Capacitor charging curve

[Sambo]

I am planing to put together PIC project to measure SMD caps.
I am not sure if the answer will help me for this project and
how I started wondering about it but it has been bugging me for
a while ( if I was hoping to discharge through a transistor
leaving .7V on it ? during repeating measurements. or was it
discharging to some voltage so it coincides with the voltage
reference point. )

Is the curve same even if the bottom is not 0V?
So from (.7V - 4.95V) * 63% for 1t.


and not to waste message numbers....

Where does the curve come from? Is it measured or calculated.

Cheers

 

[Chris Jones]

Sambo wrote:

I am planing to put together PIC project to measure SMD caps.
I am not sure if the answer will help me for this project and
how I started wondering about it but it has been bugging me for
a while ( if I was hoping to discharge through a transistor
leaving .7V on it ? during repeating measurements. or was it
discharging to some voltage so it coincides with the voltage
reference point. )

Is the curve same even if the bottom is not 0V?
So from (.7V - 4.95V) * 63% for 1t.


and not to waste message numbers....

Where does the curve come from? Is it measured or calculated.

Cheers

If you discharge it using a bipolar transistor, then it will discharge to
less than 0.7V (the collector can get pulled down to a voltage lower than
the base voltage which seems a bit weird, but is true). The actual final
voltage will be not very well defined with a bipolar transistor. A small
NMOS fet (or an open-drain CMOS gate like 74HC07 or even a port pin of the
PIC) might be better for getting it to discharge to a known value (0V).
Bear in mind the on-resistance of the transistor, this may affect the R-C
time constant.

By the way, I have been measuring surface mount capacitors with this
circuit:
http://ironbark.bendigo.latrobe.edu.au/~rice/lc/
I quite like it, though it could do with a low-battery warning, and it
cannot measure inductors as small as I would like. The one I built was
mechanically quite different from the one on that web page because I was
interested in small valued inductors and capacitors. I built the
oscillator part in a separate small box with its own output signal buffer
and its own power regulator, and a SMA connector for the device under test,
which I have fitted a probe to for measuring surface mount capacitors. It
drifts a bit with temperature (0.5pF or so), but is quite useful for
sorting surface mount caps that get mixed up on the bench.

Chris

 

[Sambo]

Chris Jones wrote:

Sambo wrote:


I am planing to put together PIC project to measure SMD caps.
I am not sure if the answer will help me for this project and
how I started wondering about it but it has been bugging me for
a while ( if I was hoping to discharge through a transistor
leaving .7V on it ? during repeating measurements. or was it
discharging to some voltage so it coincides with the voltage
reference point. )

Is the curve same even if the bottom is not 0V?
So from (.7V - 4.95V) * 63% for 1t.


and not to waste message numbers....

Where does the curve come from? Is it measured or calculated.

Cheers


If you discharge it using a bipolar transistor, then it will discharge to
less than 0.7V (the collector can get pulled down to a voltage lower than
the base voltage which seems a bit weird, but is true). The actual final
voltage will be not very well defined with a bipolar transistor. A small
NMOS fet (or an open-drain CMOS gate like 74HC07 or even a port pin of the
PIC) might be better for getting it to discharge to a known value (0V).
Bear in mind the on-resistance of the transistor, this may affect the R-C
time constant.

By the way, I have been measuring surface mount capacitors with this
circuit:
http://ironbark.bendigo.latrobe.edu.au/~rice/lc/
I quite like it, though it could do with a low-battery warning, and it
cannot measure inductors as small as I would like. The one I built was
mechanically quite different from the one on that web page because I was
interested in small valued inductors and capacitors. I built the
oscillator part in a separate small box with its own output signal buffer
and its own power regulator, and a SMA connector for the device under test,
which I have fitted a probe to for measuring surface mount capacitors. It
drifts a bit with temperature (0.5pF or so), but is quite useful for
sorting surface mount caps that get mixed up on the bench.

Chris

Great !! maybe I will put my faith in an oscillator , if it will allow measuring coils too.

Thank You.

 

[Nobody]

On Mon, 03 Sep 2007 17:19:48 -0400, Sambo wrote:

Where does the curve come from? Is it measured or calculated.

Calculated.

For an ideal capacitor, i=C.dv/dt.

