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Calculate Smoothing Capacitor

Discussion in 'General Electronics Discussion' started by NetDevil, May 8, 2010.

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  1. NetDevil


    May 6, 2010
    Hey Everybody

    Im currently making a Fullwave precision rectifier but after this i Would like to have a smothing capasitor (i know i will not get full output this way).
    After searching around i find that there are multiple formulas for calculating this but im having some trouble in which to use.

    im relativly new in the electronics world an i have designed and simulated this circuit in National Instruments Circuit dsign tool.

    The rectified signal is a low voltage(<5V), 50Hz signal and im going to ad convert this signal, thats the reason for the smothing capasitor.

    Thank you in advance :)
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    You may be better off with a peak and hold circuit, resetting it occasionally.

    Simply charging a capacitor (through a diode to prevent the output of the op-amp from also discharging the capacitor, and a resistor to ensure stability) will also act as a (poor) peak detector, but will gradually discharge over time at an unknown rate.

    You really want to use the smallest capacitor you can to ensure high speed, but large enough to maintain the voltage within your voltage tolerance at the current required to measure that voltage.

    So simply asking "what capacitor" does not provide sufficient information.

    We also have the frequency, but we need to know how much current your precision rectifier can source, what your tolerance for measuring the voltage is (1%, 0.5%, 2mV, etc), and how much current is required to measure the voltage (a moving coil meter may require up to 50uA, a digital multimeter may have an input impedance of 1M, your A-D will have it's own impedance).
  3. NetDevil


    May 6, 2010
    Hey steve thanks for the response, to be hornest im not really sure what to do :) im a bit lost, and hopping for more guidance?

    Bellow is the circuit in question:


    I guess its not possible to "just" have a capasitor there, form calculations and simulations it seems the size of this have to be >1F and well in my mind thats just cant be right.

    I already build this circuit so is there anything i can do to not have to redo, or trash it for a peak holder circuit?

    The goal of this is to measure the power consumption using a current transformer with a 1000 windings as you can see on the diagram. furthermore i do need to get the data into a µController to proces the it.

    I have by re-reading Steve's post found my answer - Thanks
    Last edited: May 10, 2010
  4. Ian

    Ian Administrator

    Aug 23, 2006
    Hi NetDevil, I've re-uploaded your circuit diagram to this site, as the link in place before had some very "not safe for work" popups appearing when clicking on it. I'm just posting this as a heads up, as I'm sure it wasn't done intentionally. :)
  5. NetDevil


    May 6, 2010
    Thanks Ian, i just used the first hit on google :)
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