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'How to simulate resistance?'
1998\05\06@012236 by

Hello everyone

I want to drive the OEM fuel gauge used on most common vehicles. Current
gauges use a resistive device to ground to drive the gauge, for example most
Chevy use a 0-90 ohm sender ( 0=full, 90=empty) , Ford 160-10 or 70-0 ohm and
a few others.How can I use a stamp or PIC to drive these gauges. I was
wondering if I could use PWM to drive the gauge to ground? I could utilize a
different table for each vehicle type. Any suggestions on how I might
accomplish this. Feel free to offer other suggestions I don't know if PWM will
work, but I am sure someone out there has figured out a slick way to do this.

Gauge----------------Sender-----------GND

Jon

At 01:20 AM 5/6/98 EDT, you wrote:
>Hello everyone
>
>I want to drive the OEM fuel gauge used on most common vehicles. Current
>gauges use a resistive device to ground to drive the gauge, for example most
>Chevy use a 0-90 ohm sender ( 0=full, 90=empty) , Ford 160-10 or 70-0 ohm and
>a few others.How can I use a stamp or PIC to drive these gauges. I was
>wondering if I could use PWM to drive the gauge to ground? I could utilize a
>different table for each vehicle type. Any suggestions on how I might
>accomplish this. Feel free to offer other suggestions I don't know if PWM
will
>work, but I am sure someone out there has figured out a slick way to do this.

Jon,
These gages really measure current so OHM's Law applies

I = E/R where I=Current in Amps
E = Voltage in Volts
R = Resistance in ohms.

The applied voltage is 12 VDC, and the resistance consists
of the resistance of the meter + the variable resistance of
the gage.  I have never measured the current through the gages
so I don't know first hand if the PIC can drive the Meter to full
scale or not.

Assuming the PIC can handle the current then it is a simple matter of
setting the pic output to a voltage level which gives the
desired reading.  No need for PWM.  My gut feeling is that you
will need to use an emitter follower circuit inplace of the
gage & drive that with the PIC analog output voltage

-Dan
kf6izoqsl.net

>
>         Gauge----------------Sender-----------GND
>
>
>Jon
>
>

At 01:20 AM 6/05/98 EDT, you wrote:
>Hello everyone
>
>I want to drive the OEM fuel gauge used on most common vehicles. Current
>gauges use a resistive device to ground to drive the gauge, for example most
>Chevy use a 0-90 ohm sender ( 0=full, 90=empty) , Ford 160-10 or 70-0 ohm and
>a few others.How can I use a stamp or PIC to drive these gauges. I was
>wondering if I could use PWM to drive the gauge to ground? I could utilize a
>different table for each vehicle type. Any suggestions on how I might
>accomplish this. Feel free to offer other suggestions I don't know if PWM will
>work, but I am sure someone out there has figured out a slick way to do this.
>
>         Gauge----------------Sender-----------GND
>
>
>Jon
>
>

Are!
1/      What are you using as the input? The fuel sender?
2/      Are you trying to drive the actual gauge? (Some form of common device)

I assume that this is correct from your comments, however the gauges that
you wish to drive seem a bit old hat. At present many instrument cluster
manufactures are using stepper motors to drive the gauge (The GMH VT Commode
is an example of this) Chrysler also don't use a resistive sender (This has
caused them countless troubles, I think that there up to Version 3 on the
Jeep!) Ford Australia also don't use a purely resistive sender. The reasons
for this are current consumption.

In the "normal" older style installation one side of the gauge is connected
to +12V and a basic resistive divider forms the sensor /sender (The gauge is
a  damped current meter). To do what you want a simple FET that connects +
of the gauge to ground could be used, then you suggestion of PWM to the gate
could be used (With the damping of the gauge). I do suggest that you filter
the PWM before it goes into the FET as it may induce noise (Not very likely
in such a low current situation) into other circuits of the auto. You will
also proberably need some feedback to compensate the FET. This is the
simplest solution that I can think of, there are others (As usual) that are
technically nicer, but this should work. Hope that it helps.

Dennis

PHXSYS wrote:

> I want to drive the OEM fuel gauge used on most common vehicles.

As long as it is the "old-fashioned" resistive sender type, then...

> I was wondering if I could use PWM to drive the gauge to ground?

Absolutely perfect!  Use a FET to drive the gauge.  Do use PWM so
that the FET dissipates virtually no power.  Do *not* condition the FET
drive other than with a 100 ohm resistor in series.  It will tend to
generate RFI which can be minimised using an inductor in series with the
output side of the FET with a diode returned to the 12V battery voltage.
Do not put any capacitors directly across the FET, you may place one on
the other side of the inductor.

Sound Okay?

Cheers,
Paul B.

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