If it discharges through a fixed resistance R, i=v/R, therefore
v/R=C.dv/dt => v=RC.dv/dt

A differential equation of the form x=k.dx/dt has the solution
x=x0.e(-t/k), so the discharge curve is:

v(t)=v0.e^(-t/RC)

where v0 is the voltage at t=0.

At t=RC, v(t)/v0 = 1/e ~= 0.37.

The curve for charging an empty capacitor up to a fixed voltage v0 is just
the discharge curve, mirrored vertically, i.e. v(t) = v0.(1-e^(-t/RC)).

At t=RC, v(t)/v0 = 1-(1/e) ~= 0.63.

Similarly, for an ideal inductor, v=L.di/dt. In circuit with a fixed
resistance R, v=iR, so iR=L.di/dt => i=(L/R).di/dt, giving:

i(t)=i0.e^(-t/(L/R))

where i0 is the current at t=0.

At t=L/R, i(t)/i0 = 1/e ~= 0.37.

 

[anirudh]

On Sep 3, 6:41 pm, Chris Jones <lugnut...@nospam.yahoo.com> wrote:

Sambo wrote:
I am planing to put together PIC project to measure SMD caps.
I am not sure if the answer will help me for this project and
how I started wondering about it but it has been bugging me for
a while ( if I was hoping to discharge through a transistor
leaving .7V on it ? during repeating measurements. or was it
discharging to some voltage so it coincides with the voltage
reference point. )

Is the curve same even if the bottom is not 0V?
So from (.7V - 4.95V) * 63% for 1t.

and not to waste message numbers....

Where does the curve come from? Is it measured or calculated.

Cheers

If you discharge it using a bipolar transistor, then it will discharge to
less than 0.7V (the collector can get pulled down to a voltage lower than
the base voltage which seems a bit weird, but is true). The actual final
voltage will be not very well defined with a bipolar transistor. A small
NMOS fet (or an open-drain CMOS gate like 74HC07 or even a port pin of the
PIC) might be better for getting it to discharge to a known value (0V).
Bear in mind the on-resistance of the transistor, this may affect the R-C
time constant.

By the way, I have been measuring surface mount capacitors with this
circuit:http://ironbark.bendigo.latrobe.edu.au/~rice/lc/
I quite like it, though it could do with a low-battery warning, and it
cannot measure inductors as small as I would like. The one I built was
mechanically quite different from the one on that web page because I was
interested in small valued inductors and capacitors. I built the
oscillator part in a separate small box with its own output signal buffer
and its own power regulator, and a SMA connector for the device under test,
which I have fitted a probe to for measuring surface mount capacitors. It
drifts a bit with temperature (0.5pF or so), but is quite useful for
sorting surface mount caps that get mixed up on the bench.

Chris

I tried out the circuit on your webpage and it worked great.But i had
to modify it in one regard.I could'nt get the oscillator to work
properly with the inductor,so i used a 555 to generate a square wave
for the capacitor.A modified 555 ckt works well for the inductor
too.It might be possible to measure lower values of inductance with
the 555 as the oscillator.
but how can the error in the measurement of the inductor and capacitor
be calculated(calculated, not measured)?

 

[Chris Jones]

anirudh wrote:

On Sep 3, 6:41 pm, Chris Jones <lugnut...@nospam.yahoo.com> wrote:
Sambo wrote:
I am planing to put together PIC project to measure SMD caps.
I am not sure if the answer will help me for this project and
how I started wondering about it but it has been bugging me for
a while ( if I was hoping to discharge through a transistor
leaving .7V on it ? during repeating measurements. or was it
discharging to some voltage so it coincides with the voltage
reference point. )

Is the curve same even if the bottom is not 0V?
So from (.7V - 4.95V) * 63% for 1t.

and not to waste message numbers....

Where does the curve come from? Is it measured or calculated.

Cheers

If you discharge it using a bipolar transistor, then it will discharge to
less than 0.7V (the collector can get pulled down to a voltage lower than
the base voltage which seems a bit weird, but is true). The actual final
voltage will be not very well defined with a bipolar transistor. A small
NMOS fet (or an open-drain CMOS gate like 74HC07 or even a port pin of
the PIC) might be better for getting it to discharge to a known value
(0V). Bear in mind the on-resistance of the transistor, this may affect
the R-C time constant.

