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Eeeeek! Another fear and question.

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jgswartout(at)earthlink.n
Guest

Posted: Fri Sep 22, 2006 1:44 pm Post subject: Eeeeek! Another fear and question.

Bob, your 9/13/06 post regarding LEDs says in part:  

   

Finally, relays are rated amongst the least reliable of  

   components. It behooves us to limit their use where ever practical.  

   

Okay.  That brings up a question.  I’m planning Z13/8 with Z32.  Z32 Heavy Duty E-Bus Feed shows the use of a relay to energize the alternate feed path.  But Z13/8 also relies on a relay to supply the SD-8’s output to the E-Bus.  That’s two relays that must work for E-Bus operations independent of battery power.  Is there a practical way to avoid the built-in weakness of two relays in the E-Bus circuit?  

   

John   

       [quote][b]

mprather(at)spro.net
Guest

Posted: Fri Sep 22, 2006 3:32 pm Post subject: Eeeeek! Another fear and question.

A couple of thoughts...



 - Relays in some applications are less reliable than some solid state

equivalents in that application.  Saying hard/fast that relays are less

reliable is probably not completely accurate.



 - When relays fail, often it's by degraded performance..  You'll notice

that once in a while a relay will fail to snap in, or disconnect.  You

exercise the controlling switch a time or two, and it starts to function.

 You make the mental note "Hmmm, that xyz relay is getting flakey; better

check it out when I get back to the hangar."  And you go on with your

flight.



 - The chances of a having a particular, properly installed/applied relay

failing on any flight might be 1/10,000.  The chances of having two

different similar relays fail on the same flight might be 1/10,000 *

1/10,000.  A very unlikely possibility.

Regards,



Matt-



 	  Quote: 	
		   Bob, your 9/13/06 post regarding LEDs says in part:



 Finally, relays are rated amongst the least reliable of



    components. It behooves us to limit their use where ever practical.



 Okay.  That brings up a question.  I'm planning Z13/8 with Z32.  Z32 Heavy

 Duty E-Bus Feed shows the use of a relay to energize the alternate feed

 path.  But Z13/8 also relies on a relay to supply the SD-8's output to the

 E-Bus.  That's two relays that must work for E-Bus operations independent

 of

 battery power.  Is there a practical way to avoid the built-in weakness of

 two relays in the E-Bus circuit?



 John

jgswartout(at)earthlink.n
Guest

Posted: Fri Sep 22, 2006 7:18 pm Post subject: Eeeeek! Another fear and question.

I *assume* that while relays may be the "least reliable" of components, they

are still "pretty" reliable, else they would not appear in the Z-drawings.

But my point is that when you combine Z13/8 and Z32, the chances of the

E-bus not working in a main-alternator-failure situation are not your

hypothetical 1/10,000 * 1/10,000, rather they are increased to 1/5,000

because BOTH relays have to work, and if either one fails (1/10,000 * 2) you

are flying on the battery only, even though the SD-8 is fine.  Usually Bob

endeavors to minimize the opportunities for a single component failure to

cause an emergency, and I wonder if he considers this daisy chain of "least

reliable components" in the Z13/8 + Z32 to pass muster.



John 



--

N6030X(at)DaveMorris.com
Guest

Posted: Fri Sep 22, 2006 7:35 pm Post subject: Eeeeek! Another fear and question.

I think that's why it's called the Endurance Bus.  So what if both 

relays fail.  If you've designed the system correctly, you have 

sufficient battery to get to your destination, and you don't even 

break a sweat.



Dave Morris



At 10:15 PM 9/22/2006, you wrote:

[quote]

<jgswartout(at)earthlink.net>



I *assume* that while relays may be the "least reliable" of components, they

are still "pretty" reliable, else they would not appear in the Z-drawings.

But my point is that when you combine Z13/8 and Z32, the chances of the

E-bus not working in a main-alternator-failure situation are not your

hypothetical 1/10,000 * 1/10,000, rather they are increased to 1/5,000

because BOTH relays have to work, and if either one fails (1/10,000 * 2) you

are flying on the battery only, even though the SD-8 is fine.  Usually Bob

endeavors to minimize the opportunities for a single component failure to

cause an emergency, and I wonder if he considers this daisy chain of "least

reliable components" in the Z13/8 + Z32 to pass muster.



