[Diy_efi] Bernd: UEGO sensor question

[Jorgen Karlsson]

Perry Harrington wrote:

> With this explanation it makes a bit more sense.  I've looked
> over the schematic and don't see where this bias is 'turned off'.
> Since the Vs output will always be 0-1.1v in operation, how
> do you sense when it has warmed up (gone from 0v to 1.1v)?

The resistors R31,R32 and R35 supply 2v to the non inverting input of UA2 as
long as the internal resistance of the sense cell are extremely high. When
the sensor heats up the resistance in the cell will only be a small fraction
of the 100k R32. The sensor itself is the switch that turn the bias off or
short-circuits it.



> Does the 2v just present an offset from the 0-1.1v, so the range
> is 2-3.1v?  Da, and the middle is 2.5v, which is your stoich reference?

Nope, the 2v does nothing when the sense cell is active.

> Is my understanding that the Cal resistor is used as a scalar for
> the current
> value.  In the DIY-WB circuit it seems that the 2.5v voltage
> would be non-linear
> since it's using -4v as ground and +4v as Vdd, this would make
> the 2.5v really
> -1.5v instead.  The whole output after the U4c is puzzling me.
> The current paths
> and voltages are not clear to me.  The bias from the Lm431 I
> don't understand.

Remember that -4v and vehicle ground are the same as a virtual dual supply
are used in this case.

The UC4 opamp is connected as a ground referencing amp with gain. It takes
the differential voltage over the current sense resistor R28, amplify it a
bit and reference it to 2.5volts over vehicle ground. That potentials is
supplied by the 431. If you would use vehicle ground instead of the 2.5v
supplied by the 431 you would have an output voltage that would be in the
range -1.1v to 0.4v instead of 1.4 to 2.9v. We would of coarse need to use a
dual supply for U4 to make it swing below 0v.

Jorgen




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