Wanting a circuit to find the average of a fluctuating DC signal ???

Discussion in 'Electronic Basics' started by jalbers@bsu.edu, Jul 23, 2008.

  1. Guest

    I am looking for a circuit that will give me the average of a
    fluctuating DC signal. I am using the term average to roughly mean a
    value in the middle but doens not have to be exact.

    A few years ago I was told by some people in this group to simply use
    a low pass filter. At that time I did not have the equipment, time,
    or knowledge of filters to mess with it. I have started to play
    around with different types of filters...

    I have done a little bit of experimenting using a scope, signal
    generator, and a simple low pass RC filter but can't get it to work to
    find the average of the signal.

    I am ultimately interested in averaging the fluctuating DC signal from
    a homemade EDM machine which is a basically a big DC relaxation
    oscillator. For various reasons, I do not want to use the actual EDM
    machine for these experiments. I have been using a function generator
    and also a NE-2 relaxation oscillator to somewhat simulate the signal
    that the EDM machine is going to be putting off. I have experimented
    with sine, triangle, square, and relaxation oscillator wave forms with
    enough DC offset to push the entire waveform completely up above the x-
    axis on the scope. To no surprize, increasing the frequency begins to
    attenuate the signal and the output of the low pass filter starts to
    hug the x-axis. The capacitor is removing the DC component and
    attenuating the AC part of the signal.

    I don't think that a LC low pass filter is going to work the way I
    want it to either. It will not remove the DC component but the
    waveform will still hug the x-axis.

    I am looking for something that will attenuate the signal as the
    frequency increases but still keep the DC component of the signal.

    Any help would be greatly appreciated. Thanks
    , Jul 23, 2008
    #1
  2. Thomas Magma Guest

    <> wrote in message
    news:...
    >I am looking for a circuit that will give me the average of a
    > fluctuating DC signal. I am using the term average to roughly mean a
    > value in the middle but doens not have to be exact.
    >
    > A few years ago I was told by some people in this group to simply use
    > a low pass filter. At that time I did not have the equipment, time,
    > or knowledge of filters to mess with it. I have started to play
    > around with different types of filters...
    >
    > I have done a little bit of experimenting using a scope, signal
    > generator, and a simple low pass RC filter but can't get it to work to
    > find the average of the signal.
    >
    > I am ultimately interested in averaging the fluctuating DC signal from
    > a homemade EDM machine which is a basically a big DC relaxation
    > oscillator. For various reasons, I do not want to use the actual EDM
    > machine for these experiments. I have been using a function generator
    > and also a NE-2 relaxation oscillator to somewhat simulate the signal
    > that the EDM machine is going to be putting off. I have experimented
    > with sine, triangle, square, and relaxation oscillator wave forms with
    > enough DC offset to push the entire waveform completely up above the x-
    > axis on the scope. To no surprize, increasing the frequency begins to
    > attenuate the signal and the output of the low pass filter starts to
    > hug the x-axis. The capacitor is removing the DC component and
    > attenuating the AC part of the signal.
    >
    > I don't think that a LC low pass filter is going to work the way I
    > want it to either. It will not remove the DC component but the
    > waveform will still hug the x-axis.
    >
    > I am looking for something that will attenuate the signal as the
    > frequency increases but still keep the DC component of the signal.
    >
    > Any help would be greatly appreciated. Thanks


    A capacitor in a low pass filter shouldn't be removing the DC component, by
    definition it is just the opposite. What voltages and frequencies are we
    talking about here? And what is this x-axis you are talking about?

    How about a simple RC filter?

    Thomas
    Thomas Magma, Jul 23, 2008
    #2
  3. Bob Monsen Guest

    <> wrote in message
    news:...
    >I am looking for a circuit that will give me the average of a
    > fluctuating DC signal. I am using the term average to roughly mean a
    > value in the middle but doens not have to be exact.
    >
    > A few years ago I was told by some people in this group to simply use
    > a low pass filter. At that time I did not have the equipment, time,
    > or knowledge of filters to mess with it. I have started to play
    > around with different types of filters...
    >
    > I have done a little bit of experimenting using a scope, signal
    > generator, and a simple low pass RC filter but can't get it to work to
    > find the average of the signal.
    >
    > I am ultimately interested in averaging the fluctuating DC signal from
    > a homemade EDM machine which is a basically a big DC relaxation
    > oscillator. For various reasons, I do not want to use the actual EDM
    > machine for these experiments. I have been using a function generator
    > and also a NE-2 relaxation oscillator to somewhat simulate the signal
    > that the EDM machine is going to be putting off. I have experimented
    > with sine, triangle, square, and relaxation oscillator wave forms with
    > enough DC offset to push the entire waveform completely up above the x-
    > axis on the scope. To no surprize, increasing the frequency begins to
    > attenuate the signal and the output of the low pass filter starts to
    > hug the x-axis. The capacitor is removing the DC component and
    > attenuating the AC part of the signal.
    >
    > I don't think that a LC low pass filter is going to work the way I
    > want it to either. It will not remove the DC component but the
    > waveform will still hug the x-axis.
    >
    > I am looking for something that will attenuate the signal as the
    > frequency increases but still keep the DC component of the signal.
    >
    > Any help would be greatly appreciated. Thanks



    As Mr Larken points out, you need to define your concept of 'average'.

