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PICList Thread
'Pic based capacitance measurement'
1999\03\01@153839 by Adam Bryant

picon face
Dear PIClister's,
Does anyone else out there have a drawer full of anonymous capacitors?  I
was wondering about the feasibility of modifying the Microchip app note
that does A/D conversion using a resistor and capacitor and using it to
measure the capacitor instead.  Using the PIC as a timer it should be
possible to get a semi-close measurement of the capacitance and you could
even do auto-ranging and select different resistance values depending on
the measurements you get.  I really don't need too exact of a
measurement, just something to tell me if a capacitor is 1uf or 1mf.  So
I think it should be possible to build something fairly low cost using a
PIC.

Ideas? Thoughts? Suggestions?  Anyone done this?

Adam Bryant (age 0x23)
spam_OUTabryantTakeThisOuTspampeaktech.com (work)
.....adamdbKILLspamspam@spam@juno.com (home)
Parker, CO, USA
Robotics, RC Airplanes, anything using a PIC

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1999\03\01@175059 by Lynx {Glenn Jones}

flavicon
face
well, if its just between 1uF and 1mF (which im guessing its not :) then
you can just build a simle audio oscilator out of a 555 and plug in the
caps as timing caps. then you will have one tone for 1uF and another for
1mF

------------------------------------------------------------------------------
A member of the PI-100 Club:
3.1415926535897932384626433832795028841971693993751
058209749445923078164062862089986280348253421170679

On Mon, 1 Mar 1999, Adam Bryant wrote:

{Quote hidden}

1999\03\01@191021 by Adam Bryant

picon face
I only used the 1uf to 1mf as an example.  I want to measure a broad
range of capacitors, but I only need to know the approximate value rather
than the exact value down to the nth picofarad.  I want to build a small
device that measures the capacitor value and displays it on a small LCD.
And being cheap I would rather build something with a PIC than purchase a
multi-meter with capacitance measurement or even go to Tech America and
buy a capacitance adapter for my multimeter for $13 bucks. :-)

Adam Bryant (age 0x23)
EraseMEabryantspam_OUTspamTakeThisOuTpeaktech.com
Parker, CO, USA
Robotics, RC Airplanes, anything using a PIC

On Mon, 1 Mar 1999 14:48:47 -0800 Lynx {Glenn Jones} <jones_glspamspam_OUTEFN.ORG>
writes:
{Quote hidden}

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1999\03\01@194316 by ryan pogge

flavicon
face
this is interesting... I would also be interested in such a
device...



>I only used the 1uf to 1mf as an example.  I want to
measure a broad
>range of capacitors, but I only need to know the
approximate value rather
>than the exact value down to the nth picofarad.  I want to
build a small
>device that measures the capacitor value and displays it on
a small LCD.
>And being cheap I would rather build something with a PIC
than purchase a
>multi-meter with capacitance measurement or even go to Tech
America and
>buy a capacitance adapter for my multimeter for $13 bucks.
:-)
>
>Adam Bryant (age 0x23)
>RemoveMEabryantTakeThisOuTspampeaktech.com
>Parker, CO, USA
>Robotics, RC Airplanes, anything using a PIC
>
>On Mon, 1 Mar 1999 14:48:47 -0800 Lynx {Glenn Jones}
<spamBeGonejones_glspamBeGonespamEFN.ORG>
>writes:
>>well, if its just between 1uF and 1mF (which im guessing
its not :)
>>then
>>you can just build a simle audio oscilator out of a 555
and plug in
>>the
>>caps as timing caps. then you will have one tone for 1uF
and another
>>for
>>1mF
>>
>>----------------------------------------------------------
--------------------
>>A member of the PI-100 Club:
>>3.1415926535897932384626433832795028841971693993751
>>058209749445923078164062862089986280348253421170679
>>
>>On Mon, 1 Mar 1999, Adam Bryant wrote:
>>
>>> Dear PIClister's,
>>> Does anyone else out there have a drawer full of
anonymous
>>capacitors?  I
>>> was wondering about the feasibility of modifying the
Microchip app
>>note
>>> that does A/D conversion using a resistor and capacitor
and using it
>>to
>>> measure the capacitor instead.  Using the PIC as a timer
it should
>>be
>>> possible to get a semi-close measurement of the
capacitance and you
>>could
>>> even do auto-ranging and select different resistance
values
>>depending on
>>> the measurements you get.  I really don't need too exact
of a
>>> measurement, just something to tell me if a capacitor is
1uf or 1mf.
>> So
>>> I think it should be possible to build something fairly
low cost
{Quote hidden}

