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switch suitability?

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CJohnston(at)popsound.com
Guest

Posted: Sat Sep 15, 2007 7:39 am Post subject: switch suitability?

hey all - 



i've spent the morning perusing the archives about switch electrical ratings, and have read the PDF by Bob on switch ratings, but i'm feeling like i'm not getting any hard and fast rules about this stuff.  in most of the posts about electrical ratings, someone invariably says "don't loose any sleep over switch ratings".  i'm losing sleep over my switch ratings.    i'm considering using some super neato (and also very high quality) switches, but the ratings are:



30 volts DC or 125 volts AC, 2.0A res., 0.5A ind.



that's the only info given about them. are these switches appropriate (safe) for use for all the things you'd find in a "technically advanced" aircraft?  i'm talking things like pitot heat, trim motors, flap motors, exterior lighting etc?  



also, what about in the place of tying electrical busses together?  i'm considering having an alternate battery feed to the E-buss, and i suspect that with a fairly heavily loaded E-buss such as the one i'm considering, these switches would be inadequate?  for reference, the E-buss on my ship has the EFIS, AHRS, one NAV, one COM, autopilot, trim, and flaps.  i'd hate to put a relay in this spot (up the parts count, etc) that is so critical.



i'm not (yet) asking a public critique of my electrical design, which i'm comfortable with, just worrying about the switches i'm thinking of going with.  any help would be greatly appreciated!



thanks

cj

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nuckolls.bob(at)cox.net
Guest

Posted: Sat Sep 15, 2007 9:17 am Post subject: switch suitability?

At 08:37 AM 9/15/2007 -0700, you wrote:



 	  Quote: 	
		  hey all -



i've spent the morning perusing the archives about switch electrical 

ratings, and have read the PDF by Bob on switch ratings, but i'm feeling 

like i'm not getting any hard and fast rules about this stuff.  in most of 

the posts about electrical ratings, someone invariably says "don't loose 

any sleep over switch ratings".  i'm losing sleep over my switch 

ratings.    i'm considering using some super neato (and also very high 

quality) switches, but the ratings are:



30 volts DC or 125 volts AC, 2.0A res., 0.5A ind.


	


   These are generally too small for anything practical

   in the DC power distribution system. These will require

   relays to boost the capability of the higher current draw

   applications.



   Switches best suited to the task will be rated at 7A or

   better. They will also be easy to acquire from more than

   one source.

 	  Quote: 	
		  that's the only info given about them. are these switches appropriate 

(safe) for use for all the things you'd find in a "technically advanced" 

aircraft?  i'm talking things like pitot heat, trim motors, flap motors, 

exterior lighting etc?



also, what about in the place of tying electrical busses together?  i'm 

considering having an alternate battery feed to the E-buss, and i suspect 

that with a fairly heavily loaded E-buss such as the one i'm considering, 

these switches would be inadequate?  for reference, the E-buss on my ship 

has the EFIS, AHRS, one NAV, one COM, autopilot, trim, and flaps.  i'd 

hate to put a relay in this spot (up the parts count, etc) that is so critical.


	


   Suggest you begin with one of the architectures published

   and then define what failure mode is not being addressed by

   the diagram as published. Read the chapter on System Reliability.

   You have too many and the wrong things on the e-bus.

        Bob . . .



        ----------------------------------------

        ( "Physics is like sex: sure, it may   )

        ( give some practical results, but     )

        ( that's not why we do it."            )

        (                                      )

        (                 Richard P. Feynman   )

        ----------------------------------------

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klehman(at)albedo.net
Guest

Posted: Sat Sep 15, 2007 10:07 am Post subject: switch suitability?

Just in case it helps Chris -the ratings you quote are  2 amps resistive 

load (steady current with no turn on surges) or 0.5 amp inductive 

(higher momentary turn on surges of current) so these are small switches.