John



--

jgswartout(at)earthlink.n
Guest

Posted: Sat Sep 23, 2006 6:17 am Post subject: Eeeeek! Another fear and question.

Well, I'm trying to understand the system I'm going to install, and some things aren't making sense to me.  

   

Following your reasoning, since I spent big bucks on a highly reliable L40 main alternator which doesn't rely on any relay to deliver power, and on two P-mags which may not require any ship's power anyway, the highly reliable SD-8 backup alternator is just excess baggage so what difference does it make whether its electrons can get to where they are needed?  That doesn't sound like correct design theory to me.  

   

I am trying to integrate Z-13/8, Z-32, and an avionics master switch circuit of Bob’s design, and am trying to figure out where and whether to use relays which are shown in Z-32 but not in the other drawings. I note that in Z-32, the up-to-8-amp Endurance Bus feeds alternate power through a relay.  (Bob has a sketch at www.aeroelectric.com/Pictures/Schematics/E-BusFatFeed.gif which says “for [E-Bus] feeds over 5A consider adding a relay to control power at the Bat Bus.”)  I seem to recall that the reason for using a relay is to avoid arcing at the switch which would possibly compromise the switch.  

   

But in Bob’s article “Avionics Master Switches:  Really Necessary?,” he shows a circuit with the caption “If you really want an avionics master switch, how about doing it this way?”  It shows an essential bus/avionics bus powering a transponder, turn coordinator, intercom, instrument flood, gps receiver, Nav/Comm, and fuel boost pump.  Both the main feed and the alternate feed are controlled by switches, not relays.  I like the idea and want to use it—but my endurance/avionics bus supports more than 5 amps, so I am “considering” the Z-32 Heavy Duty E-Bus, which uses a relay.  This made me think, if the alternate feed needs a relay, shouldn’t the main feed (avionics master switched) use a relay also?  If not, why not?  It’s doing the same thing normally that the alternate feed does if the main alternator fails—so why doesn’t it need a relay, too?  But Bob’s recent post saying that relays are among the least reliable of components makes me nervous because in the combined schematic I am contemplating my Endurance Bus would be dependent on one relay in normal mode and two relays if I were operating in main-alternator-failed mode.  Remember that the “Endurance Bus” used to be named the “Essential Bus” (for a legitimate reason).  Should anything “Essential” really be totally dependent on relays?  If not, I am looking for assistance in developing a better architecture that gives me a Heavy Duty (up to 8-amp) Essential/Avionics Bus with an avionics master switch.  

   

John     

   

--

N6030X(at)DaveMorris.com
Guest

Posted: Sat Sep 23, 2006 6:50 am Post subject: Eeeeek! Another fear and question.

Well, I'll wait for Bob to justify his own 

schematics, but I know he is in Kansas at the 

tandem-wing fly-in today and may not be able to comment for a while.



If it were ME, I would not use a relay to power 

the E-bus.  I would use a switch.  Switches 

handle larger current than 5A all the time, and 

if you're concerned about reliability, just use a 

switch rated for larger current.  The other thing 

you should do here, since you're discovering you 

have an unusual e-bus, is to ask yourself whether 

all of those items really need to be on the 

e-bus.  I had to go through several iterations 

before I finally pared my e-bus down to what was 

really going to be necessary to have running ALL 

THE TIME to complete the flight.  Remember you 

can always power up the Main Bus for a few 

minutes if you need to, while on battery power.



Dave Morris



At 09:14 AM 9/23/2006, you wrote:

[quote]Well, I'm trying to understand the system I'm 

going to install, and some things aren't making sense to me.



Following your reasoning, since I spent big 

bucks on a highly reliable L40 main alternator 

which doesn't rely on any relay to deliver 

power, and on two P-mags which may not require 

any ship's power anyway, the highly reliable 

SD-8 backup alternator is just excess baggage so 

what difference does it make whether its 

electrons can get to where they are 

needed?  That doesn't sound like correct design theory to me.



