2012
    Hi,

    As part of a uni course I have been tasked to design - "a circuit to limit a 20V rms sinusoidal voltage to a maximum positive amplitude of 10V and negative amplitude of -5V using a single 14Vdc voltage source." The basics of which are a using diode limiters and clampers possibly in conjunction with a voltage divider/doubler. I'm really stuck. Does anyone have any advice.

    Thanks
     
  2. duke37

    duke37

    5,364
    769
    Jan 9, 2011
    The specification of the task and the requiremnts are somewhat lacking.

    A resistor feeding a zener diode will limit the voltage but will need another diode in series with it to stop it conducting on the negative part of the cycle. This will give a clipped waveform.

    A 20V rms source will go from +28V to -28V, a swing of 57V.
    To get a swing of 15V, you could use an opamp with a gain of 0.26 and put in an offset to get the range you want. This will give a sinusoidal waveform.
     
  3. Engininja

    Engininja

    2
    0
    Jul 18, 2012
    I agree the details are lacking.
    I had a break-through last night (kinda). I have used a biased limiting circuit using a voltage divder off the +14Vdc supply to clip the positive cycle. I will need to do the same on the negtive cycle however I am unsure how to correctly biased that part. I wll play around with a simulation and see how i get on.

    Thanks though.
     
  4. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,268
    Nov 28, 2011
    Yes, the question is short on details.
    Simplest answer seems to be two zeners reverse-connected in series between the voltage and the 0V reference. The zener that clips the positive excursions needs to be around 9.3V (10V minus the forward voltage of the other zener) and has its cathode towards the signal. The zener that clips the negative excursions needs to be around 4.3V and has its anode towards the signal. The two zeners are connected in series, between the signal and ground. You would normally have a series resistor between the incoming signal and the zener, to limit the zener current. This arrangement doesn't need any supply voltage.

    If you're supposed to use diodes (but not zener diodes) and voltage dividers, that's easy enough for the positive excursion, but you'll need a negative voltage to clip the negative excursions.

    I suspect the trick is that the voltage source is floating, and you're supposed to balance it around the 0V reference for the sinewave using a voltage divider, so that the positive and negative ends of the 14V source sit at +9.5V and -4.5V respectively relative to 0V, then use diodes to clip the sinewave to those voltages.
     
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