Re: measurement of capacitance

[brian whatcott <inet@INTELLISYS.NET>]

At 10:35 3/22/99 -0500, In response to Martha's question, Chuck wrote:

> > How does a multimeter measure capacitance?
>
> just a quick semi-educated guess -
> the meter provides a small CONSTANT current and measures the TIME required
> for the voltage to rise by a given amount. (Constant current sources are
> easy to come up with for these limited voltage ranges)


This is certainly the case for the Fluke 'true RMS' DVM I examined today.
The instruction leaflet explains that the meter needs to be zeroed for small
values of C, and preferably used with short leads. It autoranges and ramps
as high as 1.4 volts. It uses one second as its measuring time, and uses
the resultant voltage as a measure of capacitance, so the leaflet explained.

There is a much more adventurous meter available for measuring L and C over
rather wide ranges.
 This device essentially oscillates a built in LC oscillator, then adds the
unknown C in parallel with the tuned circuit (or the unknown L in series)
and measures the change in resonant frequency. The device depends on the
computations of a built in processor, of course.
About $100 last time I looked. This is the "Almost All Digital Electronics"
L/C Meter IIB

 Although its accuracy at the low end is not outstanding, even to learn
that an inch of wire can be expected to have about 10nH inductance is
pleasing.
 By contrast, a twist of two wires may represent 10 pF - yet another
example that the electrical units are not harmonized as to magnitude (in
some sense...) in the same way that lowest practical magnitudes of
measurable current vs. PD differ by several orders.

Brian

brian whatcott <inet@intellisys.net>
Altus OK