resistive fuel sender

[Luciano]

Hi,

If possible, leave the original fuel level gauge connected to the sending unit.
(The sending unit is the potentiometer inside the tank).
Using the A/D inside the PIC, measure the voltage present on the wire
which connects the fuel level gauge to the sending unit.
(When the tank is empty you will have the the maximal voltage on this wire).
Use two resistors as voltage divider if this voltage exceeds 5V which is
the maximal voltage for the A/D inside the PIC. Protect the A/D input of
the PIC in order to limit the voltage to 5V. (If you disconnect the sender
you will have 12V through the fuel level gauge).

See attached BMP file.

Do not connect 12V or 5V directly to the sender.
If you do that, you will destroy the potentiometer and in the worst case
you will cause the explosion of the fuel tank. All the parts of the
potentiometer are in contact with the fuel.

Be careful with gasoline!

- Gasoline is toxic.
- Gasoline forms an explosive mixture with air.

Luciano

[beto]

I have setup the 10bit A/D Converter With this resistor-divider (see bmp) to read fro 0v (sender in R) to ~1.75v (max R sender)
and I use a PIc18f452 to read the resulting voltage, and send it to hyperterm
This work but no very well, manually moving the sender the readings is'nt stable there is variation, someties without move the sender!
another issue, a simple math operation I=v/r show the i aplied to my sender
i=3/510= 5mA this is safe current to prevent rupture of the sender...?

[Luciano]

If you still have the original gauge, connect it temporarily
to the sender and measure the current with an ampere meter.
Once you know the current flowing with the original gauge, then just
dimension the resistors accordingly.

Old thermally-actuated gauges use a voltage stabilizer or "vibrator" in
order to compensate the voltage changes when the generator is charging.
In this type of gauge the current will not constantly flow through the
rheostat of the sender. See URL.

http://www.nls.net/mp/volks/htm/fuel_ga.htm

Another type of gauge is the balance coil gauge.
No voltage regulation is required with this type of gauge.
In this type of gauge the current will constantly flow through
the rheostat of the sender. See above URL.

* * *

Your circuit is fine because the voltage is regulated.

* * *

When you go around a turn, or up a hill, the fuel can slosh
to one side of the tank and quickly change the float position.
If the gauge needles were to respond quickly to all of these changes,
it would be bouncing all over the place.
The capacitor C1 on my BMP was to limit this problem but you
can also take care of that in the software.

If your fuel sender is a wirewound rheostat, then don't expect
a resolution better than E, 1/4, 1/2, 3/4, F.

Example of modern fuel sender on ceramic substrate.
http://www.bourns.com/pdfs/fuel_card_apno.pdf

Again, be careful!

Luciano