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Capacitor charging curve

Discussion in 'Electronic Basics' started by Sambo, Sep 3, 2007.

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  1. Sambo

    Sambo Guest

    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
     
  2. Chris Jones

    Chris Jones Guest

    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
     
  3. Sambo

    Sambo Guest

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

    Thank You.
     
  4. Nobody

    Nobody Guest

    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.
     
  5. anirudh

    anirudh Guest

    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)?
     
  6. Chris Jones

    Chris Jones Guest

    It isn't my webpage.
    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!
    Ok, though I have never used a 555 for an LC oscillator myself.
    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).
    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
     
  7. Rich Grise

    Rich Grise Guest


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


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

    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
     
  8. Sambo

    Sambo Guest

    Yes, it does thanks.
     
  9. Jasen Betts

    Jasen Betts Guest

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

    Bye.
    Jasen
     
  10. Sambo

    Sambo Guest

    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.
     
  11. Sambo

    Sambo Guest

    But i had
    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.
     
  12. Sambo

    Sambo Guest

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

    Chris Jones Guest


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

    Chris
     
  14. neon

    neon

    1,325
    0
    Oct 21, 2006
    YOU MIGHT GET LOW saturation out of an pnp in the order of mv if used on the inverted mode. of course must use a force beta of < then 10 the the final straw is if the cap is BIG then burn out. there are limitation.
     
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