Sender: <marv@lancaironline.net> (Marvin Kaye)
To: lml
Date: Mon, 09 Sep 2002 08:49:16 -0400
Message-ID: <redirect-1724330@logan.com>
From: "Gary Casey" <glcasey@adelphia.net>
Subject: fuel probe
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<<There was a suggestion some time ago, to use a 12 VDC regulator in the
power supply for the fuel probes to eliminate one source of problems.
I noticed yesterday that, even tho my hydraulics and my fuel probes
originate from different circuits ( and circuit breakers), both fuel gages
rise to full whenever the hydraulic pump cycles.
I believe that the recommendation was to use an Archer 276-1771 voltage
regulator to isolate the circuits and to provide a constant voltage source
to the probes.   I'll comment again after I install the regulator.  Regards,
Bill Hogarty L-IVP @ 95%>>

Given the circuitry in the sensor it isn't surprising that the sensor
"rails" with the noise of the motor.  I wouldn't be surprised if it did the
same thing with a radio transmission.  Assuming the problem stems from the
fact that the fuel probes are apparently fed from an unregulated 12-volt
supply there are a couple of things that can be done.  First, I'm assuming
that the difficulty does not stem from the voltage drop during gear cycling,
but from the noise generated by the pump motor.  Does the gage read the
same, or nearly the same with the engine off?  If it does, the circuitry
behaves okay with varying supply voltages (I can't remember if we tested it
at different voltages).  Actually, the normal thing would be to feed the
fuel senders from a regulated 5-volt supply in the instrumentation - I
haven't looked into my system yet to find out if that's the way they do it.

To filter the supply you can put a resistor in series and then a capacitor
to the ground wire.  This should be as close to the sensor as practical.  If
one had a connector in the wing root as close as 6 inches from the sensor
that would be the place to put it.  A good value for the resistor would be
about 30 to 50 ohms and the capacitor should be about 0.47 mfd.  As I
recall, the metal housing is grounded through the sensor ground wire and the
ideal thing would be to put the capacitor to ground right on the housing,
but that would be a bit fussy to do.  This methodology might be more
effective that adding a voltage regulator.

Gary Casey