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Cessna 172 electrical system

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Cessna 172 electrical system

Old 30th Aug 2019, 10:55
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Cessna 172 electrical system

Hi guys
just a couple of questions about the electrical system of the C172.
i dont know how to post the schematic but i am confident that those of you able to provide me with an accurate answer are familiar with the layout.
as far as i understand when you flip the BAT side of the MASTER switch on, you ground the battery contactor, energizing the coil, driving the plunger down, completing the circuit and allowing battery power to be fed from the battery to the primary bus via the starter contactor and the ammeter. As the key is turned to the START position the the coil of the starter contactor is grounded, driving the plunger down, completing the circuit and allowing battery power to be fed to the starter motor, cranking the engine up.
as the ALT side of the MASTER switch is set to ON, the magic starts: the alternator is brought on line and power from the B terminal of the alternator is fed to the primary bus with a portion of it being fed via the alternator field cb and the alternator switch to the INPUT side of the ACU Alternator Control Unit, which in turn modulates the field charge fed to the F Field terminal of the alternator based on the positive sense input from the B terminal and on the negative sense input from the G terminal to vary the strength of the field current and thus the magnitude of the electromagnet so as to regulate the voltage output as the engine RPM’s and or the load vary.
Is the latter a correct statemennt?
now were does the current for the initial excitation of the field come from? I assume from the battery but the schematic does not show any wire going from the battery to the ACU and i am having a hard time figuring out the path.
as the alternator voltage exceeds the battery outpt, the current is fed from the bus via the starter and the battery contactors, to the battery keeping it charged.
Also, as far as i understand if the output of the generator exceeds a preset limit due to a failure of the ACU, the alternator field cb will trip, cutting off the field current to the alternator, causing the electromagnetic field to collapse and thus the alternator output to drop to zero - the cb can be reset switching the ALT side off and back on, flshing the field back, while should the draw from the bus exceed a preset limit, possibly due to an excessive draw from the bus, the ALT cb will trip isolating the bus from the alternator: if this is the case would not an excessive current draw from the bus result in the alternator field cb to trip as well as the altrnator will be coaxed to overwork?

Many thanks for your help



barrichello72 is offline  
Old 30th Aug 2019, 11:20
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Sorry to be *that* guy, but you need to get out more.
Gin Jockey is offline  
Old 30th Aug 2019, 11:20
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Jeez! With a thousand hours on 172’s, I had no idea the Master Switch did all that.
Just flick the switch, turn the key and away I went.
Capt Fathom is offline  
Old 30th Aug 2019, 17:17
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Wouldn't it be logical to assume that the ACU just gets the field current from the output side, which is energized when you switch on the alternator?

The alternator field cb most likely trips on overvoltage, not overcurrent.
Otherwise having one breaker that disconnects the alternator would suffice, wouldn't it?

But i don't have the schematic, the above is inferred from some rudimentary knowledge about electrical systems in general.
wiedehopf is offline  
Old 30th Aug 2019, 17:28
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This might help you buddy.

C172S Electrical System
jagema is offline  
Old 30th Aug 2019, 20:43
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Originally Posted by jagema View Post
This might help you buddy.

C172S Electrical System
Looking at the OP, I think he needs a different kind of help
oceancrosser is offline  
Old 31st Aug 2019, 12:39
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Hi, having been stranded at a distant airfield, with a dodgy contactor. I can say that the three contactors are not interchangeable, as some require an earth to operate, and others require a positive.

If the alternator is anything like a car alternator, then the initial start field current is supplied from the ignition warning light, just a few milliamps, to get the alternator magnetised.

Last edited by scifi; 31st Aug 2019 at 12:53.
scifi is online now  
Old 1st Sep 2019, 09:56
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Geez, what’s wrong with some of you?

The OP posts a technical question in the TECH LOG section of PPRuNe, and some of you take the p*ss.

If you are not technical yourselves, appreciate that some folk are. I bet you don’t have the same negative attitude towards the people who designed and engineered your iPhone? your laptop?, your flat screen TV?, the electronics that run your car engine? etc, etc.
Uplinker is offline  
Old 1st Sep 2019, 10:54
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Barrichello 72, if there were more pilots like you who want to know exactly how their aircraft worked there would be less incidents. Well done. The more you know the safer you will be. The people who say that all you need is to turn the key and go, should either do some study or hand in their PPLs.
bugged on the right is offline  
Old 1st Sep 2019, 11:50
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Originally Posted by jagema View Post
This might help you buddy.

C172S Electrical System
Having had to come to grips with, diagnose, and repair alternator faults in both a 35yo Toyota Landcruiser (utterly conventional for its time, mechanical external regulator, six power diodes, no exciter diodes, very similar to the 172) and a 60yo Vespa (forget everything you thought you knew, this is regulated AC, no diodes at all, no battery) in the last few weeks, I've got to say that that interactive wiring diagram is brilliant. I've forwarded the link to several automotively inclined friends as an example of what is possible.
nonsense is offline  

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