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

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

jalbers@bsu.edu
Guest
Posts: n/a

 07-23-2008, 03:14 PM
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

Thomas Magma
Guest
Posts: n/a

 07-23-2008, 04:43 PM
<(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
>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

Bob Monsen
Guest
Posts: n/a

 07-23-2008, 06:21 PM
<(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
>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

Regards,
Bob Monsen

jalbers@bsu.edu
Guest
Posts: n/a

 07-23-2008, 07:06 PM
On Jul 23, 12:48*pm, John Popelish <(E-Mail Removed)> wrote:
> (E-Mail Removed) 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?

whit3rd
Guest
Posts: n/a

 07-25-2008, 08:47 PM
On Jul 23, 11:14*am, "(E-Mail Removed)" <(E-Mail Removed)> wrote:
> I am looking for a circuit that will give me the average of a
> fluctuating DC signal. ... from

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.

Rich Grise
Guest
Posts: n/a

 07-30-2008, 10:44 PM
On Wed, 23 Jul 2008 08:14:57 -0700, (E-Mail Removed) 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

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