C172sp electrical system

diego72

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hi everyone
I am new to the forum so please dont be too harsh on me!
I am a jet jockey although i want to brush up on single engine pistons and i decided to tackle the system that used to scare me the most: the dreadful electrical system!
My questions, are related to the 172 sp model without garmin 1000 and are the following:
Looking at the schematic i can see, or should i say guess, that power from the battery is fed through the external power contactor to the battery contactor, featuring a number of terminals: one connected to the battery and supplying battery power to the hot wired item - the clock - at all times and to the contactor itself whenever theBATT side of the switch is set to ON, one to the starter and to the SENSE port of the ACU which i completely ignore what it is - not the ACU but the SENSE port! The other thing around the contactor with what i guess a diode i have no idea either!
Second question: power from the alternator is fed to the ACU which modulates the excitment charge to the axial rotor controlling the magnitude of the electromagnetic field and thus the current output. From the ACU i guess through the ALT INPUT terminal on the ACU itself goes to the ALT relay and from there to the bus network, am i right?
Third question: the ALT relay has several terminals: one from the starter and i guess controllng the power from the battery to the relay coil whenever e ALT side of the switch is set to ON, in this case the solenoid is energized and battery power is fed to the ACU power in port flashing the alternator field and bringing it online. Now is power from the alternator fed through the alternator relay and the starter relay and the battery relay back to the battery? And how is it distributed to the bus network?
So far, many thanks for your willingness and if there is anything you would like to know about the chally, i would be more than glad to assist!

Diego72
 
It does, although it does not provide a clear explanation about the schematic.
Some may argue it is pure academic and that i shouldn t trouble myself with things i wont be able to do anything about, but i love to get to know my airplane in details and if i become aware that there is an area that it is not completely clear to me i do my best to investigate it, trying to shed some light on it and i saw this forum as a valid tool.

Diego72
 
It does, although it does not provide a clear explanation about the schematic.
Some may argue it is pure academic and that i shouldn t trouble myself with things i wont be able to do anything about, but i love to get to know my airplane in details and if i become aware that there is an area that it is not completely clear to me i do my best to investigate it, trying to shed some light on it and i saw this forum as a valid tool.

Diego72
I do it for extra curricular reading as well. I haven't read the POH for a 172 for many years now but I remember it being fairly clear on explanations.
 
It does, although it does not provide a clear explanation about the schematic.
Some may argue it is pure academic and that i shouldn t trouble myself with things i wont be able to do anything about, but i love to get to know my airplane in details and if i become aware that there is an area that it is not completely clear to me i do my best to investigate it, trying to shed some light on it and i saw this forum as a valid tool.

Diego72
Then a better use of your time would be to pursue your A&P.
 
I'll try to answer as best I can, but let me preface this by saying the following is merely the guess of an uneducated guy based solely on book knowledge and no hands-on experience.

1. I think the sense wire of the ACU is used to control the output (field circuit) of the alternator by “sensing” the system voltage at the battery contactor. By sensing the voltage near the battery instead of that output by the alternator, the ACU can do a better job of judging if the alternator output is meeting the electrical needs of the entire system.

As for the diodes, I believe those two diodes between the terminals of the battery contactor constitute some type of flyback circuit. It is meant to prevent voltage spikes that could damage the contactor when it is de-energized. One is regular diode, and I think the other is a zener diode. Diodes usually allow current to flow in only one direction (the direction the triangle symbol is pointing in the schematic), but a zener diode differs in that it allows current to flow opposite if a given voltage is reached and then closely maintains that voltage. It’s useful for stabilizing voltages.

2. Yes, I believe you’re correct about the distribution path of the alternator. (Again, based only on my interpretation of the diagram – no real-world experience)

3. I don’t think current from the alternator is routed “through” the starter relay per se. It looks like it is simply connected in parallel with the battery relay. Current flows "past" the starter relay on it's way to the battery. As far as being distributed to the bus network, see the two legs labeled “A” and “B” on the upper right of the schematic? Those are just off the alternator relay and correspond to the inputs on the bus diagram on the next page of the POH.


Hope that helpful and not incorrect,
Clint
 
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