____________________________________________________________
_______
>>> You don't need to buy Internet access to use free
Internet e-mail.
>>> Get completely free e-mail from Juno at
>>http://www.juno.com/getjuno.html
>>> or call Juno at (800) 654-JUNO [654-5866]
>>>
>>
>
>___________________________________________________________
________
>You don't need to buy Internet access to use free Internet
e-mail.
>Get completely free e-mail from Juno at
http://www.juno.com/getjuno.html
>or call Juno at (800) 654-JUNO [654-5866]
>

1999\03\02@040812 by mlsirton

flavicon
face
Hi,

On  1 Mar 99 at 13:27, Adam Bryant wrote:
> Does anyone else out there have a drawer full of anonymous capacitors?  I
> was wondering about the feasibility of modifying the Microchip app note
> that does A/D conversion using a resistor and capacitor and using it to
> measure the capacitor instead.  Using the PIC as a timer it should be

Sure - it's possible.
Use 1 or more known value resistors to charge the unknown cap and
measure the time it takes to trip the PIC input threshold.  If you
want a reference you can charge a known cap through the same circuit
but for just a rough measurement you don't need one.

E.g.
R1 known resistor from RB0 to CAP+ (lets say 100K 1%, choose to give
a reasonable time constant, you can use more than one)
RB1 to CAP+
Known cap CAP+ to RB2 (Value in the middle of your measurement
range, best precision you can get, you can use more than one ref.
cap) i.e. the (-) of the cap connects to RB2 and the (+) to RB1 and
resistor.
Measured cap CAP+ to RB3

Measurement cycle:
1) Tristate RB1, RB3.
2) Set RB0 to 0.
3) Set RB2 to output 0.
4) Set RB0 to 1.
5) Measure time till RB1 reads 1. (reference capacitance)
6) Set RB0 to 0.
7) Wait measured time + some to discharge ref cap.
8) Tristate RB2.
9) Set RB3 to 0
10) Set RB0 to 1.
11) Measure time till RB1 reads 1. (measured cap)
12) Set RB0 to 0.
13) Wait for discharge.

tref = reference time, tmes = measured cap time (in seconds)
Cref  = reference cap value (In Farads), Cmes = measured cap value

tref = a*R*Cref      (a is a constant)
tmes = a*R*Cmes
tref/tmes = Cref/Cmes
Cmes = tmes*Cref/tref

Check my math and pay attention to the units...

You should probably protect the input pin (RB1) with a resistor
(1K?). For extra protection put two more resistors, one from Cref(-)
to RB2 and the other one from Cmes(-) to RB3.
You can use more the one charging resistors to measure across a
bigger range of caps quickly.  You can also use more than one
reference cap.  Also, consider the case when the cap is already
charged when connected to the PIC (with it's maximal voltage
rating...).  Have fun...

Hope this helps,
Guy - mlsirtonEraseMEspam.....inter.net.il

1999\03\02@120059 by Dwayne Reid

flavicon
face
>I only used the 1uf to 1mf as an example.  I want to measure a broad
>range of capacitors, but I only need to know the approximate value >rather
than the exact value down to the nth picofarad.

Why not check out Electronics Now issure June 1996.  It has a simple PIC
based LC meter that has the virtue of being 'self calibrating'.  The
oscillator is based upon a LM311 comparitor - it looks simple and reliable.
The PIC reads the frequency (or period - I can't remember), scales it, and
sends it to an LCD display.  It sounds like a great start for you if you
still want to roll your own version.

dwayne


Dwayne Reid   <EraseMEdwaynerspamplanet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(403) 489-3199 voice          (403) 487-6397 fax

Celebrating 15 years of Engineering Innovation (1984 - 1999)

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1999\03\02@141355 by Lynx {Glenn Jones}

flavicon
face
One thing i wonder aboujt with these capacitance mesure-ers is the
following: As i understand it, you hook up the capacitor like so

          Vcc
           |
          Res
           |
           |
Pic Pin     |
------------|
           |
          Cap
           |
          GND

now then you hold the pic pin low, then release it untill the voltage
trigers the input. However, if you do it again, hold the pic pin low, it
would seem that quite a lot more than 20ma is going to go through the pic
pin... am i mistaken?

