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RFC: My external power schematic

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mouser

Joined: 02 Feb 2010
Posts: 14
Location: Los Alamos, NM

Posted: Mon Feb 22, 2010 5:27 am Post subject: RFC: My external power schematic

I'm about to add in the external power connector to my RV-10 project and I'd like to get some feedback on my schematic before I implement it.   The idea is to have a small door in the baggage bulkhead which gives access to the standard 3-pin connector, a LED which indicates contactor status, and a switch that gives some control over how the contactor behaves.   The switch will have three positions:



FORCE:  The external power contactor is held closed by the main aircraft battery.



AUTO:  The external power contactor is actuated by the signal on the third (small) pin in the connector.



INHIBIT:  The external power contactor will not close, regardless of the signal on the connector. 



The LED next to the switch indicates the contactor status.  

I think in my installation the switch and LED will be mounted adjacent to the connector, as I won't likely need to change the external power contactor behavior while sitting up front.    The only problem with this setup is that when the switch is set to "FORCE," the LED is always on.  So if I walk away and forget about that, it's a slow drain on the main battery.



I've attached the schematic I'm currently considering.   

Comments? Advice?



Thanks,

-Richard

RV-10 #40988

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external_power_schematic.png

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External power system schematic

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nuckolls.bob(at)aeroelect
Guest

Posted: Mon Feb 22, 2010 6:58 am Post subject: RFC: My external power schematic

At 07:27 AM 2/22/2010, you wrote:

 	  Quote: 	
		  



<snip>


	


 	  Quote: 	
		  I've attached the schematic I'm currently considering.


	


     How would you use these features? What are the

     conditions that make them useful? Keep in mind

     too that a crow-bar ov module MUST be paired with

     a 2-5A breaker or breaker switch capable of

     co-existing with this technology. I.e. high interrupt

     current ratings. Commercial breakers are suspect. See:



http://aeroelectric.com/Pictures/Breakers/Breaker_Failure_1a.jpg



    Miniature breakers popular with aviation are quite

    happy working this task:



http://aeroelectric.com/Pictures/Breakers/Klixon_1.jpg



    Let's discuss the ways in which the schematic in

    Z-31A falls short of your design goals.

   Bob . . .

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mouser

Joined: 02 Feb 2010
Posts: 14
Location: Los Alamos, NM

Posted: Mon Feb 22, 2010 8:21 am Post subject: Re: RFC: My external power schematic

 	  nuckolls.bob(at)aeroelect wrote: 	
		  

    Let's discuss the ways in which the schematic in

    Z-31A falls short of your design goals.

	




First, the minor differences:   



I put an LED across the solenoid so that I have some immediate feedback that it is engaged when I think it should be (and not engaged when I think it shouldn't be).    Ran into a problem with this at work one day with a Twin Otter sitting on the ramp plugged into a generator but the small pin wasn't connected properly within the plug and never engaged the solenoid.   I had no feedback that this was the case, the ramp was too noisy to hear the solenoid click, and didn't have direct access to the solenoid to DVM the contacts.   Drained the main batteries (and a fair amount of fuel out of the generator for no reason) without realizing it.  



I also added a second diode reversed across the solenoid, similar to the one you have across the main battery contactor in Z-31A.   Is there something different about the external power contactor that makes this safety not recommended?



I do need to add the circuit breaker as you recommend, that was an oversight.



The major departure in my schematic is the addition of the switch.   The center position gives the same behavior as Z-31A.   The "force" position I put in there as a means of overcoming the problem I saw with the Otter. If I am presented with a power supply whose third pin is not engaging the contactor, I can force it closed manually.    The "inhibit" position I don't have a specific operational need for, but it came at a cost of zero complexity (the DP3T switch is internally more complex than a SPDT, I suppose... but there are no extra wires or parts).  I think the only time it would see use is if I found myself in a position of troubleshooting something and wanted to repeatedly add and remove ground power with minimal physical effort.   



With any luck, I'd just leave the switch on Auto for the lifetime of the airplane.  But weird stuff happens...

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user9253

Joined: 28 Mar 2008
Posts: 1922
Location: Riley TWP Michigan

Posted: Tue Feb 23, 2010 7:31 am Post subject: Re: RFC: My external power schematic

The whole purpose of having a diode and relay is to prevent a reversed polarity connection between the aircraft and external power.  The switch in the forced position defeats that safety feature, putting expensive avionics at risk along with a potential for fire or exploding batteries.  Compare those risks to the inconvenience of the 3d pin not making contact in Z-31A.  It isn't just inexperienced line-boys who connect batteries backwards.  Qualified people who are complacent or preoccupied make mistakes too.

Dwg Z-31B eliminates the 3d pin.  Or how about modifying your circuit to use external power through a diode to power the relay like this:

http://tinyurl.com/ylbabzd



Here is a discussion about plugs and sockets if you have not purchased them yet: http://forums.matronics.com/viewtopic.php?t=68440&sid=19d878ba84382f9b62db3fdf8411436f



Here is a link to plugs and sockets for sale: http://www.ryderfleetproducts.com/cgi-bin/ryderfp/products/srm/oid/51817/pn/Single-Pole-Socket-Female/erm/product_detail.jsp

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_________________
Joe Gores

nuckolls.bob(at)aeroelect
Guest

Posted: Thu Feb 25, 2010 2:19 pm Post subject: RFC: My external power schematic

At 10:21 AM 2/22/2010, you wrote:

 	  Quote: 	
		  

nuckolls.bob(at)aeroelect wrote:

 >

 >     Let's discuss the ways in which the schematic in

 >     Z-31A falls short of your design goals.

 >

First, the minor differences:



I put an LED across the solenoid so that I have some immediate 

feedback that it is engaged when I think it should be (and not 

engaged when I think it shouldn't be).


	


   This is what's suggested in Figure 3 of



http://aeroelectric.com/articles/grndpwr.pdf



   The press to test fixture shows when the contactor

   is energized and also proves availability of ground

   power if the press-to-test works.



 	  Quote: 	
		  I also added a second diode reversed across the solenoid, similar to 

the one you have across the main battery contactor in Z-31A.   Is 

there something different about the external power contactor that 

makes this safety not recommended?


	


    Why would you want to close the ground power contactor

    from a battery that's okay? As another reader pointed

    out, this negates the protection for reversed polarity

    in the ground power source.

 	  Quote: 	
		  The major departure in my schematic is the addition of the 

switch.   The center position gives the same behavior as 

Z-31A.   The "force" position I put in there as a means of 

overcoming the problem I saw with the Otter.


	


    I can't figure out how the schematic cited above

    doesn't address this.



 	  Quote: 	
		  With any luck, I'd just leave the switch on Auto for the lifetime of 

the airplane.  But weird stuff happens... 


	


    Why not replace the switch breaker with a pullable

    breaker in Figure 3 and probably leave that

    breaker OUT for the lifetime of the airplane?



    Bob . . .

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