737 Fuel valve position indicators

[allendc]

I have a question regarding Fuel valve position indicators on the fuel overhead panel. The way I understand their operation, is that they have three levels of indication.

Off = Valve Closed ( 0 Volts)
Bright = Valve in transit or Valve Switch disagreement ( 28 Volts)
Dim = Valve open ( 16 Volts)

On the main instrument panel there is the Bright / Dim / Test switch. In the Bright position 28 volts is supplied to the indicators. With the Bright / Dim / Test switch in the Dim position 16 volts supplied to the indicators.

I assume with the Bright / Dim / Test switch in the Dim (16 Volt) position the Bright function of the "Bright" Fuel valve position indicator now at 16 volts.

Now for my question:
With the Bright / Dim / Test switch in the Dim (16 Volt) position what is the voltage of the Valve position indicator indicating "Dim" showing the valve is open?

Regards

David C. Allen
www.737FlightSim.com

[aztruck]

Havent a clue mate. I'll ask an engineer and see if he knows. The main thing is that it works!!
Basically you want to know how the bulb manages to be bright when its in the dim position, is that correct?

[Stealth]

>Off = Valve Closed ( 0 Volts)
>Bright = Valve in transit or Valve Switch disagreement ( 28 >Volts)
>Dim = Valve open ( 16 Volts)

This statement is ok as position indicators lamps actually have a ground (power seeking) and come on when an electrical power is on the output terminal ie 16 or 28V.

>On the main instrument panel there is the Bright / Dim / Test >switch. In the Bright position 28 volts is supplied to the >indicators. With the Bright / Dim / Test switch in the Dim position >16 volts supplied to the indicators.

Now, the dimming module does not alter the voltage be that 16 or 28V as it employs Zener Diodes to cause large changes in current while voltage change is minimal. I think that the Bright position of the Bright/Dim/Test switch allows the voltage (16/28V) to be applied to the lamp directly while the Dim position is diverting it through the Zener diode reducing the brightness in both scenarios (open valve or in transit).

>Now for my question:
>With the Bright / Dim / Test switch in the Dim (16 Volt) position >what is the voltage of the Valve position indicator >indicating "Dim" showing the valve is open?

You are implying in your question that the dimming module generates the ‘open valve’ signal which is incorrect.
Have fun.
;)

[Flight Detent]

Hi Guys,
My understanding is a little different -
DIM - valve closed
BRIGHT - valve in transit
OFF - valve open

-- and the dim switch selection does not effect it at all.

-- the test switch gives it full 28v, and therefore bright indication,
whenever it's selected, via a seperate input.

But, as I say, that's as per a B747 Classic.

[allendc]

My prior question was to complex:

Could someone open the crossfeed valve and after the valve has opened, could you while watching the crossfeed valve indicator move the Bright / Dim / Test switch from Bright to Dim.

Did the intensity of the crossfeed valve indicator change brightness?

Thanks

David
www.737FlightSim.com

[allendc]

Thanks to everyone for all the help on this question, in summary for the 737 NG aircraft:

Bright = valve in transit or valve switch disagree

Crossfeed Valve (Valve open) Indicator
Dim = crossfeed valve open

Engine Valve & Spar Valve (Valve Closed) Indicator
Dim = related valve is closed

With the Bright / Dim / Test switch in Bright the and the indicators indicating a dim condition, if the Bright / Dim / Test switch is moved to Dim the dim condition will reduce in brightness.

Thanks

David

[aztruck]

Tried it the other night for you on a -700. The dim/bright switch makes no difference to the dimness or brightness of the light during operation of the crossfeed valve.
Hope this helps.