------------------------------------------------------------------------------
A member of the PI-100 Club:
3.1415926535897932384626433832795028841971693993751
058209749445923078164062862089986280348253421170679

1999\03\02@143906 by paulb
flavicon
face
Lynx {Glenn Jones} wrote:

> As i understand it, you hook up the capacitor like so

>            Vcc
>             |
>            Res
> Pic Pin     |
> ------------|
>             |
>            Cap
>             |
>            GND

> it would seem that quite a lot more than 20ma is going to go through
> the pic pin... am i mistaken?

 Two options; 5V/ 20mA is conservatively, 270 ohms.  Put that in series
with the pin (will undoubtedly be much less than Res).  Put Res on the
PIC pin side so there's no voltage divider effect.

 Alternately, use Res istelf to limit the current.  Hold it low for
*much* longer than the measuring interval, then alternate between
holding it high for a few cycles and sampling it as an input.

 The standard "POT" function ( http://www.dontronics.com/see.html#pot )
in PIC basic variants works this way.
--
 Cheers,
       Paul B.

1999\03\02@155602 by Adam Bryant

picon face
Glenn,
True, you should worry about the current going in to the PIC pin.  The
actual wiring looks like the following (and I apologize in advance for my
ASCII art).

Pic RA0 ----------------|
                       |
                       ResA
                       |
Pic RA1 -----ResB-------|
                       |
                       Cap
                       |
                       |
                       GND

To discharge the cap set RA0 and RA1 to outputs and drive them low.  To
do the measurement RA0 is set as output and driven high.  RA1 is set as
input.  As soon as the Capacitor has charged to the threshold voltage of
RA1, RA1 will go high.  Resistor A is chosen to give an RC time constant
measurable by the PIC (I am currently using 1K) and Resistor B will limit
the current into RA1 and can be a fairly high value (100K is what I am
using) to provide high input impedance on RA1.

My initial tests with this setup last night were very encouraging.  My
program basically runs in a loop and does the discharge/measurement over
and over, and after the first couple of cycles the measured value settled
down into a very consistent value.  I still have some work to do to tweak
the code a little and to better interpret the results, but for the small
amount of time I have spent on it I am very pleased with the results.

Adam Bryant (age 0x23)
RemoveMEabryantEraseMEspamEraseMEpeaktech.com (work)
RemoveMEadamdbspam_OUTspamKILLspamjuno.com (home)
Parker, CO, USA
Robotics, RC Airplanes, anything using a PIC

On Tue, 2 Mar 1999 11:12:02 -0800 Lynx {Glenn Jones} <RemoveMEjones_glTakeThisOuTspamspamEFN.ORG>
writes:
{Quote hidden}

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Get completely free e-mail from Juno at http://www.juno.com/getjuno.html
or call Juno at (800) 654-JUNO [654-5866]

1999\03\02@162321 by hmiller

picon face
Dwayne Reid wrote:
>
> >I only used the 1uf to 1mf as an example.  I want to measure a broad
> >range of capacitors, but I only need to know the approximate value >rather
> than the exact value down to the nth picofarad.
>
> Why not check out Electronics Now issure June 1996.  It has a simple PIC
> based LC meter that has the virtue of being 'self calibrating'.  The
> oscillator is based upon a LM311 comparitor - it looks simple and reliable.
> The PIC reads the frequency (or period - I can't remember), scales it, and
> sends it to an LCD display.  It sounds like a great start for you if you
> still want to roll your own version.
>
> dwayne
=====================

Is there a way of getting a copy of that article?

Harley L. Miller     EraseMEhmillerspamspamspamBeGonesound.net
WB0ROQ

1999\03\03@064157 by Octavio Nogueira

flavicon
face
>Why not check out Electronics Now issure June 1996.  It has a simple PIC
>based LC meter that has the virtue of being 'self calibrating'.  The
>oscillator is based upon a LM311 comparitor - it looks simple and reliable.
>The PIC reads the frequency (or period - I can't remember), scales it, and
>sends it to an LCD display.  It sounds like a great start for you if you
>still want to roll your own version.
>
>dwayne

This project is fantastic. I built one and it's still
working great.
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1999\03\03@084923 by Larry N. Fraysier

flavicon
face
Adam,
Go to this web page and you may find what you are looking for....
http://www.aade.com/

See you later,
Larry

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