    You need to define

    1) The period over which you want the average taken
    2) The frequencies your input signal contains
    3) How you want to read the average value obtained (ie, what kind of a
    circuit will use the output).
    4) How much the 'averaging' circuit may affect the input.

    That is all I can think of, but Mr Larken may have more ideas. Once the
    requirements are defined, it'll be easier to help you.

    Regards,
    Bob Monsen
    Bob Monsen, Jul 23, 2008
    #3
  4. Guest

    Re: Wanting a circuit to find the average of a fluctuating DC signal???

    On Jul 23, 12:48 pm, John Popelish <> wrote:
    > wrote:
    >
    > (snip)
    >
    > > I am ultimately interested in averaging the fluctuating DC signal from
    > > a homemade EDM machine which is a basically a big DC relaxation
    > > oscillator.   For various reasons, I do not want to use the actual EDM
    > > machine for these experiments.  I have been using a function generator
    > > and also a NE-2 relaxation oscillator to somewhat simulate the signal
    > > that the EDM machine is going to be putting off.  I have experimented
    > > with sine, triangle, square, and relaxation oscillator wave forms with
    > > enough DC offset to push the entire waveform completely up above the x-
    > > axis on the scope.  To no surprize, increasing the frequency begins to
    > > attenuate the signal and the output of the low pass filter starts to
    > > hug the x-axis.  The capacitor is removing the DC component and
    > > attenuating the AC part of the signal.

    > (snip)
    > > I am looking for something that will attenuate the signal as the
    > > frequency increases but still keep the DC component of the signal.

    >
    > Swap the positions of the R and C in your filter.  The R
    > should connect to the oscillator signal and the C should
    > connect to ground.
    >
    > --
    > Regards,
    >
    > John Popelish


    Thanks for figuring out from the info I had given that I had the
    connections switched around. The circuit seems to work correctly, as
    the frequency of the the signal produced by the function generator
    increases the voltage across C decreases but is centered around the DC
    offset and the voltage across R increases but is centered around zero.

    I am however having a little trouble using this same RC low pass
    filter on the NE-2 relaxation oscillator circuit. The relaxation
    oscillator is producing a nice sawtooth wave centered around 70 VDC on
    the scope but when I connect the output of the relaxation oscillator
    to the RC low pass filter and chage the frequency, I am not seeing the
    voltage across C (sawtooth centered around 70 VDC) decreasing in
    amplitude and the components in the low pass filter are srewing with
    the frequency of the relaction oscillator.

    The relaxation oscillator I am using consists of a 1M resistor wired
    in series with NE-2 wired parallel to a capacitor and all of this is
    conned to a 150VDC supply. I am using a .47uF, 11nF, and 1.9nF to
    generate 9Hz, 333Hz, and 2500Hz signals. The RC filter consists of a
    1K resistor and .1 uF capacitor. What size resistor and capacitor
    shoud I be using in the RC low pass filter to be able to observe the
    wave form decreasing in amplitude around the 70VDC offset?
    , Jul 23, 2008
    #4
  5. whit3rd Guest

    Re: Wanting a circuit to find the average of a fluctuating DC signal???

    On Jul 23, 11:14 am, "" <> wrote:
    > I am looking for a circuit that will give me the average of a
    > fluctuating DC signal. ... from
    > a homemade EDM machine


    The gap voltage peak and gap voltage average have a useful
    relationship,
    or the average current might be useful, so let's consider those
    first.

    Current can be related simply to the power supply input current; just
    measure the input current if you want that (the power supply
    filter will keep fluctuations relatively slow).

    The gap voltage rings/oscillates and has fast-slewing character, so it
    isn't
    nice to attach a long wire to this; you will want to have a filter
    near
    the spark gap that attenuates RF (maybe .01 uF/100 kohm resistor),
    as well as a slow filter (two seconds time constant?) using an op
    amp as an integrator. Analog (moving-needle) meters are possibly
    slow enough to just connect directly, using the inertia of the moving
    assembly.

    It's possible, too, that your concern is with the area under a single
    pulse
    X = integral(gap_voltage * dT| T= 0 to T = end-of-pulse)
    (irrespective of time separation of pulses) which requires a
    resettable
    circuit. First, discharge a capacitor. Then, set a timer to open a
    gate
    when a pulse is detected, and integrate (onto the capacitor) current
    proportional to the gap voltage until the timer closes the gate. Then
    display the capacitor's voltage on a meter, using a sample/hold
    circuit. It sounds complicated, but a dual op amp and quad analog
    switch
    will do the job.
    whit3rd, Jul 25, 2008
    #5
  6. Rich Grise Guest

    On Wed, 23 Jul 2008 08:14:57 -0700, wrote:

    > I am looking for something that will attenuate the signal as the
    > frequency increases but still keep the DC component of the signal.
    >
    > Any help would be greatly appreciated.


    Signal in >-----[R]-----+--------> DC out
    |
    [C]
    |
    GND

    Determine the R and C values by the formula f = 1/(2 * PI * SQRT(R * C))
    approx; then tweak until you get the result you want. (a higher-value
    resistor OR a higher-value capacitor (or both) will decrease the cutoff
    frequency.

    Have Fun!
    Rich
    Rich Grise, Jul 30, 2008
    #6

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