On Fri, 10 Sep 2004 08:24:46 -0400, Olin Lathrop
<olin_piclist@embedinc.com> wrote:
> PicDude wrote:
> > - The sender is a fuel sender, GM style (240-33 ohm).  I use a 240 ohm
> > resistor for the upper part of the voltage divider (between the PIC
> > A/D input and +5V), and the fuel sender forms the lower part of the
> > voltage divider.
> > So the max A/D input signal voltage is 2.5V, and the impedance is 120
> > ohms max.
 
I have a similar project that uses 240 ohm fuel senders, but I did not
run that high of a voltage to the sender.  The divider from 5v is a
24K resistor so that the voltage at the sender is 0-50mV, not 2.5V. 
In an automotive environment the load dump conditions are horrendous
(example: turn off the headlights and the regulator overshoots by 300
volts for a few uS).  With that in mind I thought it was a safe idea
to use a very low voltage in the sender/divider circuit (not that it
would really matter since my supply is filtered), then use opamps to
isolate and scale/offset for .5-4.5 volt range at the adc.  Any
voltages above or below that means there's a fault in the system.  I
believe 0-50mV is the "standard" for automotive senders.

Regards,
Bob
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