C measurement tool

[Paul Schreiber]

Don't hold your breath for a MOTM DVM! <grin>

The Schreiber/Curtis chip in the RadioShack DVM uses switched laser-trimmed
current
sources and a 16-bit timer.

EE-101 >>> I = C dv/dt

So, we switch in a known I (there are 5 current sources to cover the cap
ranges of 10pf to 100mf),
 measure dv/dt (that's the change in voltage over time) and calculate C with
the on-board 6502 uP.

The ever clever reader will immediately note this is how a standard sawtooth
VCO works!
In 'DVM lingo' this is a "single-slope integration".

Yes, the DVM is a hidden VCO controlled by an Apple II! Wonders never cease.

Paul Schreiber
Synthesis Technology


-----Original Message-----
From: Paul Perry <pfperry at melbpc.org.au>
To: synth-diy at mailhost.bpa.nl <synth-diy at mailhost.bpa.nl>
Date: Monday, April 19, 1999 4:48 AM
Subject: Re: C measurement tool


>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
>
>