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'Homemade Cap Meter'
1999\06\23@082103 by Thomas McGahee

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School is finally over (I teach Electronics and Computer
Technology), and I have had a little time to finish
up one of the projects I began earlier in the year. It is
a Capacitance Meter that measures from fractions of a
picofarad out to just a little over 16,000 microfarads.

I use a PIC 16F84 as a controller, and have a 16x2 LCD
readout for displaying results and messages. The unit
allows for both AutoRange and Manual Modes of operation.
A special Comparison Mode is also provided for.

I use an analog front-end that pumps a constant current
into (and out of) the capacitor under test. The resultant
voltage is then compared with a reference and used to
switch the polarity of the constant current. This results
in two signals. One of these is a triangle wave, and
derived from this (via the comparator section) is a nice
clean square wave whose PERIOD is directly proportional
to the capacitance being measured.

I use a sneaky trick to help eliminate error due to
any changes in the bi-polar voltage reference. In fact,
the technique works so well I use a normal red LED as
my voltage reference. For those who want to eke an
extra digit of precision out of the circuit, you can
substitute the LED with a Zener.

The accuracy of the circuit is dependant on several factors:

1) The type of quad Operational Amplifier used. I chose
  to use a TL084CN opamp because it gave me a decent
  performance/price ratio. This is an industry standard
  pinout device, so you can substitute a better version
  opamp if desired. The limiting factors as regards the
  opamp are its slew rate, input bias current, and
  available output current.

2) The accuracy of the resistors used in the
  voltage-to-current section of the circuit. I chose to
  use .1% precision resistors for the two resistors that
  determine the current for the lowest scales. I used
  1% resistors and trimmer pots for the remaining
  scales so that I could nullify some of the errors that
  creep in at the higher scales. Note that I used
  *two* .1% resistors so that I could get the desired
  250k value using standard precision resistor values
  (249k + 1k)

3) the accuracy of the resistors used in the comparator
  section. I used .1% precision units.

4) The accuracy of the clock. I used a 4.000 Mhz crystal
  rather than my more usual 4.00 Mhz ceramic resonator.

5) The error due to the difference between the number of
  cycles that occur between the time a period begins
  and ends, and the number of instruction cycles it takes
  before the count is accumulated.

6) The error due to stray capacitance in the internal
  wiring and also any external stray capacitance caused
  by the measurement leads, etc.

I attempt to eliminate most of the error coming from #5
and #6 by initially accumulating a count when there is
no external capacitor attached, and then subtracting
that value from subsequent readings. These initial "zero"
values are measured whenever a reset or power-up
condition occurs. They are retained until the next
reset, or they may be re-acquired at any time by pushing
the ZERO button.

Pushing the ZERO button causes the unit to enter Manual
Mode and causes the unit to acquire a reading and place
this value in the current subtraction register set.

So, if you happen to have, say, a .1 uf capacitor on the
leads and you push the ZERO button, then the display goes
to zero. If you were to remove the .1 ufd capacitor, the
display would now read negative .1 ufd to indicate the
DIFFERENCE between the current capacitance and the value
that was in effect when the ZERO button was pushed. This
Compare Mode is quite useful for "matching" capacitors,
or for tracking the *changes* in capacitance that may
be occuring.

An UP and a DOWN button are available which cause the unit
to enter Manual Mode (if not already in that mode), and
move to the Next or Previous range. The buttons are
"sticky" in that they go to the limit and stay there.
They do NOT "wrap around".

There are indicators on the display that show when the unit
is waiting for synchronization, when it is accumulating a
count, and when it has updated the numerical display. In
addition there are indicators for +/- for Comparison
Mode, and an Auto/Manual Mode indicator.

Messages are displayed when there are Over-Range values
in Manual Mode, and when the unit is AutoRanging from
one range to another in Auto Mode. There is also a
message displayed when the unit has exceeded the 16
million count limit.

AutoRange mode has hysteresis, so that small changes don't
cause the circuitry to constantly change range. The
upper limit for switching ranges in AutoRange Mode is
approximately a 500,000 counts. The lower limit is
approximately 400,000 counts. The exceptions are the
highest range, which goes up to the maximum count of
approximately 16,000,000   and the lowest range, which
can go down to .00 picofarads.

The counts are displayed on the upper line of the LCD,
and the unit designators are displayed on the second
line of the LCD. Typical displays would look something
like this:

+  15,123.456 XX    15,123.456 uf AutoMode New Count
      uf  nfA**

-      23,321.54    23,321.54 pf Manual Mode
  uf  nf  pfM X    <-> means Below Reference Value
                   Blinking X means Count In Progress.