I am trying to integrate Z-13/8, Z-32, and an 

avionics master switch circuit of Bob’s design, 

and am trying to figure out where and whether to 

use relays which are shown in Z-32 but not in 

the other drawings. I note that in Z-32, the 

up-to-8-amp Endurance Bus feeds alternate power 

through a relay.  (Bob has a sketch at 

<http://www.aeroelectric.com/Pictures/Schematics/E-BusFatFeed.gif>www.aeroelectric.com/Pictures/Schematics/E-BusFatFeed.gif 

which says “for [E-Bus] feeds over 5A consider 

adding a relay to control power at the Bat 

Bus.”)  I seem to recall that the reason for 

using a relay is to avoid arcing at the switch 

which would possibly compromise the switch.



But in Bob’s article “Avionics Master 

Switches:  Really Necessary?,” he shows a 

circuit with the caption “If you really want an 

avionics master switch, how about doing it this 

way?”  It shows an essential bus/avionics bus 

powering a transponder, turn coordinator, 

intercom, instrument flood, gps receiver, 

Nav/Comm, and fuel boost pump.  Both the main 

feed and the alternate feed are controlled by 

switches, not relays.  I like the idea and want 

to use itbut my endurance/avionics bus supports 

more than 5 amps, so I am “considering” the Z-32 

Heavy Duty E-Bus, which uses a relay.  This made 

me think, if the alternate feed needs a relay, 

shouldn’t the main feed (avionics master 

switched) use a relay also?  If not, why 

not?  It’s doing the same thing normally that 

the alternate feed does if the main alternator 

failsso why doesn’t it need a relay, too?  But 

Bob’s recent post saying that relays are among 

the least reliable of components makes me 

nervous because in the combined schematic I am 

contemplating my Endurance Bus would be 

dependent on one relay in normal mode and two 

relays if I were operating in 

main-alternator-failed mode.  Remember that the 

“Endurance Bus” used to be named the “Essential 

Bus” (for a legitimate reason).  Should anything 

“Essential” really be totally dependent on 

relays?  If not, I am looking for assistance in 

developing a better architecture that gives me a 

Heavy Duty (up to 8-amp) Essential/Avionics Bus with an avionics master switch.



John



--

nuckollsr(at)cox.net
Guest

Posted: Sat Sep 23, 2006 7:08 am Post subject: Eeeeek! Another fear and question.

At 05:43 PM 9/22/2006 -0400, you wrote:



 	  Quote: 	
		  Bob, your 9/13/06 post regarding LEDs says in part:



Finally, relays are rated amongst the least reliable of



    components. It behooves us to limit their use where ever practical.



Okay.  That brings up a question.  I m planning Z13/8 with Z32.  Z32 Heavy 

Duty E-Bus Feed shows the use of a relay to energize the alternate feed 

path.  But Z13/8 also relies on a relay to supply the SD-8 s output to the 

E-Bus.  That s two relays that must work for E-Bus operations independent 

of battery power.  Is there a practical way to avoid the built-in weakness 

of two relays in the E-Bus circuit?


	


    This thread has expanded into a predictable level of

    excitement for lack of perspective . . .



    Note the key word in the quotation above . . . PRACTICAL



    About 20 years ago, I was designing the first of a series

    of electronically controlled rate controllers for

    pitch trim on the Learjets. While conducting a Mil-HBK-217

   MTBF study, it was interesting to note that when only

    the electronic components, wires, solder joints and connectors

    were factored into the study, MTBF for the device was in

    the thousands of hours . . . about 7,000 as I recall.



    When adding the last component, a mil-spec, 4-pole, double

    throw, hermetically sealed 25A relay to the study, the predicted

    MTBF fell to 900 hours for the system.



    Mil-HBK-217 suggests and even accommodates de-rating of

    parts to decrease stresses with commensurate increases in

    MTBF. I was able to design the system so that the relay

    was closed and bounce free before current flowed in the

    contacts. Further, I was able to shut off all current flow

    electronically before the relay contacts were opened.



