Re: Accurately measure C of electrolytic capacitors ?

From: gmv (noemail_at_please.123)
Date: 02/07/05 

Date: Mon, 07 Feb 2005 15:51:05 GMT

"Watson A.Name - "Watt Sun, the Dark Remover"" <NOSPAM@dslextreme.com> wrote in message news:110cnbn267cd7fe@corp.supernews.com...
>
> "gmv" <noemail@please.123> wrote in message
> news:jJdNd.21541$C24.9629@attbi_s52...
>> Interesting,
>> I always thought the opamp buffered the output
>> so you could hook it to anything between 2K
>> and infinity.
>> I will keep in mind what you have said.
>> I have noticed a terrible ringing effect at Q
>> 5 and above.
>> Maybe that is what you are talking about.
>> In any case I really need to see things between .5Hz
>> and 2Hz. It is only the noise in my local area that
>> keeps me from doing this as far as I can tell.
>>
>> http://home.mchsi.com/~gmvoeth/index.html
>>
>> will show a bit more of what I am doing.
>
> Buy the RS printed circuit board that has the same layout as the
> breadoard. Build the circuit on that board, and put it inside of a
> metal enclosure, so that most of the noise is kept out of the circuit.

What is "RS printed circuit board " ?

I have settled on using only a single filter
as is now posted to my web site. This will
limit one to only seeing P waves but in
this noisy environment it is the best I can do.
I agree with everything you say except I can not
put a preamp outside with the Geophone.
There is a circuit I have included not on the
schematic and it is a 567 tone decoder on the back
end to decode WWV calibration marks.

Thanks for your advice, I can see a lot of
common sense to it. But I still do not
know how to buy a circuit board that already
has my layout on it. I figure I will have
to do the layout and etching etc... myself.

