Problem with my WB

[Phil Hunter]

> > Does a free air test give an output of ~ 4.0V? If
> so I'd say that it
> > would seem the DIY-WB is working correctly. 
> 
> Free air gives an output of 4.06v.  A little on the
> high side.....

[snip]

> Is there a practical limit on the length of the
> leads from the WB to the sensor.  I used 18g wire
> for all leads, but they are about 16 ft long in
> their current temporary routing.

Depending on how you've done it, that much wiring
could explain the free-air difference/error. 18ga
wire has 6.3ohms/1000ft resistance, call it 0.1 ohm
for 16ft, 1 amp of heater current thru the ground wire
will generate 0.1V ( I x R ), 3.93V + 0.1V = 4.03V

For the positive supply wire(s), the power supply IC's
will compensate for the minor voltage drop.

For the rest of the wires, so little current flows
thru them that 18ga is huge, I x R = 0.0V

2 techniques I can think of to minimize errors. One
is to run both Vout and Gnd wires from the PCB to your
meter, don't just run a Vout wire and use a convenient
nearby ground. Since the meter draws no current, there
is no error voltage this way, you can use very small
guage wire too.

The other technique that I'm doing is to run seperate
Gnd wires from both Gnd connections on the PCB using
"zip cord". This seperates the small current for the
control circuitry from the large current for the
heater circuit, hence minimal error voltages. Since I
intend to interface the DIY-WB to the ECM, I'm running
the control circuit ground to the same connection the
ECM uses for the NB sensor ground, for my convenience
I'm running the heater ground to the same spot (both
leads smoldered to the same lug), but I could also
hook them up to seperate places, like the heater Gnd
to battery ground terminal instead.

You can use either or both techniques.

HTH,
philh
(digest)


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