Leading zeroes are suppressed up to the decimal point.
Commas, decimal point, and space are used to separate
number groups. Thus the last example can be easily
seen to also be equal to 23.32154 nf.

Unit was

1999\06\23@083555 by Thomas McGahee

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Sorry...
I accidentally hit the send key before I had entirely
finished the message about my Cap Meter.

I wired the unit up using two single-sided PC boards
(and a fair number of jumpers!) that are mounted
piggy-back to conserve space.

If anyone is interested in AutoCad drawings of the
schematic and PC boards and/or the assembly listings
for the PIC, please e-mail me *directly*. Please
do *not* post requests to the list, as this
inconveniences everybody.

If anyone wants to put the plans on their web site,
that is OK with me. In that case once you have
them available on your site post the url to the
list.

I will be in and out the next few days, so if you
make a request and don't get it answered immediately
please wait a few days before sending an additional
request.

To aid me in locating your requests among the
hundreds of e-mails I receive each day, please
use the subject header Re:Homemade Cap Meter.

Fr. Tom McGahee

1999\06\23@084144 by Jack Shidemantle

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Sounds like a well thought out design. Are you willing to share any of the
details?

Thanks

@
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06/22/99 05:15 PM AST
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To: .....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU
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bcc:
Subject: Homemade Cap Meter


School is finally over (I teach Electronics and Computer
Technology), and I have had a little time to finish
up one of the projects I began earlier in the year. It is
a Capacitance Meter that measures from fractions of a
picofarad out to just a little over 16,000 microfarads.

I use a PIC 16F84 as a controller, and have a 16x2 LCD
readout for displaying results and messages. The unit
allows for both AutoRange and Manual Modes of operation.
A special Comparison Mode is also provided for.

I use an analog front-end that pumps a constant current
into (and out of) the capacitor under test. The resultant
voltage is then compared with a reference and used to
switch the polarity of the constant current. This results
in two signals. One of these is a triangle wave, and
derived from this (via the comparator section) is a nice
clean square wave whose PERIOD is directly proportional
to the capacitance being measured.

I use a sneaky trick to help eliminate error due to
any changes in the bi-polar voltage reference. In fact,
the technique works so well I use a normal red LED as
my voltage reference. For those who want to eke an
extra digit of precision out of the circuit, you can
substitute the LED with a Zener.

The accuracy of the circuit is dependant on several factors:

1) The type of quad Operational Amplifier used. I chose
Ê to use a TL084CN opamp because it gave me a decent
Ê performance/price ratio. This is an industry standard
Ê pinout device, so you can substitute a better version
Ê opamp if desired. The limiting factors as regards the
Ê opamp are its slew rate, input bias current, and
Ê available output current.

2) The accuracy of the resistors used in the
Ê voltage-to-current section of the circuit. I chose to
Ê use .1% precision resistors for the two resistors that
Ê determine the current for the lowest scales. I used
Ê 1% resistors and trimmer pots for the remaining
Ê scales so that I could nullify some of the errors that
Ê creep in at the higher scales. Note that I used
Ê *two* .1% resistors so that I could get the desired
Ê 250k value using standard precision resistor values
Ê (249k + 1k)

3) the accuracy of the resistors used in the comparator
Ê section. I used .1% precision units.

4) The accuracy of the clock. I used a 4.000 Mhz crystal
Ê rather than my more usual 4.00 Mhz ceramic resonator.

5) The error due to the difference between the number of
Ê cycles that occur between the time a period begins
Ê and ends, and the number of instruction cycles it takes
Ê before the count is accumulated.

6) The error due to stray capacitance in the internal
Ê wiring and also any external stray capacitance caused
Ê by the measurement leads, etc.

I attempt to eliminate most of the error coming from #5
and #6 by initially accumulating a count when there is
no external capacitor attached, and then subtracting
that value from subsequent readings. These initial "zero"
values are measured whenever a reset or power-up
condition occurs. They are retained until the next
reset, or they may be re-acquired at any time by pushing
the ZERO button.

Pushing the ZERO button causes the unit to enter Manual
Mode and causes the unit to acquire a reading and place
this value in the current subtraction register set.

So, if you happen to have, say, a .1 uf capacitor on the
leads and you push the ZERO button, then the display goes
to zero. If you were to remove the .1 ufd capacitor, the
display would now read negative .1 ufd to indicate the
DIFFERENCE between the current capacitance and the value
that was in effect when the ZERO button was pushed. This
Compare Mode is quite useful for "matching" capacitors,
or for tracking the *changes* in capacitance that may
be occuring.

