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EXPBUS Circuit Failure

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hmanvel

Joined: 05 Jul 2016
Posts: 3

Posted: Tue Jul 05, 2016 10:38 am Post subject: EXPBUS Circuit Failure

I found several conversations on this site relating to the Expbus system, and Electronic Circuit Breakers.  I wanted to relate my recent experience.  I have two Expbus systems in my Rutan Defiant.  They have operated flawlessly for 16 years, or about 870 hours on the airplane.  Recently one circuit, with two devices on it, started shutting down intermittently, but with no indication of a short on the indicator board.  I found the problem the other day, by pulling the two devices (terra nav and Gemini backup efis) and checking voltage at each connector.  Both read normal 12+ volts.  I then plugged in the Gemini, checked voltage at the nav connector, now it was 3.4 volts.  I plugged in the nav radio, pulled out the Gemini, and IT was now showing about 3.4 volts.  Apparently the one circuit/breaker was dropping out with any sort of load on it.  Not sure why, but I have fixed it by wiring an inline fuse to the devices, and disconnecting from the output on the EXPBUS.

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_________________
Harry Manvel
Defiant N2HM
PTK / Pontiac, MI

nuckolls.bob(at)aeroelect
Guest

Posted: Wed Jul 06, 2016 7:37 am Post subject: EXPBUS Circuit Failure

 	  Quote: 	
		   Apparently the one circuit/breaker was dropping out with any sort of load on it.  Not sure why, but I have fixed it by wiring an inline fuse to the devices, and disconnecting from the output on the EXPBUS. 	


   I don't know what the failure modes are for the

   polyfuses . . . we looked at them on two separate

   occasions at Beech and just couldn't figure out

   how to integrate them into system wiring . . .

   

   Had the idea progressed any further, we proably

   would have done some endurance/halt testing on

   the devices to see if their weaknesses would

   present . . . but given the service life demonstrated

   on your airplane, I'm not sure we could have 

   'turned that rock over' as a part of preliminary

   design studies.



   Those polyfuses are widely available parts. If

   you choose to identify and replace the one that

   went belly up, I can help you find a replacement.



 

   Bob . . .

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