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.