Accurately measure C of electrolytic capacitors ?

[gmv]

Is there anyway to accurately measure the
capacitance of an electrolytic capacitor
in the range of 10uf to 100uf.

These capacitors are to be used at frequencies
under 1 Hz.

Everytime I try to use my meter to measure, the
capacitance will change from one range scale
to the next, I guess because the frequency changes too.
When I use a typical non-electrolytic capacitor
these changes in readings are not there.

I have been thinking of making
a simple RC inverter oscillator and using
the resulting frequency to determine the
true capacitance in the small signal
application I want to apply them to.

Maybe there are non-electrolytic capacitors
in the 10uf to 100uf range but if so
I have never heard of them before.

I do not know if any of you have ever seen
the SCI FI movie THIS ISLAND EARTH
but I am in need of several of those
fantastic capacitors.

I am trying to get a reading
within 1% of reality.

Help here is appreciated.


--
Regards;
gmv

 

[loedown]

There are non polarized caps, mainly used in loudspeaker crossover networks,
some at 100uF if that helps?

Paul

 

[loedown]

If you need really closely matched bandpass filters, have you considered
crystal filters. I know very little about them, but I do know they have a
very sharp cutoff / roll off

Paul

 

[Watson A.Name - \"Watt Su]

"gmv" <noemail@please.123> wrote in message
news:%nIMd.48003$EG1.44889@attbi_s53...

They dont work at freqs below 10Hz
unless they are very large.
I have never seen a 1/20Hz crystal before.

You're right, a xtal would have to be huge to get down to that freq.
However you could multiply the low freq many times and then filter it.

"loedown" <zero242@internode.on.net> wrote in message
news:42034d8b$1@duster.adelaide.on.net...
If you need really closely matched bandpass filters, have you
considered crystal filters. I know very little about them, but I do
know they have a very sharp cutoff / roll off

Paul

 

[Jim Backus]

On Thu, 3 Feb 2005 21:37:45 UTC, "gmv" <noemail@please.123> wrote:

I am trying to get a reading
within 1% of reality.

I'm not sure reality allows measuring electrolytics to 1%

I've never considered electrolytics to be stable components. Their
typical manufacturing tolerance is -20% +50%.

--
Jim Backus OS/2 user since 1994
bona fide replies to j <dot> backus <the circle thingy> jita <dot>
demon <dot> co <dot> uk

 

[Watson A.Name - \"Watt Su]

"Jim Backus" <jhb@nospam.co.uk> wrote in message
news:TpquPuPd0tCd-pn2-SgulLeF9nLcd@localhost...

On Thu, 3 Feb 2005 21:37:45 UTC, "gmv" <noemail@please.123> wrote:

I am trying to get a reading
within 1% of reality.

I'm not sure reality allows measuring electrolytics to 1%

I've never considered electrolytics to be stable components. Their
typical manufacturing tolerance is -20% +50%.

--
Jim Backus OS/2 user since 1994

But those ceramic caps that are .1 uF or more have tempcos that seem to
make them great temp sensors. I put my fingers on a cold one and it
changes by ten percent or more. Makes me think that I could use one in
a chirping cricket circuit and just like a cricket, it will speed up as
it gets warmer.

 

[Fred Abse]

On Fri, 04 Feb 2005 20:18:02 +0000, Jim Backus wrote:

I'm not sure reality allows measuring electrolytics to 1%

Reality allows measuring electrolytics to better accuracy than that.

Unfortunately, however, it doesn't allow *making* them to such tolerances.
Capacitance can also vary with polarizing voltage.


--
Then there's duct tape ...
(Garrison Keillor)

 

[Watson A.Name - \"Watt Su]

"Fred Abse" <excretatauris@cerebrumconfus.it> wrote in message
newsan.2005.02.05.12.21.24.536446@cerebrumconfus.it...

On Fri, 04 Feb 2005 20:18:02 +0000, Jim Backus wrote:

I'm not sure reality allows measuring electrolytics to 1%

Reality allows measuring electrolytics to better accuracy than that.