An UP and a DOWN button are available which cause the unit
to enter Manual Mode (if not already in that mode), and
move to the Next or Previous range. The buttons are
"sticky" in that they go to the limit and stay there.
They do NOT "wrap around".

There are indicators on the display that show when the unit
is waiting for synchronization, when it is accumulating a
count, and when it has updated the numerical display. In
addition there are indicators for +/- for Comparison
Mode, and an Auto/Manual Mode indicator.

Messages are displayed when there are Over-Range values
in Manual Mode, and when the unit is AutoRanging from
one range to another in Auto Mode. There is also a
message displayed when the unit has exceeded the 16
million count limit.

AutoRange mode has hysteresis, so that small changes don't
cause the circuitry to constantly change range. The
upper limit for switching ranges in AutoRange Mode is
approximately a 500,000 counts. The lower limit is
approximately 400,000 counts. The exceptions are the
highest range, which goes up to the maximum count of
approximately 16,000,000 Ê and the lowest range, which
can go down to .00 picofarads.

The counts are displayed on the upper line of the LCD,
and the unit designators are displayed on the second
line of the LCD. Typical displays would look something
like this:

+ Ê15,123.456 XX Ê Ê15,123.456 uf AutoMode New Count
Ê Ê Ê uf ÊnfA**

- Ê Ê Ê23,321.54 Ê Ê23,321.54 pf Manual Mode
Ê uf Ênf ÊpfM X Ê Ê<-> means Below Reference Value
Ê Ê Ê Ê Ê Ê Ê Ê Ê ÊBlinking X means Count In Progress.

Leading zeroes are suppressed up to the decimal point.
Commas, decimal point, and space are used to separate
number groups. Thus the last example can be easily
seen to also be equal to 23.32154 nf.

Unit was

1999\06\23@090301 by Laszlo Kohegyi

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face
Hi,

>If anyone is interested in AutoCad drawings of the
>schematic and PC boards and/or the assembly listings
>for the PIC, please e-mail me *directly*. Please

Your project sounds interesting and I'd like to see it.
Could you also include a .dxf file as I don't always have
ACad handy?

>Fr. Tom McGahee

Best regards

Les

1999\06\23@101635 by Andy Kunz

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>picofarad out to just a little over 16,000 microfarads.

16,384 uF?

Sounds pretty interesting.

[OT personal note]

Say, did I mention our newest addition to you?  Heidi Anne arrived 5/17.  I
forget if you were on the baby announcement list we put out.

Andy

==================================================================
  Andy Kunz - http://www.montanadesign.com - andyspamKILLspammontanadesign.com
          Life is what we do to prepare for Eternity
==================================================================

1999\06\23@195608 by Antonio L Benci

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Sounds excellent. Is this a private/personnel project or a commercial
proposal. I would more than intrested in looking at and possibly
integrating your idea/design in some instrumentation we are developing
for inhouse use in our research labs. I am currently looking into
simplifying a capacitive dillatometer (measures the thermal expansion of
alloys). The curent setup uses 6 pieces of VERY costly hardware
(~$35,000). My design replaces every thing except the capacitance
bridge, hence my interest :-)

Would you be willing to share ???

Thomas McGahee wrote:
{Quote hidden}

Nino.
--
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1999\06\23@232010 by adastra

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Please send information packet to:
fosterspamspam_OUTadastran.com
Thank you!
Foster Brashear

{Quote hidden}

1999\06\24@011337 by Chris Eddy

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Andy..  You win the contest!
You can quit now, nobody is trying to keep up.
How many is that now?  Shooting for an even dozen?

Congratulations..
Chris Eddy

Andy Kunz wrote:

> [OT personal note]
>
> Say, did I mention our newest addition to you?  Heidi Anne arrived 5/17.  I
> forget if you were on the baby announcement list we put out.
>
> Andy
>

1999\06\24@104153 by Andy Kunz

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At 01:11 AM 6/24/99 -0400, you wrote:
>Andy..  You win the contest!
>You can quit now, nobody is trying to keep up.
>How many is that now?  Shooting for an even dozen?

7 living, 1 died.

No, we stopped "trying" at 3.  God wasn't finished with us yet, though.

My sister is still trying.

Andy
==================================================================
  Andy Kunz - http://www.montanadesign.com - RemoveMEandyTakeThisOuTspammontanadesign.com
          Life is what we do to prepare for Eternity
==================================================================

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