    Mil-HDK-217 had no prediction formula for my proposed scenario

    but obviously, it was going to be as high as was PRACTICALLY

    possible for incorporation of the relay. Twenty years later,

    inquiries to the overhaul shop for this product tell me that

    relays have NEVER been replaced in a unit returned for repair.



    Even as I write these words, I'm designing the next new

    product to appear in the AeroElectric Connection catalog

    and the design incorporates a RELAY. Why not replace it

    electronically? PRACTICALITY - it's technologically possible

    but costs more . . . makes the product more expensive to

    build and doesn't make it perform any better. It MIGHT

    keep the system from needing maintenance attention before

    the original owner sells the airplane . . . but I doubt

    it.



    The decision as to whether a relay or contactor is

    PRACTICAL involves consideration of much more data than

    the gross reliability factors. Cost of ownership and

    FMEA for the SYSTEM figure heavily into the thought

    process. We still use $25, el-cheeso contactors in our

    airplanes because in spite of their need for periodic

    maintenance, we have crafted a Plan-B to deal with those

    failures. Hence my recommendation to many builders over

    the years that 'upgrading' to Mil-Spec contactors in

    their RV is dollars probably not well spent.



    So before anyone goes of the end of the plank in

    excitement or fear, know that suggestions for the

    selection of ANY component need to be considered against

    your ability to deal sweat-free with a failure of that

    component. If your Plan-B is solid, then use of that

    component is PRACTICAL and as free of excitement and

    concerns as we know how to do.



    Bob . . .

nuckollsr(at)cox.net
Guest

Posted: Sat Sep 23, 2006 7:51 am Post subject: Eeeeek! Another fear and question.

At 09:47 AM 9/23/2006 -0500, you wrote:



 	  Quote: 	
		  



Well, I'll wait for Bob to justify his own schematics, but I know he is in 

Kansas at the tandem-wing fly-in today and may not be able to comment for 

a while.



If it were ME, I would not use a relay to power the E-bus.  I would use a 

switch.  Switches handle larger current than 5A all the time, and if 

you're concerned about reliability, just use a switch rated for larger 

current.  The other thing you should do here, since you're discovering you 

have an unusual e-bus, is to ask yourself whether all of those items 

really need to be on the e-bus.  I had to go through several iterations 

before I finally pared my e-bus down to what was really going to be 

necessary to have running ALL THE TIME to complete the flight.  Remember 

you can always power up the Main Bus for a few minutes if you need to, 

while on battery power.



Dave Morris


	


    The suggestion for adding an e-bus relay had nothing to do

    with current carrying capacity of the e-bus switch. There's

    be a long standing tradition in TC aircraft to limit the size

    of always-hot feeders in aircraft to those protected by 5A or

    smaller breakers.



    Since the "FAT" e-bus feeder needs to carry more than the few

    amps originally considered when the e-bus was conceived, the

    idea for adding a 'mini-contactor' at the e-bus for larger

    feeders was offered.



    Now, fuses are much faster than breakers, and for Z-13/8

    were a continuous e-bus load of 8A is PRACTICAL, then

    assuming the e-bus alternate feed is fuse-fed from the

    battery bus, going up to 10A fuse is a useful thing to

    consider without thumbing our noses too vigorously at tradition.



    Too many folks are treating the Z-figures as carved-in-

    stone recommendations for details of a proposed electrical

    system. These drawings are STARTING POINTS that illustrate

    architectures that will in-turn drive your Plan-A/Plan-B

    thinking.



    Don't get yourselves wrapped around any axles here based

    on sizes of wires, sizes of fuses/breakers or which

    devices are fed from which busses.  The drawings were

    never intended to be a recipe for success, only a tool

    for development. I get a half-dozen requests a week for

    "Gee Bob, I like Figure Z-x but it doesn't . . . and

    I want to  . . . Would you please craft me a Z-y that

    takes care of all my desires/worries?"