>
> All preamplification should be done as close as possible to the
> geophone, to get the signal well above the noise _before_ it goes any
> distance.
>
> Filtering should be done at the other end, to get rid of noise in the
> long transmission line between the sensor and the PC.
>
> One other important point. The outside can of the electrolytic is at
> signal level, and is a large capacitance to the outside world. Just
> shielding these from outside noise should help. You might consider
> putting some aluminum foil over the ones at the input, and grounding the
> foil to common. Just to see how much it will help to put the whole
> thing in an enclosure.
>
>> "Watson A.Name - "Watt Sun, the Dark Remover"" <NOSPAM@dslextreme.com>
> wrote in message news:1109v26sjt1i70c@corp.supernews.com...
>> >
>> > "gmv" <noemail@please.123> wrote in message
>> > news:EJ1Nd.51210$eT5.4698@attbi_s51...
>> >>
>> >> "Watson A.Name - "Watt Sun, the Dark Remover""
> <NOSPAM@dslextreme.com>
>> > wrote in message news:11097hnlc4n3q25@corp.supernews.com...
>> >> >
>> >> > "gmv" <noemail@please.123> wrote in message
>> >> > news:f_QMd.49525$eT5.27123@attbi_s51...
>> >> >> Sorry, Do not know what a DSP/FIR is.
>> >> >> I am using only 8 bit data.
>> >> >> Surprising enough the filters work quite well
>> >> >> between 1Hz and DC just like they do between
>> >> >> 1Hz and infinity. The only problem I have is
>> >> >> getting those capacitors in a non-electrolytic form.
>> >> >> I have found I can get pretty good matches in frequency
>> >> >> simply if I can know the true value of the capacitors.
>> >> >> The design matches the theory if only I can
>> >> >> match the capacitors.
>> >> >> I used to laugh at precision and accuracy but
>> >> >> I have found that is the only way to go when
>> >> >> dealing with filter circuits and possibly
>> >> >> oscillator circuits too.
>> >> >> The only way I will play with things like FIR or
>> >> >> DSP is if it will handle data from a non-linearI still think
>> >> >> kind of sensor that tends to overdrive the
>> >> >> preamp at high freqs making it impossible to
>> >> >> look at the low freqs also it got to be cheap.
>> >> >> I need to filter out
>> >> >> the upper freqs right off the bat or it is
>> >> >> impossible to get enough gain to look at the low.
>> >> >
>> >> > I still think you need to raise the impedance of the circuits
>> >> > up high enough so that the capacitors are much smaller.
>> >>
>> >> That increases the noise.
>> >>
>> >> >
>> >> > Using FET input opamps and megohm resistors, you should be able
>> >> > to get down to very low freqs with caps of a microfarad.
>> >>
>> >> I think I need BJT types if I remember right
>> >> they are lower in noise. I have found chopper
>> >> amps to be out of the question.
>> >>
>> >> >
>> >> > I offered that Action Electronics web page with the motor
>> > capacitors,
>> >> > but I don't know if they are electrolytic or not. Maybe someone
>> >> > could help. They also have tantalom caps, which are more stable
>> >> > than 'lytics. http://www.action-electronics.com/capacitr.htm
>> >>
>> >> motor caps are too high power for my needs
>> >> I am just using small signal here.
>> >> NP caps are what i need.
>> >
>> > Well, the motor caps are non-polarized. That's what you want.
>> > And they are not electrolytic, I believe, but I'm not certain.
>> > And the power doesn't make any diff, but they are large, and
>> > that might be a problem. As I already mentioned, there are
>> > ceiling fan capacitors at the local hardware store that are
>> > about 10 uF, and are not very large. ANd reasonably priced IIRC.
>> >
>> > About noise, you decrease noise by reducing the bandwidth. You
>> > reduce the bandwidth by reducing the load on the tuned circuit.
>> > You do this by increasing the impedance of the load on the tuned
>> > circuit, IOW use a higher resistance, and that means FETs or
>> > other very high impedance amps.
>> >
>> > I think you are getting more noise because you are decreasing
>> > the bandwidth by keeping the load the same impedance, but
>> > increasing the tuned circuit impedance. That's backwards
>> > from what you should be doing.
>> >
>> >> I think the ceramic ones would be perfect.
>> >> I just want ones that will read consistant with my C meter.
>> >>
>> >>
>> >> Thanks to everyone for your answers
>> >> but like always I must seek a solution myself
>> >> because it is so rare I really find any answers
>> >> on these newsgroups.
>> >> It is as if the Engineers and Scientists
>> >> avoid this internet stuff like it is the plague.
>> >> I just tried building my own cap and a monster
>> >> size it was and only 3.3nf...they must be very
>> >> hard to make at 10uf or higher and be small.
>> >> You need to use the same technology that makes
>> >> ICs. Thin layer of metal, thin layer of insulator,
>> >> do this repeatedly until you get the right value.
>> >> Surely there must be a breakthrough in technology
>> >> to make decent high value caps.
>> >> Sort of like the movie THIS ISLAND EARTH
>> >> I need to find that engineers supplier of
>> >> interocitor parts.
>> >>
>> >>
>> >>
>> >>
>> >> >
>> >> >
>> >> >> Yes, a log amp is a possibility, I have already
>> >> >> considered that. But the signals I now receive
>> >> >> are good enough for myself and all I wish to do
>> >> >> is to bring closure to my circuit be acquiring what
>> >> >> I consider to be decent capacitors. These electrolytics
>> >> >> are seemingly only for one frequency because
>> >> >> funny things start happening if you read the
>> >> >> capacitance with different frequencies.
>> >> >> Like I get a lower capacitance at a higher frequency.
>> >> >> I have found no such problems with regular
>> >> >> caps that do not use an electrolytic form.
>> >> >> Your solution would be possible if I had a sensor
>> >> >> resonant at 20 seconds but the Geophone I am using
>> >> >> is resonant at something over 1 Hz.
>> >> >> Geospace has just the device I would like to have
>> >> >> they have a .5 Hz Geophone but they want about
>> >> >> $3000 for one and that is way too high for someone
>> >> >> living on Social Security.
>> >> >>
>> >> >>
>> >> >> "Rob Gaddi" <rgaddi@bcm.YUMMYSPAMtmc.edu> wrote in message
>> >> > news:cu0elc$t4c@gazette.corp.bcm.tmc.edu...
>> >> >> > While I hate to give my usual answer, have you considered just
>> >> > digitizing the data and running it through a DSP? You're talking
>> >> >> > about some massively slow filters to be trying to make with
> any
>> >> > reliable tolerance in analog, and with component values that
> large
>> >> >> > it's not going to be cheap. Plus you can use a linear FIR
> filter
>> >> > and not have to worry about the phase distortion you were
>> >> >> > mentioning.
>> >> >> >
>> >> >> > If your noise is really swamping the signal, and dynamic range
> is
>> > a
>> >> > concern, at the low speeds you're talking about you could run
>> >> >> > the signal through a log amp before you A/D it.
>> >> >> >
>> >> >> >>
>> >> >> >> Lucky for me different phases of Earthquakes
>> >> >> >> seem to prefer different frequencies.
>> >> >> >> But my picture will never be as good as the
>> >> >> >> big boys.
>> >> >> >> Just too darn noisy at this location.
>> >> >> >> I get a terrible noise at maybe 15 Hz or so.
>> >> >> >> Need to filter like crazy to get rid of it.
>> >> >> >> It drowns out just about everything i try to do.
>> >> >> >> I suspect but can not prove that my geophone
>> >> >> >> is out of spec and not working at the 1Hz
>> >> >> >> it is supposed to.
>> >> >> >> The only adjustment I can make is to center the
>> >> >> >> mass in the housing. If there is a way to lower the
>> >> >> >> resonant freq I would sure like to know.
>> >> >> >> I am thinking of finding some weights to put on
>> >> >> >> the adjustment screw as much weight as can still be
>> >> >> >> centered.
>> >> >> >> But it is my experience that for a spring to
>> >> >> >> lower its frequency it needs to extend or compress
>> >> >> >> farther in distance. similar to a pendulums length.
>> >> >> >> T=2PiSqRt(length/gravity)
>> >> >>
>> >> >>
>> >> >
>> >> >
>> >>
>> >>
>> >
>> >
>>
>>
>
>

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