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jskiba(at)icosa.net
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 Posted: Sat Feb 04, 2017 10:43 am Post subject: Why is this diode there ? |
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Why is this diode in the Alternator B line ?
Is it old thinking from way back in the day and old tech maybe to protect the older alternators ?
[img]cid:image001.png(at)01D27EE2.E05BB1A0[/img]
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Eric M. Jones
Joined: 10 Jan 2006
Posts: 565
Location: Massachusetts
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| Posted: Sat Feb 04, 2017 11:37 am Post subject: Re: Why is this diode there ? |
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Yes, it looks strange. It probably prevents the ammeter from slamming backwards when the external ground power is applied?
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113 Brentwood Drive
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Bob McC
Joined: 09 Jan 2006
Posts: 258
Location: Toronto, ON
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| Posted: Sat Feb 04, 2017 12:32 pm Post subject: Why is this diode there ? |
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Wouldn't the diodes within the alternator serve the same purpose? You can't "back feed" the alternator (and thus the ammeter) as its internal diodes would prevent current in that direction. That "strange" diode would appear to serve no purpose.
Bob McC [quote]
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Bob McC
Falco #908
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jskiba(at)icosa.net
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| Posted: Sat Feb 04, 2017 2:45 pm Post subject: Why is this diode there ? |
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Is it possibly used to limit the output of the alternator???? I see no Breaker in the diagram which is weird as standard practice today would be to use a circuit breaker to protect the alternator output wire. Typical 60-80 amps depending………….
From: owner-aeroelectric-list-server(at)matronics.com [mailto:owner-aeroelectric-list-server(at)matronics.com] On Behalf Of Robert McCallum
Sent: Saturday, February 4, 2017 2:30 PM
To: aeroelectric-list <aeroelectric-list(at)matronics.com>; Eric M. Jones <emjones(at)charter.net>
Subject: Re: AeroElectric-List: Re: Why is this diode there ?
Wouldn't the diodes within the alternator serve the same purpose? You can't "back feed" the alternator (and thus the ammeter) as its internal diodes would prevent current in that direction. That "strange" diode would appear to serve no purpose.
Bob McC [quote]
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MMiller
Joined: 07 Jul 2016
Posts: 8
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| Posted: Sun Feb 05, 2017 5:10 am Post subject: Re: Why is this diode there ? |
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The diode is minor, why do you want the starter contactor to self hold?
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nuckolls.bob(at)aeroelect
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| Posted: Sun Feb 05, 2017 6:39 am Post subject: Why is this diode there ? |
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At 12:33 PM 2/4/2017, you wrote:
| Quote: | Why is this diode in the Alternator B line ?
Is it old thinking from way back in the day and old tech maybe to protect the older alternators ?
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What is the source of this drawing? This diode MAY
have been offered as an alternative to b-lead protection
for failure/crash faults on the line to the alternator.
but it's in error as to how the starter contactor is
wired. There's also a curious diode in the ground power
contactor wiring.
This drawing has the 'smell' of having been copied from
some more credible source as a 'simplified' wiring diagram
not unlike those in the Cessna Pilot Handbooks of yesteryear.
A guy named C.E. Fraley used to sit right behind me in Cessna's
Tech Pubs department. He used to do all of the POH for the Pawnee
Plant. On several occasions I was asked to vet figures that he
distilled from the production wiring documents.
[img]cid:.0[/img]
The simplification process needs to be conducted carefully
lest errors creep in. I suspect that the drawing you cite
may have fallen victim to errors in transcription.
Bob . . .
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jskiba(at)icosa.net
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| Posted: Wed Feb 08, 2017 6:23 pm Post subject: Why is this diode there ? |
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Okay the original diagram was from a maintenance manual, here’s one from the POH
I think the big diode in the B line is used to limit / protect the alternator line (aka no circuit breaker) but now what the heck is the power relay circuit at the bottom ???
And the diode is still in the ground power, Maybe to protect from reverse polarity ??
[img]cid:image001.png(at)01D28248.6C6BB560[/img]
From: owner-aeroelectric-list-server(at)matronics.com [mailto:owner-aeroelectric-list-server(at)matronics.com] On Behalf Of Robert L. Nuckolls, III
Sent: Sunday, February 5, 2017 8:36 AM
To: aeroelectric-list(at)matronics.com
Subject: Re: Why is this diode there ?
At 12:33 PM 2/4/2017, you wrote:
| Quote: |
Why is this diode in the Alternator B line ?
Is it old thinking from way back in the day and old tech maybe to protect the older alternators ?
|
What is the source of this drawing? This diode MAY
have been offered as an alternative to b-lead protection
for failure/crash faults on the line to the alternator.
but it's in error as to how the starter contactor is
wired. There's also a curious diode in the ground power
contactor wiring.
This drawing has the 'smell' of having been copied from
some more credible source as a 'simplified' wiring diagram
not unlike those in the Cessna Pilot Handbooks of yesteryear.
A guy named C.E. Fraley used to sit right behind me in Cessna's
Tech Pubs department. He used to do all of the POH for the Pawnee
Plant. On several occasions I was asked to vet figures that he
distilled from the production wiring documents.
[img]cid:image002.jpg(at)01D28248.6C6BB560[/img]
The simplification process needs to be conducted carefully
lest errors creep in. I suspect that the drawing you cite
may have fallen victim to errors in transcription.
Bob . . .
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nuckolls.bob(at)aeroelect
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| Posted: Fri Feb 10, 2017 4:18 pm Post subject: Why is this diode there ? |
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At 08:20 PM 2/8/2017, you wrote:
| Quote: | Okay the original diagram was from a maintenance manual, here’s one from the POH
I think the big diode in the B line is used to limit / protect the alternator line (aka no circuit breaker) but now what the heck is the power relay circuit at the bottom ???
And the diode is still in the ground power, Maybe to protect from reverse polarity ?? |
Not that one . . . the diode and resistor in series
around the open battery contactor is indeed part
of the ground power kit. It is intended to supply
a small 'charge path' for a dead battery after
the bus comes up under ground power.
If the battery is too weak to close the contactor,
this sneak-path will bring the battery's open
terminal voltage up enough to close the contactor
so that the battery can be charged by ground
power.
The first POH drawing depicted the single-terminal
ground power connector popular on Piper models
(and others). The drawing I posted was typical
of Cessna POH. It depicts the legacy military
style ground power connector.
[img]cid:.0[/img]
Note that this drawing also features a 'sneak
path' diode around the battery contactor. Another
diode is shown in series with the ground power
contactor coil. This is repeated in the
drawing on my website:
http://tinyurl.com/gtqp6tk
Note that this drawing does not include the sneak-path
diode. This is because I recommend applying ground
power directly to the battery allowing it to be
charged without powering up the whole aircraft.
Bob . . .
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