    All these requests get the same reply which is not

    much different that what I've written above. Nobody

    promised anyone a decision free solution. One is

    always on firm footing to clone the system out of

    a C-172 and drive on. Any variations from that theme

    WILL require thought, exploration, crafting/answering

    of questions and finally understanding of the final

    configuration which WILL be uniquely applicable to

    YOU and your airplane.



    Bob . . .

jgswartout(at)earthlink.n
Guest

Posted: Sat Sep 23, 2006 7:48 pm Post subject: Eeeeek! Another fear and question.

Okay -- I think I get it.

First, I forgot the chief reason to use a relay in the first place -- to

keep fat wires short and if possible, out of the cockpit.

Second, I lost sight of the over-arching fact that just HAVING a back-up

alternator on board is a huge leap in reliability, not to mention STILL

having the last ditch option of keeping the lights on with the battery for a

couple of hours.  < (smiley face with dunce cap)



I think the simple answer to my specific question is YES, use a relay in the

H.D. E-Bus normal feed line if that's the only way to keep the run from the

Main Bus to the switch and from the switch to the E-Bus under 6 inches or

so.



It is useful to know, however, that the relays might have a service life

short enough to consider replacing them at, say 500-hour intervals.  Mine

will be living in a pretty benign environment, two of them just fwd of the

panel, and two just aft of the firewall.



In any conglomeration of components, it's a truism that one of them will be

the weakest link.  Upgrade or eliminate it, and something else will

automatically become the weakest link.  Although 900 hours MTBF seems pretty

short -- any ONE relay failing during a given flight will not produce a

life-threatening emergency in my plane.  And I can carry a spare, as Bob

suggests in AEC.



Thanks Bob.



John 



--

khurst(at)taroom.qld.gov.
Guest

Posted: Sun Sep 24, 2006 6:48 pm Post subject: Eeeeek! Another fear and question.

Bob,



You said :

 	  Quote: 	
		   I was able to design the system so that the relay

 was closed and bounce free before current flowed in the

 contacts. Further, I was able to shut off all current flow

 electronically before the relay contacts were opened.


	


You have me with my thinking cap on now !  If the current didn't flow

before the contacts were closed and was shut off before the contacts

opened, what was the purpose in having a relay ?  It seems you had

complete control over the current flow without the relay !!  Just

wondering ??



Regards

Kingsley in Oz.

nuckollsr(at)cox.net
Guest

Posted: Mon Sep 25, 2006 4:47 am Post subject: Eeeeek! Another fear and question.

At 12:48 PM 9/25/2006 +1000, you wrote:

 	  Quote: 	

<khurst(at)taroom.qld.gov.au>

Bob,

You said :

 > I was able to design the system so that the relay

 > was closed and bounce free before current flowed in the

 > contacts. Further, I was able to shut off all current flow

 > electronically before the relay contacts were opened.

You have me with my thinking cap on now !  If the current didn't flow

before the contacts were closed and was shut off before the contacts

opened, what was the purpose in having a relay ?  It seems you had

complete control over the current flow without the relay !!  Just

wondering ??

   I was wondering if someone would pick up on that. You

   get the prize.

   When you hook motors to things that control flight

   surfaces, you need to craft a fail passive architecture.

   In this case, the electronics were a necessary component

   of accurate control of pitch trim rate. There's

   a fat transistor in series with the motor that takes

   intelligence from other electronics to modulate supply

   current as needed to maintain the required speed.

   Obviously, the same transistor can turn the motor OFF

   as well. I.e., all solid state control. However, several

   failure modes would cause the fat transistor to stay ON

   continuously causing a pitch trim runaway. We needed

   a SECONDARY means for disconnecting pitch trim power

   and the lowly relay seemed a good candidate due to its

   very low on-resistance.

   So, the relay was added but programmed via electronics

   to close first and open last thereby offering VERY

   long life contacts. However, monitor circuitry that

   watches for a "stuck" fat transistor would also open

   the relay, break power to the motor and illuminate a warning

   light.

