Re: Accurately measure C of electrolytic capacitors ?

From: gmv (noemail_at_please.123)
Date: 02/06/05 

Date: Sun, 06 Feb 2005 00:41:51 GMT

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.

"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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