Unfortunately, however, it doesn't allow *making* them to such
tolerances.
Capacitance can also vary with polarizing voltage.

The thought occurred to me that at those very low freqs, the polarized
electrolytic may have a charge on it long enough during each cycle so
that the chemicals have time to do something that changes the
capacitance.

> --

 

[Fred Abse]

On Sat, 05 Feb 2005 09:21:02 -0800, Watson A.Name - "Watt Sun, the Dark
Remover" wrote:

The thought occurred to me that at those very low freqs, the polarized
electrolytic may have a charge on it long enough during each cycle so that
the chemicals have time to do something that changes the capacitance.

Quite true.

Interesting question: When does low frequency AC become varying DC?

--
Then there's duct tape ...
(Garrison Keillor)

 

[Charlie+]

On Sun, 06 Feb 2005 00:59:10 GMT, "gmv" <noemail@please.123> wrote as
underneath my scribble :

Problem is getting your absolute value with a homemade solution unless
you have a calibrated standard to go from/compare with especially with
large caps as there are residual voltage issues.
I have an old homemade oscillator based L/C bridge measurer which has
very good repeatability and accuracy with small values but relies on
comparison in any range with calibrated standards - it is rough at
anything over 1 uF as far as repeatability goes especially with
electrolytics, over 5uF forget it! ; and this unit was not
particularly cheap or simple to make at the time!
Charlie+

Exactly what I was considering.
Possibly a simple RC inverter oscillator.

 

[Watson A.Name - \"Watt Su]

"Fred Abse" <excretatauris@cerebrumconfus.it> wrote in message
newsan.2005.02.05.19.01.32.904972@cerebrumconfus.it...

On Sat, 05 Feb 2005 09:21:02 -0800, Watson A.Name - "Watt Sun, the
Dark
Remover" wrote:

The thought occurred to me that at those very low freqs, the
polarized
electrolytic may have a charge on it long enough during each cycle
so that
the chemicals have time to do something that changes the
capacitance.

Quite true.

Interesting question: When does low frequency AC become varying DC?

--

Or more like when does the variation of the current become slow enough
to allow the chemicals time to react, and change the capacitor?

 

[loedown]

Hi All,
I know very little about radio and the like, but could some form
of superheterodyning be done in this instance, the idea of AM, or FM to
combine the slow signal with much higher frequency?

Paul

 

[Watson A.Name - \"Watt Su]

"loedown" <zero242@internode.on.net> wrote in message
news:4207ea40$1@duster.adelaide.on.net...

Hi All,
I know very little about radio and the like,

Yeah. I guess not.

but could some form
of superheterodyning be done in this instance, the idea of AM, or FM
to
combine the slow signal with much higher frequency?

If you mix the signal with a much higher freq, then the bandwidth of the
filter has to be much, much narrower. Which then makes filtering that
much more difficult.

> Paul

 

[Hal Murray]

I must filter out the high stuff first because my sample rate
is only 65539.3 samples per hour and my dynamic range is only
8 bits. Like I told someone else my sensor has a non-linear
response in the frequency range of interest.
2Hz to 1/20 Hz.
It would be so wonderful to have a dead end at 2HZ kind
of filter then most of my troubles would evaporate.

What is your signal/noise ratio? (without the filter)

Can you get an A/D with enough bits so that you can see
the signal in the bottom bits when the gain is set so that
the noise doesn't saturate the A/D? If so, then consider
the DSP suggestion. It's easy to do the sort of filtering
you want in software, especially at such slow speeds.

--
The suespammers.org mail server is located in California. So are all my
other mailboxes. Please do not send unsolicited bulk e-mail or unsolicited
commercial e-mail to my suespammers.org address or any of my other addresses.
These are my opinions, not necessarily my employer's. I hate spam.