   Last time I talked to the guys in overhaul, they'd seen

   only one failure in the total fleet of Lears that might

   have caused a pitch trim runaway wherein the monitor was

   called upon to do its job. Interestingly enough, there's

   a fair number of items returned for repair . . . virtually

   all needed refurbishment of connection technologies. This

   device is located high in the vertical fin of the airplane

   . . . second worst environment in the airplane (hell hole

   is #1). Products of corrosion and temperature cycles

   have proven to be the biggest test of our design. The

   stuff everyone worries about the most (electronics) have

   proven quite robust and long lived.

   Bob . . .

frank.hinde(at)hp.com
Guest

Posted: Mon Sep 25, 2006 9:14 am Post subject: Eeeeek! Another fear and question.

Replace alt feed relay with a switch.

  

 The SD8 relay is one of those things that is a second order  failure...at least it is assuming you are using the SD8 as a backup to your main  alternator....In other words the chances of the relay failing as well as the  main alternator failing is extremly small.

  

 Of course it means you need to test your SD8 operation  regularly.....And you really DON't want to turn off your main alt with the  engine running.

  

 Frank



   From:  owner-aeroelectric-list-server(at)matronics.com  [mailto:owner-aeroelectric-list-server(at)matronics.com] On Behalf Of John  Swartout

Sent: Friday, September 22, 2006 2:43 PM

To:  aeroelectric-list(at)matronics.com

Subject: Eeeeek!  Another fear and question.

 

  

Bob, your 9/13/06 post regarding LEDs says in  part: 

  

Finally, relays are rated  amongst the least reliable of 

   components. It  behooves us to limit their use where ever practical. 

  

Okay.  That brings up a  question.  I’m planning Z13/8 with Z32.  Z32 Heavy Duty E-Bus Feed shows the use of  a relay to energize the alternate feed path.  But Z13/8 also relies on a  relay to supply the SD-8’s output to the E-Bus.  That’s two relays that  must work for E-Bus operations independent of battery power.  Is there a  practical way to avoid the built-in weakness of two relays in the E-Bus  circuit? 

  

John 

[quote]



matronics.com/Navigator?AeroElectric-List



[b]

wgill10(at)comcast.net
Guest

Posted: Mon Sep 25, 2006 9:42 am Post subject: Eeeeek! Another fear and question.

What harm is there in turning off the alternator (open the field) with the engine running?

  

 Bill

  

 [quote]-------------- Original message -------------- 

From: "Hinde, Frank George (Corvallis)" <frank.hinde(at)hp.com> 

  (at)font-face { 	font-family: Georgia; } (at)page Section1 {size: 8.5in 11.0in; margin: 1.0in 1.0in 1.0in 1.0in; } P.MsoNormal { 	FONT-SIZE: 12pt; MARGIN: 0in 0in 0pt; FONT-FAMILY: "Times New Roman" } LI.MsoNormal { 	FONT-SIZE: 12pt; MARGIN: 0in 0in 0pt; FONT-FAMILY: "Times New Roman" } DIV.MsoNormal { 	FONT-SIZE: 12pt; MARGIN: 0in 0in 0pt; FONT-FAMILY: "Times New Roman" } A:link { 	COLOR: blue; TEXT-DECORATION: underline } SPAN.MsoHyperlink { 	COLOR: blue; TEXT-DECORATION: underline } A:visited { 	COLOR: purple; TEXT-DECORATION: underline } SPAN.MsoHyperlinkFollowed { 	COLOR: purple; TEXT-DECORATION: underline } SPAN.EmailStyle17 { 	FONT-WEIGHT: normal; COLOR: windowtext; FONT-STYLE: normal; FONT-FAMILY: Georgia } DIV.Section1 { 	page: Section1 }   Replace alt feed relay with a switch.

  

 The SD8 relay is one of those things that is a second order failure...at least it is assuming you are using the SD8 as a backup to your main alternator....In other words the chances of the relay failing as well as the main alternator failing is extremly small.

  

 Of course it means you need to test your SD8 operation regularly.....And you really DON't want to turn off your main alt with the engine running.

