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battery contactor vs starter contactor wiring

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kenryan

Joined: 20 Oct 2009
Posts: 426

Posted: Sat Oct 08, 2016 10:05 am Post subject: battery contactor vs starter contactor wiring

Reviewing 13/8 I notice that the battery contactor jumps power from the big terminal and then switches the coil ground, but the starter contactor switches coil positive. Why do it differently on one than the other?



Ken

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kenryan

Joined: 20 Oct 2009
Posts: 426

Posted: Tue Oct 11, 2016 10:17 am Post subject: battery contactor vs starter contactor wiring

I haven't seen any responses to this. Did I ask a dumb question?



<snip>

Reviewing 13/8 I notice that the battery contactor jumps power from the big terminal and then switches the coil ground, but the starter contactor switches coil positive. Why do it differently on one than the other?



<snip>

On Sat, Oct 8, 2016 at 9:44 AM, Ken Ryan <keninalaska(at)gmail.com (keninalaska(at)gmail.com)> wrote:

 	  Quote: 	
		  Reviewing 13/8 I notice that the battery contactor jumps power from the big terminal and then switches the coil ground, but the starter contactor switches coil positive. Why do it differently on one than the other?



Ken

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kjashton(at)vnet.net
Guest

Posted: Tue Oct 11, 2016 12:32 pm Post subject: battery contactor vs starter contactor wiring

The difference is the fuse.  In both of them, grounding the coil circuits actuates the solenoids but there is no fuse on the battery contactor circuit.  If you visualize the starter contactor wired the same way as the battery contactor, you would have to bring the output side of the actuating coil back to the cockpit to the starter switch and thence through a single dedicated fuse.  Wiring it the way it is allows you to use a fuse or CB in the fuse panel.  



Just a guess.

-Kent

 	  Quote: 	
		   On Oct 11, 2016, at 2:15 PM, Ken Ryan <keninalaska(at)gmail.com> wrote:

 

 I haven't seen any responses to this. Did I ask a dumb question?

 

 <snip>

 Reviewing 13/8 I notice that the battery contactor jumps power from the big terminal and then switches the coil ground, but the starter contactor switches coil positive. Why do it differently on one than the other?

 <snip>

 

 On Sat, Oct 8, 2016 at 9:44 AM, Ken Ryan <keninalaska(at)gmail.com> wrote:

 Reviewing 13/8 I notice that the battery contactor jumps power from the big terminal and then switches the coil ground, but the starter contactor switches coil positive. Why do it differently on one than the other?

 

 Ken

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millner(at)me.com
Guest

Posted: Tue Oct 11, 2016 1:38 pm Post subject: battery contactor vs starter contactor wiring

 	  Quote: 	
		  >   In both of them, grounding the coil circuits actuates the solenoids but there is no fuse on the battery contactor circuit.  If you visualize the starter contactor wired the same way as the battery contactor, you would have to bring the output side of the actuating coil back to the cockpit to the starter switch and thence through a single dedicated fuse.


	


Not so!



The battery contractor coil is hard tied to the battery on one end. The other end of the coil is grounded through the master switch. No fuse.



The starter contractor coil is tied to ground on one end... The other end gets power via the mag/starter switch, typically via some breaker. On my Cessna it's the electric fuel pump breaker.



Paul

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