 

[gmv]

All I can tell you is the noise is way way up there
and I need to filter most all of it out before I can see
anything of interest. You need to understand the nature
of a Geophone resonant at 1Hz to understand what I am dealing
with. The device acts like a high pass filter with a rolloff
at the resonant freq.
There are noises in the ground at about 15 Hz it seems that
swamp all else. All I can think of is there are some
very big motors working fast and furious all the time
in my town.
I can not evaluate the noise because my sample rate is only good
for up to 9Hz.
The area I need to look is between .5Hz and 2Hz so what I really
need is a .5Hz Geophone but it will be a cold day in hell
before I will pay $3000 to buy one.
I have been thinking of building a great little
pendulum sensor but that will be too large for me to
hide well in my back yard.
I have come to hate being forced to live
with the rat race kind of people because of all
the noise they generate.


"Hal Murray" <hmurray@suespammers.org> wrote in message news:jZKdndS0X8axz5jfRVn-vw@megapath.net...

I must filter out the high stuff first because my sample rate
is only 65539.3 samples per hour and my dynamic range is only
8 bits. Like I told someone else my sensor has a non-linear
response in the frequency range of interest.
2Hz to 1/20 Hz.
It would be so wonderful to have a dead end at 2HZ kind
of filter then most of my troubles would evaporate.

What is your signal/noise ratio? (without the filter)

Can you get an A/D with enough bits so that you can see
the signal in the bottom bits when the gain is set so that
the noise doesn't saturate the A/D? If so, then consider
the DSP suggestion. It's easy to do the sort of filtering
you want in software, especially at such slow speeds.

--
The suespammers.org mail server is located in California. So are all my
other mailboxes. Please do not send unsolicited bulk e-mail or unsolicited
commercial e-mail to my suespammers.org address or any of my other addresses.
These are my opinions, not necessarily my employer's. I hate spam.

 

[gmv]

You make an electrolytic capacitor sound like
it is a very large veractor diode.


"Fred Abse" <excretatauris@cerebrumconfus.it> wrote in message newsan.2005.02.05.12.21.24.536446@cerebrumconfus.it...

On Fri, 04 Feb 2005 20:18:02 +0000, Jim Backus wrote:

I'm not sure reality allows measuring electrolytics to 1%

Reality allows measuring electrolytics to better accuracy than that.

Unfortunately, however, it doesn't allow *making* them to such tolerances.
Capacitance can also vary with polarizing voltage.


--
Then there's duct tape ...
(Garrison Keillor)

 

[gmv]

The attention should be focused on
measuring the C of an electrolytic capacitor.
Can you tell me how to be sure within 1%
the true value of a 10Uf or 100uf cap
when your meter will give you different readings
at different frequencies.
at 80Hz I might get 10u and at 8Hz
I get 11u so which one do I trust ?
I have found the best way to be sure
is to make a filter ring and see if it
rings at the designed center frequency.
I lack the proper test equipment so I got be
imaginative.



"Fred Abse" <excretatauris@cerebrumconfus.it> wrote in message newsan.2005.02.05.19.01.32.904972@cerebrumconfus.it...

On Sat, 05 Feb 2005 09:21:02 -0800, Watson A.Name - "Watt Sun, the Dark
Remover" wrote:

The thought occurred to me that at those very low freqs, the polarized
electrolytic may have a charge on it long enough during each cycle so that
the chemicals have time to do something that changes the capacitance.

Quite true.

Interesting question: When does low frequency AC become varying DC?

--
Then there's duct tape ...
(Garrison Keillor)

 

[Fred Abse]

On Sun, 06 Feb 2005 16:39:34 +0000, gmv wrote:

The attention should be focused on
measuring the C of an electrolytic capacitor. Can you tell me how to be
sure within 1% the true value of a 10Uf or 100uf cap when your meter will
give you different readings at different frequencies.
at 80Hz I might get 10u and at 8Hz
I get 11u so which one do I trust ?

Both, if your measuring instrument is any good. The capacitance of
electrolytics *does* vary with applied frequency, though usually not as
much as 10% over a decade. What sort of instrument is it? Does it polarize
the capacitor?

I have found the best way to be sure
is to make a filter ring and see if it rings at the designed center
frequency. I lack the proper test equipment so I got be imaginative.

That's a sensible approach.


--
Then there's duct tape ...
(Garrison Keillor)