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DC electrical system according to the FAA

Charlie Longley

Charlie Longley

MEMBER
Anchorage, AK
I know there are a lot of different views and theories on how a DC electrical system works in an aircraft. But the FAA only sees it one way. It’s important to know it because all the manufacturers wiring diagrams are in accordance with it. These pics are from AC65-9A or a Cessna service manual.

First off the FAA says current flows from negative to positive. This is important because of the placement of diodes, LED’s and other polarity sensitive appliances.
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The forward bias, or high current, direction is always against the arrow of the symbol on a diode.
F64F9C0B-0985-4E89-B84D-44939F21E41A.jpeg

This is important because of the way that FAA approved maintenance manuals are written. For example on the Cessna 172 external power there are diodes to help prevent hooking up an incorrect external power.
2F026AC5-0335-4319-9861-6CB71140434C.jpeg

I am willing to answer any questions people have.

Charlie
 

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Charlie Longley said:
I know there are a lot of different views and theories on how a DC electrical system works in an aircraft. But the FAA only sees it one way. It’s important to know it because all the manufacturers wiring diagrams are in accordance with it. These pics are from AC65-9A or a Cessna service manual.
Click to expand...

Did the Cessna service manual diagram include the red markings or did you add them?
 
That's a standard NATO DC power plug. When wired correctly, there are large gauge positive and negative cables and a small gauge jumper to the small socket contact. This small contact is connected to the positive cable and is used to operate relays wired for external power connection and reverse polarity protection.

For example, when the external power plug is inserted, if an external power relay is installed, no external power will be applied to the bus until that relay is energized. This can happen automatically if the small contact from the plug is wired to the relay coil. On some aircraft (usually larger types that spend lots of time on the ground electrically powered up for specific purposes) that same circuit is used but with a switch added at a convenient location to control when the external power is applied or removed. An added benefit of using the positive voltage of the small contact to power the external power relay is that it acts as reverse polarity protection. If that relay is looking for a positive voltage to energize it's coil but the external power is connected backwards, then a negative is applied to the relay which will NOT energize it. In that case no damage will be done as reverse polarity voltage will not pass the relay and enter the bus system.

Web
 
Charlie Longley said:
Anything red I’ve added.
Click to expand...

So if I took the same Cessna diagram and added a red arrow in the opposite direction it would be entirely consistent with "conventional" current flow. I just don't see how referencing the Cessna diagram supports your case.

An internet search for "diode symbol current flow direction" will return many references that say the current (conventional current) flows in the direction of the diode symbol's arrow.

Here is a diode description from Fluke (probably generally regarded as a competent test instrument manufacturer) -

"Strange but true: The diode symbol's arrow points against the direction of electron flow. Reason: Engineers conceived the symbol, and their schematics show current flowing from the positive (+) side of the voltage source to the negative (-). It's the same convention used for semiconductor symbols that include arrows—the arrow points in the permitted direction of "conventional" flow, and against the permitted direction of electron flow."

ref https://www.fluke.com/en-us/learn/blog/electrical/what-is-a-diode

I am bemused by the assertion that using the direction of electron flow is an aid to circuit diagnosis. In any electronics symbol that includes an arrow (e.g. diode, NPN or PNP bipolar transistor) that arrow always indicates the direction of conventional current flow. What could be easier than following the arrows?
 
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frequent_flyer said:
So if I took the same Cessna diagram and added a red arror in the opposite direction it would be entirely consistent with "conventional" current flow. I just don't see how referencing the Cessna diagram supports your case.

An internet search for "diode symbol current flow direction" will return many references that say the current (conventional current) flows in the direction of the diode symbol's arrow.

Here is a diode description from Fluke (probably generally regarded as a competent test instrument manufacturer) -

"Strange but true: The diode symbol's arrow points against the direction of electron flow. Reason: Engineers conceived the symbol, and their schematics show current flowing from the positive (+) side of the voltage source to the negative (-). It's the same convention used for semiconductor symbols that include arrows—the arrow points in the permitted direction of "conventional" flow, and against the permitted direction of electron flow."

ref https://www.fluke.com/en-us/learn/blog/electrical/what-is-a-diode

I am bemused by the assertion that using the directron of electron is an aid to circuit diagnosis. In any electronics symbol that includes an arrow (e.g. diode, NPN or PNP bipolar transistor) that arrow always indicates the direction of conventional current flow. What could be easier than following the arrows?
Click to expand...

I am just telling you how the FAA sees it and I am trained as an A&P mechanic. The picture of the diode diagram is taken right out of the AC. All I did is underline their statement. I doubt any A&P on here will disagree with it.

i would also like to point out that the first edition of AC65-9 was in 1970, 53 years ago. A lot has changed in state of the art since then.

But you be you.
 
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wireweinie said:
That's a standard NATO DC power plug. When wired correctly, there are large gauge positive and negative cables and a small gauge jumper to the small socket contact. This small contact is connected to the positive cable and is used to operate relays wired for external power connection and reverse polarity protection.

For example, when the external power plug is inserted, if an external power relay is installed, no external power will be applied to the bus until that relay is energized. This can happen automatically if the small contact from the plug is wired to the relay coil. On some aircraft (usually larger types that spend lots of time on the ground electrically powered up for specific purposes) that same circuit is used but with a switch added at a convenient location to control when the external power is applied or removed. An added benefit of using the positive voltage of the small contact to power the external power relay is that it acts as reverse polarity protection. If that relay is looking for a positive voltage to energize it's coil but the external power is connected backwards, then a negative is applied to the relay which will NOT energize it. In that case no damage will be done as reverse polarity voltage will not pass the relay and enter the bus system.

Web
Click to expand...

Yep I have that exact switch in the 767 I fly. :)
 
Geez you guys! Current flows positive to negative, but electrons flow negative to positive. It makes absolutely no difference how you look at it.

