Forums

Capacitor charging curve

Started by Sambo September 3, 2007
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
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
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.
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.
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)?
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
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
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.
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
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.