  

 Frank



   From: owner-aeroelectric-list-server(at)matronics.com [mailto:owner-aeroelectric-list-server(at)matronics.com] On Behalf Of John Swartout

Sent: Friday, September 22, 2006 2:43 PM

To: aeroelectric-list(at)matronics.com

Subject: Eeeeek! Another fear and question.

  

  

Bob, your 9/13/06 post regarding LEDs says in part: 

  

Finally, relays are rated amongst the least reliable of 

   components. It behooves us to limit their use where ever practical. 

  

Okay.  That brings up a question.  I’m planning Z13/8 with Z32.  Z32 Heavy Duty E-Bus Feed shows the use of a relay to energize the alternate feed path.  But Z13/8 also relies on a relay to supply the SD-8’s output to the E-Bus.  That’s two relays that must work for E-Bus operations independent of battery power.  Is there a practical way to avoid the built-in weakness of two relays in the E-Bus circuit? 

  

John 

 	  Quote: 	
		  



matronics.com/Navigator?AeroElectric-List





	
[b]

nuckollsr(at)cox.net
Guest

Posted: Mon Sep 25, 2006 10:23 am Post subject: Eeeeek! Another fear and question.

At 05:39 PM 9/25/2006 +0000, you wrote:



 	  Quote: 	
		  What harm is there in turning off the alternator (open the field) with the 

engine running?



Bill


	


   IF . . . IF you're opening the field . . . no harm whatsoever.

   On all TC aircraft, one may turn the alternator ON and OFF at

   will under any conditions without risk to hardware.



   Aircraft adaptations of internally regulated alternators pose

   some risk of damage to the alternator's built in regulator due

   to load dump if the b-lead is disconnected while the alternator

   is under load.



   Now, if the battery is fully charged -AND- all electro-whizzies

   are OFF, the alternator is not significantly loaded and breaking

   the b-lead a la Z-24 is low risk. So it's not so much a matter of

   whether or not the alternator is being spun but a matter of

   loads on the alternator at b-lead disconnect time.



   Bob . . .

nuckollsr(at)cox.net
Guest

Posted: Mon Sep 25, 2006 10:27 am Post subject: Eeeeek! Another fear and question.

At 12:13 PM 9/25/2006 -0500, you wrote:



 	  Quote: 	
		  Replace alt feed relay with a switch.


	


    The relay was added to facilitate OV protection . . .



 	  Quote: 	
		  

The SD8 relay is one of those things that is a second order failure...at 

least it is assuming you are using the SD8 as a backup to your main 

alternator....In other words the chances of the relay failing as well as 

the main alternator failing is extremly small.


	


    Right. Probability of dual failures on any give tank

    of fuel is exceedingly small.

 	  Quote: 	
		  

Of course it means you need to test your SD8 operation regularly.....And 

you really DON't want to turn off your main alt with the engine running.


	


    Yup, preflight testing of all on-board energy sources is

    a good idea. Most aircraft adaptations of automotive

    alternators (internally regulated) cannot be turned off

    once they've been turned on. This makes it difficult to

    test an SD-8 except to make it the FIRST alternator you

    turn on at preflight, then OFF, then turn the main alternator

    ON.



    If you're main alternator is an externally regulated device,

    then it may be turned ON and OFF at will under any conditions

    for any purpose.



    Bob . . .

khurst(at)taroom.qld.gov.
Guest

Posted: Mon Sep 25, 2006 11:17 pm Post subject: Eeeeek! Another fear and question.

 	  Quote: 	
		   > I was able to design the system so that the relay

 > was closed and bounce free before current flowed in the

 > contacts. Further, I was able to shut off all current flow

 > electronically before the relay contacts were opened.



You have me with my thinking cap on now !  If the current didn't flow

before the contacts were closed and was shut off before the contacts

opened, what was the purpose in having a relay ?  It seems you had

complete control over the current flow without the relay !!  Just

wondering ??


	


Bob,



Thank you for the explanation, I would never have worked it out myself.

Much obliged



Kingsley



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