By the way, I have been measuring surface mount capacitors with this
circuit:http://ironbark.bendigo.latrobe.edu.au/~rice/lc/
I quite like it, though it could do with a low-battery warning, and it
cannot measure inductors as small as I would like. The one I built was
mechanically quite different from the one on that web page because I was
interested in small valued inductors and capacitors. I built the
oscillator part in a separate small box with its own output signal buffer
and its own power regulator, and a SMA connector for the device under
test,
which I have fitted a probe to for measuring surface mount capacitors.
It drifts a bit with temperature (0.5pF or so), but is quite useful for
sorting surface mount caps that get mixed up on the bench.

Chris

I tried out the circuit on your webpage and it worked great.
It isn't my webpage.

But i had
to modify it in one regard.I could'nt get the oscillator to work
properly with the inductor,so i used a 555 to generate a square wave
for the capacitor.
The circuit oscillated for me no problem. In fact I have seldom built any

circuit with LM311 comparators that didn't oscillate, even when they were
not supposed to!

A modified 555 ckt works well for the inductor
too.
Ok, though I have never used a 555 for an LC oscillator myself.

It might be possible to measure lower values of inductance with
the 555 as the oscillator.
One day I might investigate a discrete oscillator with a divide-by-100

prescaler (frequency divider) (to scale the L and C values both down by 10
times, without changing the software).

but how can the error in the measurement of the inductor and capacitor
be calculated(calculated, not measured)?
I think that if it were easy to calculate then it would have been corrected

for in the software so it would no longer be an error. That circuit is
pretty accurate anyway. It is fine for sorting capacitors which is what I
use it for.

Chris

 

[Rich Grise]

On Mon, 03 Sep 2007 17:19:48 -0400, Sambo wrote:

I am planing to put together PIC project to measure SMD caps.
I am not sure if the answer will help me for this project and
how I started wondering about it but it has been bugging me for
a while ( if I was hoping to discharge through a transistor
leaving .7V on it ? during repeating measurements. or was it
discharging to some voltage so it coincides with the voltage
reference point. )

Is the curve same even if the bottom is not 0V?
So from (.7V - 4.95V) * 63% for 1t.

It's the same curve, maybe scaled or offset, depending on circumstances.

and not to waste message numbers....

This is USENET. There is no such thing as "message numbers".

Where does the curve come from? Is it measured or calculated.

It simply happens. It was measured, and somebody else came up with
e, which just happens to be the base of the natural logs.


As to "why", probably for the same reason that the sky is blue, birds
sing, and water flows downhill - that's just the way stuff came out, this
bigbang. ;-)

Hope This Helps!
Rich

 

[Sambo]

Rich Grise wrote:

It's the same curve, maybe scaled or offset, depending on circumstances.



and not to waste message numbers....



This is USENET. There is no such thing as "message numbers".



Where does the curve come from? Is it measured or calculated.


It simply happens. It was measured, and somebody else came up with
e, which just happens to be the base of the natural logs.


As to "why", probably for the same reason that the sky is blue, birds
sing, and water flows downhill - that's just the way stuff came out, this
bigbang. ;-)

Hope This Helps!
Rich

Yes, it does thanks.

 

[Jasen Betts]

On 2007-09-05, Rich Grise <rich@example.net> wrote:

This is USENET. There is no such thing as "message numbers".

oh yes there is. They are probably different on every server but they are
part of the NNTP specification.

If you see an "xref:" header that gives the message number(s) of the
message.

that said there is no scarsity - they are a renewable resource

Where does the curve come from? Is it measured or calculated.

It simply happens. It was measured, and somebody else came up with
e, which just happens to be the base of the natural logs.

not only that, e^(i*pi/2*i) = 0 ( where i^2 = -1 )

As to "why", probably for the same reason that the sky is blue, birds
sing, and water flows downhill - that's just the way stuff came out, this
bigbang. ;-)

yeah.

Bye.
Jasen

 

[Sambo]

Jasen Betts wrote:

On 2007-09-05, Rich Grise <rich@example.net> wrote:


This is USENET. There is no such thing as "message numbers".


oh yes there is. They are probably different on every server but they are
part of the NNTP specification.

If you see an "xref:" header that gives the message number(s) of the
message.

that said there is no scarsity - they are a renewable resource


yeah , it was more of an excuse to cram another question in there,

but even though they went from 16 bits to ...??? 32? or x? number of digits,
fairly regularly ( about a month) in some of the more active groups I get message from
netscape saying that there are 5 -10 000 new messages (and do I want to download them,
which I am guessing is at the time of the rollover.

cheers.

