How is it powered?

[AF330]

Hi, I wanted to understand how different computers were powered.
So here are my questions:

1) When we press the master switch, there is a switch that get's pressed and it let's the courant pass threw it. It sends an electric signal. For the master switch, there can only be two of them: 28V = master switch pressed. 0V = master switch not pressed. Then the PEPDC will receive the 28V DC and will understand that it has to open the LP fuel valve. Airbus/Boeing has programmed the PEPDC, so that it knows the amount of courrant needed to open the LP fuel valve. It will then open anothee switch and close it when the LP valve would be open. Am I right here?

2) The ECU receives 28 DC V. But does it use 28V DC? Let's say that it needs to send 10V DC to the HMU, it needs 10V to calculate the amount of courant to give to the HMU and 2V DC to open the FMV. So in this case, it uses 22V DC. Where does the 6V DC go? Heat?

I am still learning, thanks to help! ;)

[Amadis of Gaul]

Sweet Whistlin' Geronimo, are you trying to fly an airplane or build one?!

[ChristiaanJ]

AF330,
I don't mind trying to answer your questions, but what is a PEPDC ?

[AF330]

Well... It's a computer that controls the DC part of the plane.
@ Amadis of Gaul: Well, I don't want the overall picture, when you see F-COM with the transfer valves in the CTR TANK.... And when you see all these valves, I still want to know how they are controlled, for e.g: The HP fuel pump of the HMU is on the acessory gear box connected to the turbine... but in the tanks, the pumps are electrically powered! Do you remember, we had talked about the CTR TANKS? I hadn't understood how it had priority on Inner tanks! I had to understand the engine feed stuff with the sequence valves. Now , I think: how are the sequence valves controlled? Electrically? (switches, servomotors....) ;)

[spannersatcx]

Well... It's a computer that controls the DC part of the plane. Well having worked on a variety of Boeing/Airbus a/c I have never heard of a PEPDC?
DC is normally supplied by a DC BATT bus or the DC buses are through TRU's.

[Amadis of Gaul]

I'm starting to see your "main error" or perhaps "main miscalculation". You admitted to not wanting the overall picture, which is exactly what you should want. Instead, you're getting hung up on REALLY minute stuff that's completely irrelevant to a pilot.

[AF330]

Well having worked on a variety of Boeing/Airbus a/c I have never heard of a PEPDC?
DC is normally supplied by a DC BATT bus or the DC buses are through TRU's.

So how do you control DC? For example, you have to charge the batteries. The batteries have to send a signal somewhere to get connected with DC1 and/or DC2! How are the BCL controlled?

@Amadis of Gaul: Are you an airline pilot? Just wanted to know! ;)

[EEngr]

So how do you control DC?

Simple relay logic. The main battery supplies voltage to a few flight deck control switches. One of them is the 'Battery' switch. Closing this provides a signal to a DC contactor (big relay) to connect the battery to the battery bus. Many of the systems control computers (including electrical bus control logic) are powered from this bus.

One can also plug external power into the airplane. This powers a utility bus and a few other systems used for ground operations like lights, cargo door controls, etc. In this case, the main battery is not involved.

This is a general, high level explaination. The specifics depend on exactly which model you are interested in.

[AF330]

So the batteries send an electric signal to the DC BAT?

[spannersatcx]

No, have you got access to basic AC/DC system schematics?

[AF330]

No prob, I talked with a 777 engineer and he explained it! ;) It's the BCL...