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Misprint in Z-16, v.12/ OV with Rotax generators?

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klehman(at)albedo.net
Guest

Posted: Mon Sep 24, 2012 11:52 am Post subject: Misprint in Z-16, v.12/ OV with Rotax generators?

I assumed that Sacha was referring to my example that I think 

effectively had no battery in the circuit Bob. My little AGM battery was 

completely flat. Afterwards it seemed that I could run the voltage up 

and the battery would not absorb any current until it sat on the charger 

for quite some time.



thanks for the review though. I for one really appreciate your real 

world info and stories.



Ken



On 24/09/2012 2:57 PM, Robert L. Nuckolls, III wrote:

 	  Quote: 	
		   

 <nuckolls.bob(at)aeroelectric.com>



 At 12:28 PM 9/24/2012, you wrote:

 



 Hi Sacha



 I might be a bit optimistic with that comment. My thoughts are that when

 my OVM activates it immediately shorts out the relay but it is still

 connected to the buss and is clamping the buss voltage until the circuit

 breaker pops.



 Actually, it's a healthy battery that "clamps

 the bus". During OVM reaction time the alternator

 goes into max v current limited output with the

 ship's accessories taking part of the load

 and the battery working to soak up the rest.



 Once the OVM trips, the current draw on the bus

 WILL go up to 50+ amps . . . so I suppose there's

 some additional clamping going on. A 5A breaker

 with typical OVM loads opens in 15 to 50 milliseconds

 so it's all over in a blink of an eye.



 So you are correct that with a slow opening relay and a fast acting CB

 there could be a few milliseconds delay during which time the voltage

 will be rising again.



 Relay, cb and ovm timing 'races' don't figure into

 system functionality at all. Your electro-whizzies

 manufactured to DO-160 design goals will stand off

 20 volts for 1 second. The battery will keep bus

 volts down to 17 volts or so. This give the OVM and

 associated components all the time in the world to

 do their intended job.



 Probably shouldn't mention it since I can't claim they are significant

 but I also have a few TVS diodes on the buss. Since they are fast acting

 and cheap they seemed like a good idea (several years ago) with an

 electrically dependent engine.



 They don't hurt a thing . . . don't do anything

 useful either. The TVS is designed to mitigate

 low energy, fast rise, high voltage transients

 that do not exist on your ship's DC bus . . . and

 again, devices built to DO-160 intent are designed

 to stand off the classic 'transient spike test'.



 Years ago I spent a fist full of the boss's money for

 my man Cecil to build a test fixture required for

 GENERATION of the DO-160 test spikes. Cool gadget.



 When we got it finished and calibrated, I began to

 explore vulnerability for my favorite bus power

 conditioning circuits.



 I was astounded to find that when the device under

 test had a simple capacitor of 1uF or more across

 the incoming line . . . Cecil's Evil Spike Thrower

 couldn't put a spear past it.



  From that time onward, the fixture was never again

 pulled down from the shelf . . . 'cause all our products

 already featured the necessary capacitor. Had I

 endeavored to analyze the fact mathematically

 (or simply brass-boarded the circuit) I could have

 used Cecil's time more productively.



 Short answer is that the much dreaded and poorly

 understood 'spikes' are already being run to ground

 by MULTIPLE input capacitors that already exist

 in your system. I have chased the elusive 'spike'

 in airplanes from C-150 to Beechjets and have

 never been able to capture one in the wild. The

 ones I generated on the bench turned out to be

 wimps.

 Bob . . .

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uuccio(at)gmail.com
Guest

Posted: Mon Sep 24, 2012 3:26 pm Post subject: Misprint in Z-16, v.12/ OV with Rotax generators?

 	  Quote: 	
		   I for one really appreciate your real world info and stories.


	


Me too, thanks again to both of you. 



Sacha

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