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Interpreting those electrical system schematic!

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Interpreting those electrical system schematic!

Old 23rd Dec 2014, 10:13
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Interpreting those electrical system schematic!

A nightmare for many if us who don t have a master in electricity - if such thing exists!
I am referring to the electrical system of the C172, a basic trainer and i have to take note that i am having a far harder time interpreting that schematic than the Challenger electrical system!
For instance how does the battery get charged? By the main bus through the starter contactor and the battery contactor? Cos i dont see any line going from the battery contactor straight to the bus network! So my guess is that battery power is made available to battery contactor which in turn distributes it to the starter contactor, amongst other items, which feeds it to the starter when the key is turned in the START position and to the bus prior to starting the engine and bringing the rngine driven alternator online. Once the engine is running the alternator supplies the bus which in turn recharges the battery through the contactors.
Am I wrong? Anyone would care shining a light on the matter?
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Old 23rd Dec 2014, 10:51
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I guess we are all trying to 'imagine' what the diagram looks like - without much success. Want an answer?
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Old 23rd Dec 2014, 11:04
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Usually not easy to find a battery-alternator arrangement that is wired in series but they are wired in parallel.
The battery is connected to the buss thru a battery relay and an ammeter. The alternator is connected directly to the buss thru a circuit breaker.
The ammeter measures charge or discharge of the battery and when it is being charged, current is flowing from the alternator to the battery and the buss.
Hope this helps but I think on G1000 something could be different.

BAT and ALT are both connected to the Main Distribution Bus. In flight, the alternator is supplying all the electrical power to that bus and is charging the battery (by virtue of having a higher voltage output).

The Cessna-style (+)(-) and (0) ammeter works because it is literally wired in series with the (+) battery lead wire, which is routed behind the panel between the Battery Contactor (operated by the Master Switch) and the Primary Bus Bar. When current is flowing from the bus bar into the battery, the ammeter shows a (+) deflection. If the alternator fails or is switched off with the ALT half of the Master Switch, current flows through the ammeter in the opposite direction - from the (+) terminal of the battery into the Primary Bus Bar - thus the needle is deflected the opposite direction towards (-) (also the ACU detects a low voltage condition and illuminates the "Low Voltage" warning light). If the battery charge is topped off and the electrical system is in happy equilibrium then the needle is vertical at (0), because no current is flowing either into or out of the battery.


Last edited by axl76fg; 23rd Dec 2014 at 11:17.
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Old 23rd Dec 2014, 11:31
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Schematic

Hi axl, you must be an angel sent from heaven!! And a very knowledgeble one! I have thought about including the diaghragm but i couldn t figure out how - i am using an ipad -.
But allow me to take a bit more advantage of your knowledge and willingness:
The GPU or external power cart is plugged through a receptacle and then it connects to the battery contactor - where does it go from there? Is it used to recharge the battery through the contactor or to start the engine cranking it up and then to recharge e battery once the engine driven alternator is brought on line?
Power from the alternator is fed to the bus via the B pole - i guess - through the 60amp CB. The other two posts on the alternator labelled G and F what do they stand for? F for field indicating the trickle charge fed by the ACU to the rotor of the alternator through the slip rings to modulate the magnitude of the current induced in the stator as the load and/or engine RPM vary? What about the G post? And also the overvoltage relay is it based on the output of the generator so that if the output of the generator exceeds whatever value it will open interrupting the trickle charge from the ACU to the rotor causing the electromagnetic field to collapse and thus the output of the alternatir to drop? What is the function of the 5 amp GEN FIELD CB? Will it pop if the trickle charge exceeds 5 amps indicating aproblem with the ACU prior to the 60amp CB popping, preventing the itmes from becoming overloaded? Shouldn t the alternator drop offline as well?
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Old 23rd Dec 2014, 12:40
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The GPU helps the main battery during ground starts as you guessed-
F and G I suppose are as in the diagram the Field and Ground.
I'm not an Angel...I understand what you understand, just trying to compare my knowledge with yours-
But what you understood here above is the same as me!
For other specifics..contact any engineer
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Old 23rd Dec 2014, 12:58
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Are you barrichello72 or Baobab72 ...or BOTH
I searched in PPRuNe and you opened a similar thread recently in the past years...
I'm so confused....

Welcome to the Multiple account! LOL
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Old 23rd Dec 2014, 14:39
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My identity

Hi axl i am both.
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