Other issues are connections to switches and how many diferent types you 

wish to use.  1/4" wide tab connections for push on AMP PIDG connectors 

are very convenient.  Solder connections will drive you crazy.  Screw 

terminals are somewhere in between  

Ken

Chris Johnston wrote:



 	  Quote: 	
		  hey all - 



i've spent the morning perusing the archives about switch electrical ratings, and have read the PDF by Bob on switch ratings, but i'm feeling like i'm not getting any hard and fast rules about this stuff.  in most of the posts about electrical ratings, someone invariably says "don't loose any sleep over switch ratings".  i'm losing sleep over my switch ratings.    i'm considering using some super neato (and also very high quality) switches, but the ratings are:



30 volts DC or 125 volts AC, 2.0A res., 0.5A ind.



that's the only info given about them. are these switches appropriate (safe) for use for all the things you'd find in a "technically advanced" aircraft?  i'm talking things like pitot heat, trim motors, flap motors, exterior lighting etc? 



also, what about in the place of tying electrical busses together?  i'm considering having an alternate battery feed to the E-buss, and i suspect that with a fairly heavily loaded E-buss such as the one i'm considering, these switches would be inadequate?  for reference, the E-buss on my ship has the EFIS, AHRS, one NAV, one COM, autopilot, trim, and flaps.  i'd hate to put a relay in this spot (up the parts count, etc) that is so critical.



i'm not (yet) asking a public critique of my electrical design, which i'm comfortable with, just worrying about the switches i'm thinking of going with.  any help would be greatly appreciated!



thanks

cj

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CJohnston(at)popsound.com
Guest

Posted: Sat Sep 15, 2007 10:48 am Post subject: switch suitability?

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CJohnston(at)popsound.com
Guest

Posted: Sat Sep 15, 2007 12:24 pm Post subject: switch suitability?

Thanks for the reply Bob, and i really appreciate the insight from someone who knows more about this stuff than me.  i've been studying the connection for awhile now, and with my proposed architecture i don't mean to pretend i'm more knowledgable than i am.  i am learning new stuff daily, so be patient!  your reply that i have too many and the wrong things is curious to me.  the aircraft has no "conventional" instruments, so the EFIS and AHRS are essential.  In my application, the AHARS includes a GPS receiver, (GADAHARS) so i've got the GPS without the draw of a big panel mounted Garmin style unit.  the NAV/COM (SL30) isn't too spendy in terms of power (as long as i don't feel the need to transmit in a long winded fashion!) and gets me quite a bit in terms of navigational ability.  The autopilot is possibly excessive, but as you say in the chapter on electrical system reliability, it's got an off switch, and if i don't need it, it can be turned off.  The flaps and trim could be considered excessive, but my my thinking, they're only as "electrically spendy" as you want them to be (use sparingly), and in a fast slippery aircraft, my personal opinion is that these items are essential.  with my proposed 28Ah main battery and the 17Ah aux battery, the E-bus should keep the electrons flowing for quite awhile right? 



The reason i'm trying to sort out an alternate feed to the E-bus from a second battery is that from what i understand, there is an issue with the EFIS screens rebooting while you're cranking to start the engine.  as i type this, i'm thinking that it's possible that this is a mythical problem that i just took for granted to be real.  i'll have to investigate that one...  but the before start procedure would be to turn on the EFIS, then do the pre-flight, and by the time i'm ready to hop in and start the engine, the EFIS is booted up and ready to go.  also, this way i have the engine gauges during engine start.  the last piece of the puzzle has to do with starter kickback caused by the lightspeed ignition if the voltage it's getting drops too low during engine cranking.  this was a motivator for having a second battery in the aircraft, and an alternate feed to the ignition.  the ignition, by the way, would be on the always hot aux battery, or the always hot main battery (switchable) for the reasons that you state in the connection.  



just to be clear, i'm not one of these guys who has tons of flight experience.  i'm a new pilot with an instrument rating, and for my mission, it is important to me that i have every advantage if i have an emergency in the cockpit.  in the event of a charging malfunction, i'd be landing as soon as it was reasonably safe to do so, even if i had the electrical endurance to continue much farther.  i'm not talking about shouting "MAYDAY!" and setting it down in a field, but i would choose to land at the first airport that i saw.  i'm kinda funny that way.



thanks for all your help and insight...  please set me straight if necessary!



cj

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