Measure AC Voltage across capacitor

Discussion in 'The Projects Forum' started by MatheusLPS, Jan 23, 2014.

  1. MatheusLPS

    Thread Starter Member

    Jul 16, 2011
    Hi guys.

    I am doing a project that follows:

    I want to charge a capacitor through a SCR. This part I already did. Controlling the angle of the SCR I can control how much voltage will be on the capacitor.

    I am using the circuit below.

    The problem now is to measure this voltage across the capacitor and send it to a uC.

    I am trying to do with a voltage divider but this is not working because I getting strange signal on the middle of the voltage divider.


    Any advice?

  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
    To measure the voltage across the capacitor you would have to use a differential amplifier with a high common-mode range. The bottom end of the capacitor is not referenced to ground, but has a huge deltaV as the SCR switches.
  3. MatheusLPS

    Thread Starter Member

    Jul 16, 2011
    Thanks MikeML for the info. I made a little search here and you are right. This kind of amplifier would be perfect for me.

    But, they are so hard to find here in my country!

    Any other workaround? Maybe make a differential amplifier with custom parts?

    Since that i am using a uC to control the SCR, I can measure the top end of the capacitor and the botton end. In the code, subtract them. Can this work using voltage dividers******?


    ****** Thinking a little further, putting a voltage divider on the botton end of the capacitor will not work, it will be a direct path to ground..... :(

    Last edited: Jan 24, 2014
  4. t06afre

    AAC Fanatic!

    May 11, 2009
    What is the purpose of this circuit and what do you want to achieve with it
  5. inwo

    Well-Known Member

    Nov 7, 2013
  6. MatheusLPS

    Thread Starter Member

    Jul 16, 2011

    I am trying to modify my actual magnetizer. in this moment, mine operates Like this:

    Capacitor is charged fully from 220V with a SCR. The machine start to beep. After 5 seconds, it disable the SCR and activate another SCR that is in series with a coil. So the energy stored on the capacitor is fully discharged on the coil.

    This machine has 20 capacitors of 4700uF.

    But I am building a small one with 5 x 4700uF and I want to control how much voltage will be on the capacitor. That is why I need to measure this voltage.

    An example:

    I will enable the SCR on the right angle. The caps will start to charge. Lets say that when it has 100V I will disable the SCR charger, enable the SCR discharger and magnetize the coil.

    On the simulation I can see that 100V from the example. I just make a subtraction:

    On Proteus, when we use a voltage probe, it will measure the voltage to the ground. The only thing I do is place a probe on the top end of the capacitor and another probe on the bottom end. After that, I subtract one from another and plot the graphic and there will be my 100V CC across the capacitor.

    But how to make in on real life? I thought that this task could be more easy.

    I do not need isolation for now. All the parts can be live.


    Thanks for the link. I will that a look.

  7. BillB3857

    Senior Member

    Feb 28, 2009
    Unless I'm missing something, one major problem I see is that when SCR shuts off, there will be half wave rectified version of the AC source supplied to the inputs of the uProcessor. With no path to ground through the SCR, everything above it will ride right along without reduction.
  8. MatheusLPS

    Thread Starter Member

    Jul 16, 2011