10 LED O2 monitor

[Aron Lopes Petrucci]

If we are talking about led voltmeter, it's good we remember something about
they, in
special about the voltmeter input inpedance.

The Voltmeter input impedance does not depend of the analog or digital kind.
This (analog or digital) means the aspect of result presentation (by digits
or by a pointer over an scale), and, of course, some aspect related to the
max. precision for reading it.

The input impedance, (ie. that ohm/volt indication), means how much the
metering instrument "loads" the device under test. So an old, an poor,
"non-electronic" voltmeter can show to the device 5,000 ohm/volt, ie. at an
scale wich has 5 volts of full range, the meter is something like a shunt of
25,000 ohm to the device under test. Note that the determination is made by
the deep of the scale and is valid for all that range).

Now, you can repeat the idea above, with an old, and analogical, electronic
voltmeter, (like that one with 10M). But, pay attention: some electronic
voltmeter, however, have a fixed input impedance (look, it's 10 Mohm, and
not 10 Mohm/V).

This missundestood is made by the fact that there are not non-electronic
digital  voltmeters, the analog voltmeters were (or are) made in these 2
systems.

But, to measure a signal like the O2 sensor voltage, where I'm not intersted
only in the voltage, but  in a sight of its variation too, a digital
voltmeter, with the same input impedance, can be 'more poor' than an analog
unit (some digital voltmeters have a "analog scale", something like a VU
meter to fix this problem). So, the Led voltmeter would be a good chice.

At end, 1'd like to remember that a low input impedance voltmeter can show
values
lower than that wich are actually present. Its easy to explain:

Look at picture (the atachment in this message).

The O2 sensor can be treated as an association of a battery (wich represent
the
voltage that we are interested in) in a serial connection with a resistor
(wich
represent the internal resistence of an O2 sensor). At fact there are not a
battery and a resistor inside O2 sensor, but its behavior is like this.

By the other hand, a real voltmeter is like an ideal voltmeter, (an unit
that
shows voltage values with no current trhough it), shunted by a resitor (that
internal resistence.I told about it above).

If the resistence "Riv" is low, there are more drainage of current through
"Ri",
then there is a higher "loss of voltage" in Ri, when less voltage is
remainning for
Riv, just the voltage wich the voltmeter is showing.



Thankx for attention, I hope that this explanation can be useful, for a
begginer, at least.

and I apologize for my brazilian K12 english ;-)

Aron L. Petrucci
aronlp at onda.com.br


and I apologize for my brazilian K12 english ;-)



----- Original Message -----
From: Bruce Plecan <nacelp at bright.net>
To: <gmecm at diy-efi.org>
Sent: Tuesday, November 14, 2000 3:04 PM
Subject: Re: 10 LED O2 monitor


>
> Some of the old stuff was like less then 10.
> Hmm, radio shack.
> I got away from the analogs for auto use 15+ years ago.
> may have to revisit that.
> Bruce
>
>
>
> > Do the analog VOMs have that low of an input impedance?  I have a radio
> > shack analog that uses an FET front end, it claims to have a 10M input
> > impedance for voltage measurement
> >
> >  On Tue, 14 Nov 2000, Bruce Plecan wrote:
> >
> > >
> > > O2 sensor is not hardly designed to drive such a low resistance devise
> as a
> > > VOM.  Shouldn't be any less then 20K ohms per volt for reading an O2.
> > >
> > > Best idea in not even bothering with trying to read a switching O2
other
> > > then >.4 rich, <.4 lean.  Other then that your just kidding yourself
> > > Bruce
> > >
> > >
> > >
> > > >why
> > > > even screw around with an led display when all you need is a 0-1V
> analog
> > > > volt meter....try to find an old simpson 260 or triplett 310 VOM.
> it'll do
> > > > just fine and will probably have a better response time for that
> matter.
> > >
> > >
> >
>
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