Stratomaster smart single

[Ronnie L]

Please can anyone help
I have just fitted a Stratomaster FF1 Fuel managment system to my RAF2000, it has a fuel level in a type of bar graph along the bottom and it also gives a readout of the fuel level in numbers eg 60 lts. This thing is set up by putting measured amounts of fuel in at pre determined points untill its full, you know the type of thing. The problem is it seems to be affected by the engine running.
First of all I set it up with just the battery voltage, as I drained measured amounts of fuel out after calibration it seemed very accurate and with 35 lts in I started the engine, it then went up to full. So I did the calibration thing again with a battery charger connected to try and lift the voltage to similar to when the alternator is charging, and now it still goes up with the engine running but not as much.
Has anyone any ideas please.
Ive checked the alternator and it's charging at 14.3 volts

Ronnie Legge
RAF 2000
G-BYIN

[banaari]

Hi Ronnie,
if as you suspect the issue is voltage-related, try running the FF1 off it's own 12V regulator (fed in turn from the aircraft regulator), specifically a 7812 3-terminal device... that'll supply up to 1A, you need 70mA max. That way you hopefully remove voltage fluctations from the system. Please email me if you need more info.

I don't know if you're using a capacitive or resistive level sensor, but I'd also look at shielding the cables running to it.

cheers,
John

[PW_Plack]

Guys,

A 7812 won't solve the problem, because 78xx regulators require a minimum 2-volt input/output differential to hold regulation. If the maximum charging voltage reaches 14.3, then a 7812 will lose regulation as soon as the alternator drops below 14.

Ronnie, see if you can get an accurate reading on the battery voltage with the engine off for a few minutes, and let us know what it is. (If it's below about 12.2, you may need a new battery.)

Stratomaster says the FF1 can operate at 8 volts, so a 7808 could work. That will hold regulation down to 10 volts, at which point your battery is permanently damaged anyway. Place a 1 uF capacitor from the regulator's input side to ground, and a .1 uF on the output side to ground.

I doubt that voltage regulation is your real problem anyway...

I would suspect you have some kind of noise coming in the power lead. If there's a substantial AC component superimposed on the DC from the alternator, it can confuse digital instruments, and even cause digital radios to change or lose their frequency programming. If you can hear a high-pitched "whining" sound in your radio which rises and falls with engine RPM, or which gets worse when you add load to the electrical system, this could be an issue. An alternator with brushes going bad can make this worse. The power lead filters used to keep whine out of car stereos may help, if the alternator's good.

Be careful not to let voltage spikes from an electric starter reach avionics. That can be instant death to solid-state regulators.

[banaari]

A 7812 won't solve the problem, because 78xx regulators require a minimum 2-volt input/output differential to hold regulation. If the maximum charging voltage reaches 14.3, then a 7812 will lose regulation as soon as the alternator drops below 14.

Bugger... forgot that! :)

Ronnie tells me he's using a capacitive sensor... and the FF1 won't accept more than 5V from such a beast... it might pay to investigate & regulate the power supply to the sensor itself.

[Ronnie L]

Thanks guys I'll try your ideas.

Ronnie