nuckollsr(at)cox.net
Guest
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 Posted: Sun Jan 28, 2007 10:02 am Post subject: a simple question |
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At 11:55 AM 1/28/2007 -0500, you wrote:
| Quote: |
On 28 Jan 2007, at 11:26, jdalton77(at)comcast.net wrote:
>Listers,
>
>With all of these these complex questions being discussed I often
>feel intimidated to ask simple ones. You see, I'm a neophyte. If
>it's woodworking, I could write the book. If it's software, I'd go
>toe-to-toe with Bob's eletrical wizardry any time any place. But
>I'm new to this. So please bear with me as I continue to ask
>simple questions and don't flame me, tell me I should "look in the
>archives," read the book, or anything else that would sour my
>experience. By the way, I've made similar comments on the RV-10
>board, and on Vans Airforce, and you would be surprise how many
>emails I got from people like me thanking me for it. 'nuff said on
>that.
>
>Now my newbie question (and yes, I have the book and have read
>it). The way I read about contactors, they need power to the
>"small" terminal to move the switch and allow the two large
>terminals to join which in turn allows the "real power" to pass
>through them. But I was confused by the batter contactor (and the
>ground power contactor), which seems to only need a switch with a
>connection to the gound at the firewall to be activated. I'm not
>sure I get how that works. This is not the case for the starter
>contactor, which needs power to activate. I know I could wire it
>up and make it work, I just want to know "why" so I am confident in
>my own work.
With the battery contactor, there is an internal connection that
allows it to use the power from the battery to energize the coil on
the solenoid that pulls the contactor closed. No power flows through
the coil in the solenoid until you provide a ground by closing the
battery master switch. As soon as you have provided a ground, now
the current can flow from the battery, through the coil, which
creates a magnetic field that pulls the solenoid closed. This makes
the connection that allows battery power to now flow to the rest of
the aircraft.
Kevin Horton
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Good put Kevin. I might elaborate on Kevin's explanation
by linking you to this drawing:
http://www.aeroelectric.com/Pictures/Schematics/Contactors_PU_and_PD.pdf
I've traced the power paths for energizing both the battery
contactor (in red) and the starter contactor (in green). I did
not mark the pathways through the grounds common to both systems.
Note that while the starter contactor's coil power runs through a
lot more switches (and the battery contactor) we still satisfy
the need to get 12 volts or so impressed across the contactor's
coil terminals by means of switched power that PULLS UP from
ground. The battery contactor has it's switch in the ground lead
and is said to provide a PULL DOWN to ground to close the coil's
power pathway. The PULL down architecture for battery contactors
eliminates a need to fuse the circuit . . . there are no faults
of wiring that have potential for burning a wire. On the other
hand, the starter contactor supply circuit does need a fuse to
protect wiring between the bus and contactor. So the key expansion
of Mr. Dalton's question is to consider BOTH terminals of the
contactor coil . . . and the need to impress 12v or so across
the coil. I think he was getting confused by not tracing the
full pathway for energizing the coil through both ends of
the coil.
Bob . . .
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