Malcolm Walker wrote:
> Hmm... a sound-card equipped computer can almost be used as a 'scope...
> <ponder>
Well, a scope is like a MIG welder, once you get one you
are surprised you lived without.
However the sound card ones have very limited input ranges,
among other limitations.
> > (in other words, analog meters actually use power from the circuit
> > to move the needle, and the signal produced from the oxygen sensor
> > is too weak to move the needle. Digital meters draw almost no
> > current from the circuit they measure)
>
> Ugh. I think you're right. My cheap-o meter (the REAL reason why I
> bought an analog) only goes down to 5V DC and 10V AC. That means I have
> 1/5 of the scale to watch for 1 volt.
Actually, it's a little worse than that.
The problem is that the analog meters use the current from the circuit
they are watching to move the needle. The voltage in the circuit causes
a current in the windings of the needle movement that causes the needle
to move.
Because you are using a voltage to create a current, work is done and
therefore power is consumed.
Enter the humble O2 sensor, which creates a voltage based on some
fancy chemistry but can not supply any meaningful amount of current. It
can only supply the voltage to an open or near-open circuit, clip a
voltmeter across it and the voltage will dip to about nothing.
The term is "high impedance", which digital meters are pretty much
universally, where only a few specially designed analog meters are
able to be. Very very little current flows through the probes, so
you don't disrupt the circuit you are watching.
> When I'm rich and famous I'll buy a Fluke or similar beast. For now it's
> the Radio Shack Special. :-(
I'm a big fan of radio shack specials actually. I have a fluke 87
which
is a somewhat fancy fluke with backlighting and bar graphs and true RMS
reading and all sorts of doo dads. I also have one of those credit card
sized radio shack digital meters that runs on watch batteries. Guess
which
one gets used the most?
--
Trevor Boicey
Ottawa, Canada
tboicey@brit.ca
http://www.brit.ca/~tboicey/
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