O2 Sensor Voltage

[John Dammeyer]

Hi all,

I think you have to consider the input impedance of the measuring device when
checking out the O2 sensor.  Unlike parts available from semiconductor
manufacturers there doesn't seem to be a lot of information out there about O2
sensors.  However,  measuring the voltage the way you are is fine as long as the
load the meter presents isn't too low.  Normally a standard digital meter is
adequate.  As for an isolated ground... this isn't really needed unless the O2
sensor is also still connected to the Engine Control Unit.

This is because you don't know what the electronics inside the ECU looks like.
Both sides may be isolated above ground so connecting one side to ground will do
strange things.  But as far as measuring the ECU output relative to ground,  I
use a standard LM324 operation amplifier to isolate the low input impedance of
the uC A/D converter from the O2 sensor and once side of the O2 sensor is
connected to ground.

What I have also determined empirically is that 0.5V is 14.7:1 ratio and that
0.7 is about 14.3:1 and 0.3 is about 14.9:1 or thereabouts.  Note too that some
A/D devices (like your digital meter display module),  may require a stable
signal in order to create a valid reading.  This is why some systems use a
Sample and Hold (S/H) circuit to freeze the value while the A/D performs the
conversion.

Cheers,

John