C measurement tool

[Gene Zumchak]

Guys,

    This is how I would do it.  It assumes that you can measure time
very accurately.  A micro with a capture input on a timer would do the
job.  At time t=0 switch a discharged capacitor to a plus voltage
through a precision resistor.  A comparator is set to fire when the cap
charges to .634 volts which occurs at one time constant, tau = RC.  If
your R is 10.00K or 100.0K then you can directly read the capacitance
from the measured time.  The whole process is best intiated and
controlled by a micro.

Gene Z.


Paul Perry wrote:

> At 09:58 AM 19/04/99 +0200, Martin Czech  wrote:
>
> >I think this is the way that cheap multimeters do it.
>
> I had a feeling that a VERY cheap multimeter ran an
> AC current thru the cap, and measured like a resistance, but
> I could be very wrong... perhaps the designer of the worlds
> best 'budget' multimeter can tell us!
> Perhaps we could have a MOTM multimeter (not THAT crazy an idea).
>
> You would think that if you had say a 10k resistor in series with
> an unknown cap, and sequentially applied 10v AC at say 10Hz, 100 Hz,
> 1K etc and looked at the voltages developed across it, you could
> work out esily and fairly acccurately not only the capacitance but
> also the effective series resistance.
>
> paul perry (Frostwave P/L) Melbourne Australia