Most say current flows plus to minus - think about the holes the elecrons leave flowing opposite to the electrons.
So the FAA and the Navy teach electron flow. The first thing those grizzled old CPOs said in class is that it makes no difference how you look at it. None.
 
bob turner said:
Geez you guys! Current flows positive to negative, but electrons flow negative to positive. It makes absolutely no difference how you look at it.

Most say current flows plus to minus - think about the holes the elecrons leave flowing opposite to the electrons.
So the FAA and the Navy teach electron flow. The first thing those grizzled old CPOs said in class is that it makes no difference how you look at it. None.
Click to expand...

Until you take a written or oral with the FAA or an A&P school. But otherwise I agree it doesn’t really make any difference on how you look at it.
 
I have taken them all. Never had a problem. They don't do trick questions. I admit - I missed one on my IA exam - embarrassing for a Decathlon owner! It was the one on compression cracks.

You know that 20 question test for BasicMed? My buddy is a dentist, and missed only one - the dental question. We are both pretty good at what we do.
 
What’s your point Bob? I agree the FAA doesn’t do trick questions. But distinctly remember getting asked to explain the external power diagram during my O&P. They teach and test you on the DC system as I’ve described. Do you disagree?
 
Nope. Did they ask you which way current flows?

My orals were about 15 minutes long - I had my test scores forwarded to an outfit in OKC that guaranteed an A&P within two weeks. Took three hours. Most of that was driving to the examiner’s house.

I am able to visualize it either way. If I had to, I could discuss electron flow. But nobody has asked me.

Is there such an exam question? There was not anything like that in 1976.
 
Charlie Longley said:
But distinctly remember getting asked to explain the external power diagram during my O&P.
Click to expand...

A diode passes current when it is forward biased and not when it is reverse biased. For the diode to be forward biased the anode must be connected to a positive voltage and the cathode must be connected to a negative voltage. In the symbol used to depict the diode the cathode is the bar and the anode is the base of the triangle.

Current will only flow through the diode and the ground service contactor when external power is connected with the correct polarity. When external power is connected with reverse polarity the diode does not pass current, the ground service contactor is not closed, and no aircraft components can be damaged by the incorrect external power connection.

There you are. A simple explanation that does not require a definition of what is meant by current flow.
 
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The most important thing about electricity I learned long ago…. It moves faster than I do, so don’t touch it…
 
bob turner said:
Nope. Did they ask you which way current flows?

My orals were about 15 minutes long - I had my test scores forwarded to an outfit in OKC that guaranteed an A&P within two weeks. Took three hours. Most of that was driving to the examiner’s house.

I am able to visualize it either way. If I had to, I could discuss electron flow. But nobody has asked me.

Is there such an exam question? There was not anything like that in 1976.
Click to expand...

Yep I went through a 147 school in the mid 90’s. They asked which way the current flow was and which direction the diode stopped it. In reality you can think of the current flow and diode in either direction. I was just taught negative to positive so that’s the easiest way for me to think of it.

I looked through the latest FAA handbooks from 2018 and they just gloss over the whole subject.
 
I have found this discussion very interesting and kind of thought that the little prong on my 180K could be hooked up to the positive to make the big prongs work but wasn't 100% sure. A few years ago an Air Tractor driver was desperate for a three prong plug in because he left the master on. He found one but I was thinking at the time that he could improvise. Here is an interesting Youtube on which way electrons flow.
https://www.youtube.com/watch?v=0vzcWOEKvjI
 
wireweinie said:
And make sure it says 'Fluke' on it!

Oh crap! Here goes ANOTHER argument.

Web
Click to expand...

Actually it was just a tasteless poke at this thread by the placement of the leads.
Screenshot 2023-02-01 at 10.03.09 AM.jpg
What I did not know is that the FAA uses electron current flow as part of the A&P test. That answers some things. I have known about the Navy's NEETS modules as you can't endeavor the learn this stuff without stumbling across it as it is a highly regarded resource. It does seem odd though that both the Navy and the FAA went with electron current flow in their instruction material. I'm not smart enough to understand why they would do that.

Jerry
 

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I just talked with a retired Boeing electrical engineer friend of mine. (Major Mike) He helps me when I have to do a deep dive into avionics. I asked him to describe to me electrical current and how it interacts with a diode. He said, “Well the convention is electrons move negative to positive, current moves positive to negative and flows through the diode in the direction of the arrow.” I asked him why the FAA has it taught the other way? He said, “Because they’re the FAA.”

Welp I think we’ve beaten this dead horse long enough. Thanks for playing. ;)
 
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cubflier said:
Actually it was just a tasteless poke at this thread by the placement of the leads.
View attachment 64546
What I did not know is that the FAA uses electron current flow as part of the A&P test. That answers some things. I have known about the Navy's NEETS modules as you can't endeavor the learn this stuff without stumbling across it as it is a highly regarded resource. It does seem odd though that both the Navy and the FAA went with electron current flow in their instruction material. I'm not smart enough to understand why they would do that.

Jerry
Click to expand...

Come on Jerry! It doesn't matter with the leads. Just make sure the diode tests good one way and blocks current the other. lol

Web
 
Very true.

And I've seen many newbies get in trouble because they didn't pay attention to the + or - sign, on digital meters.

Web
 
Charlie Longley said:
“Because they’re the FAA.”
Click to expand...
That pretty well covers it!

A couple of things that I have gotten out of this:

First, V=I*R actually works for either definition of current flow. I had thought it would end up as V = -I*R, but it doesn't. So, my mistake on that.

Second, if someone had asked me what an anode was, I would have said the "positive end". While that is correct for a diode, it is not for a battery because it isn't the correct definition.
 
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