 

[Sambo]

But i had
to modify it in one regard.I could'nt get the oscillator to work
properly with the inductor,so i used a 555 to generate a square wave
for the capacitor.

The circuit oscillated for me no problem. In fact I have seldom built any
circuit with LM311 comparators that didn't oscillate, even when they were
not supposed to!

Oh , if you follow the following link for the 'LC006d.txt' version:
http://ironbark.bendigo.latrobe.edu.au/~rice/lc/CM/index2.html

The schematic there is missing the 100K feedback resistor, I noticed that ( perhaps
because of this message) but forgot there was a diagram on the original site until
finally ended up going back.

The problem I am having is that the 6d and 6d32 versions of the software wont compile.

Make: The target "H:\Devel\MicrC-PICS\16f628\LC-meter-christi\lc006d.o" is out of date.

Executing: "D:\Program Files\Microchip\MPASM Suite\MPAsmWin.exe" /q /p16F628A "lc006d.asm" /l"lc006d.lst" /e"lc006d.err"
Error[108] H:\DEVEL\MICRC-PICS\16F628\LC-METER-CHRISTI\LC006D.ASM 3 : Illegal character (
)
Error[129] H:\DEVEL\MICRC-PICS\16F628\LC-METER-CHRISTI\LC006D.ASM 4 : Expected (END)
Halting build on first failure as requested.
BUILD FAILED: Sun Sep 23 06:45:44 2007
the LC006D32.ASM acctualy doesn't complain about '(' and ecpects 'END' on line 2 (comented out or not)?????

; Test ; for debugging purposes only

;#define Frequency 1 ; insert start_up sequence to test oscillator
#define Input_Calib 2 ; insert start_up sequence to display
One of them contains the floating point code but ...

The lc007.txt doesn't compile because I could not find the modified library mentioned somewhere
in the text, and including the original FP32.A16 revealed that all(?) functions have been renamed.

Cheers.

 

[Chris Jones]

Sambo wrote:

But i had
to modify it in one regard.I could'nt get the oscillator to work
properly with the inductor,so i used a 555 to generate a square wave
for the capacitor.

The circuit oscillated for me no problem. In fact I have seldom built
any circuit with LM311 comparators that didn't oscillate, even when they
were not supposed to!



Oh , if you follow the following link for the 'LC006d.txt' version:
http://ironbark.bendigo.latrobe.edu.au/~rice/lc/CM/index2.html

The schematic there is missing the 100K feedback resistor, I noticed that
( perhaps because of this message) but forgot there was a diagram on the
original site until finally ended up going back.

The problem I am having is that the 6d and 6d32 versions of the software
wont compile.


Make: The target "H:\Devel\MicrC-PICS\16f628\LC-meter-christi\lc006d.o" is
out of date. Executing: "D:\Program Files\Microchip\MPASM
Suite\MPAsmWin.exe" /q /p16F628A "lc006d.asm" /l"lc006d.lst"
/e"lc006d.err"
Error[108] H:\DEVEL\MICRC-PICS\16F628\LC-METER-CHRISTI\LC006D.ASM 3 :
Illegal character ( )
Error[129] H:\DEVEL\MICRC-PICS\16F628\LC-METER-CHRISTI\LC006D.ASM 4 :
Expected (END) Halting build on first failure as requested.
BUILD FAILED: Sun Sep 23 06:45:44 2007


the LC006D32.ASM acctualy doesn't complain about '(' and ecpects 'END' on
line 2 (comented out or not)?????

; Test ; for debugging purposes only
;#define Frequency 1 ; insert start_up sequence to test oscillator
#define Input_Calib 2 ; insert start_up sequence to display


One of them contains the floating point code but ...

The lc007.txt doesn't compile because I could not find the modified
library mentioned somewhere in the text, and including the original
FP32.A16 revealed that all(?) functions have been renamed.

Cheers.

I did not try assembling, I just took the hex file (I think I took
lc007a.hex).

Chris

 

[Sambo]

I did not try assembling, I just took the hex file (I think I took
lc007a.hex).

Chris

lol, so no worry I'll get ambitious trying to convert it to say 2320,
so I can add zener tester and never finish.