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latching relays

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fklein(at)orcasonline.com
Guest

Posted: Thu Mar 10, 2011 12:38 pm Post subject: latching relays

Hi Bob,



I'm following your Figure Z-19 schematic as a guide to the electrics  

to service an MPEFIed Sube EA81 derivative put out by Ron Carr at RAM  

Performance...it is powering a Europa XS, well along in its  

construction.



With the FWF complete (well, almost complete), a lot of the wiring  

done for ancillary systems (fuel pumps, lighting, antennas, etc.) I'm  

taking a hard look at the power side of things and have just ordered  

35 feet of Super-4 fatwire, and am planning on dual Odyssey batteries.



And I have a couple of the generic contactors which Acft. Spruce sells  

as Master Relays (p/n 111-226), purchased w/ the intention of using  

one at each battery.



A pal...a savvy guy for MARINE electrical systems...has suggested that  

I use "latching relays" which, he sez, will add to flight safety. I've  

not seen any references to them in your discussion of battery  

contactors in your book, though you do include some cautionary words  

about proper installation and potential contactor failure.



What do you have to say about "latching relays" functioning as battery  

contactors?



Thanks in advance for any wisdom you can offer...I confess to be one  

of those airplane builders w/ scant knowledge of the electrical side  

of things,



Fred

markcs

Joined: 14 Feb 2011
Posts: 3
Location: England

Posted: Fri Mar 11, 2011 5:03 am Post subject: Re: latching relays

Fred,



I know you asked Bob for his opinion on self latching relays, but thought I would through my two penneth in.



Self latching relays operate in a number of different ways including the use of permanent magnets and cam mechanisms to achieve the latching.  I suppose that there are two main perceived advantages of these devices for an aircraft:



1.  Once latched they do not require power to hold them open (but with your Sube you have a fairly meaty alternator so that should not be an issue for you, especially if you install an essential bus that does not require a made contactor to provide power to your ignition system).

 

2.    Once latched they are closed unless they are positively unlatched and so any failure in the wiring etc will not cause them to unlatch, but see comment above ref the use of an essential bus.



Contactors are very simple devices and have proved extremely reliable in practice.  One of the principle benefits of a simple battery contactor is its function as your last port of call isolation device for your main electrical systems.  If you have a fault either with a stuck starter on the the ground or perhaps an electrical fault in the air that causes smoke in the cockpit your ultimate get out of jail card is to switch off the battery contactor, and in flight with an electrically dependent engine operate the essential bus depending on your electrical architecture.  If the wiring fails to a latching relay at the same time as your fault you cannot release it but if the wiring fails to a battery contactor it will release under its own steam.



If I am sailing a boat, the issue is somewhat different.  With a major fault and unable to release the latching relay I can normally gain access to the battery and release the positive terminal or if that isn't possible most boats I have sailed have battery isolation switches for all battery banks which would isolate the fault - both scenarios are not possible in an aircraft.  So your fault isolation logic would seem to me to need to be different

Just food for thought.



Mark

user9253

Joined: 28 Mar 2008
Posts: 1921
Location: Riley TWP Michigan

Posted: Fri Mar 11, 2011 5:04 am Post subject: latching relays should fail safe

A designer needs to ask, "if this part fails, how will I deal with it?"  If a latching relay fails to open when commanded to, how will the electrical power be shut off?  In case of a fire or imminent forced landing, the power needs to be shut off immediately.

What is safer, a master contactor that fails open or one that fails closed?  The engine will keep running with a failed contactor.  And the essential bus will keep powering critical equipment.  Your marine friend might not realize that an airplane can keep flying with the master contactor off.  Simpler is better.

Joe

_________________
Joe Gores

nuckolls.bob(at)aeroelect
Guest

Posted: Fri Mar 11, 2011 6:43 am Post subject: latching relays

 	  Quote: 	
		  

What do you have to say about "latching relays" functioning as battery

contactors?


	


   A couple of List members have accurately captured the thought

   processes unique to aircraft systems design. The

   well considered failure modes effects analysis speaks

   to Plan-B . . .  mitigating the effects of any

   particular failure. It also seeks to identify whether or

   not the failure is passive (fail safe) or active (oh (at)#$(at)#!

   how do I turn this thing off!).



   I've considered the latching contactor often since

   it was introduced to us on the List a few weeks back.

   I've not yet discovered the right words and reasoning

   one might apply to say, a design review at Hawker-Beechcraft

   suggesting incorporation of the device onto one of their

   airplanes.

   Bob . . .

fklein(at)orcasonline.com
Guest

Posted: Fri Mar 11, 2011 9:45 am Post subject: latching relays

Mark, Joe, Bob,



Your wisdom is very much appreciated...thank you...I have much to learn.



Though I've been active on the matronics Europa-list, I've just joined  

here and look forward to a time when I might have some wisdom to share  

as well...til then, I fear I'll just be askin questions.



Fred



do not archive

nuckolls.bob(at)aeroelect
Guest

Posted: Fri Mar 11, 2011 3:34 pm Post subject: latching relays

At 12:39 PM 3/11/2011, you wrote:

 	  Quote: 	
		  



Mark, Joe, Bob,



Your wisdom is very much appreciated...thank you...I have much to learn.



Though I've been active on the matronics Europa-list, I've just joined

here and look forward to a time when I might have some wisdom to share

as well...til then, I fear I'll just be askin questions.


	


    Good . . . 'cause for sure, we can't build your

    airplane for you. Here is where you'll find lots

    of recipes for success and some help on sorting through

    the options. What ever you decide to do will perform

    as advertised. Questions are the place to start . . .



   Bob . . .

jan_de_jong(at)casema.nl
Guest

Posted: Sun Mar 13, 2011 2:03 am Post subject: latching relays

May I offer a thought in support of latching relays.



How likely is it that the fault that causes smoke in the cockpit 

coincides with the failure of the relay to open?

Wouldn't they be generally independent failures?



I do agree that if you have plenty of amps it is hard to find a 

compelling reason to use them.

On the other hand - Tyco must see a market for automotive power saving.



For the minimalist system with PM alternator I think the notion of using 

latching relays can be played with.



Jan de Jong

user9253

Joined: 28 Mar 2008
Posts: 1921
Location: Riley TWP Michigan

Posted: Sun Mar 13, 2011 6:25 am Post subject: Master contactor reduced voltage holding

Another option for using a conventional master contactor is to energize it at full voltage, then hold it at reduced voltage.  I believe that Bob was working on a microprocessor based device that would do that.  One could also make their own reduced voltage holding circuit using a resistor or diodes to drop the voltage along with a 3-position master switch: off, low voltage, and normal voltage.

Or how about this IC along with a Power MOSFET?

http://ww1.microchip.com/downloads/en/DeviceDoc/21448c.pdf

I was going to use this IC to control the elevator trim servo.  I think it would also work with the master contactor.  It applies full voltage for a timed interval, then puts out a PWM voltage, for a price of $1.38.

Joe

_________________
Joe Gores

nuckolls.bob(at)aeroelect
Guest

Posted: Sun Mar 13, 2011 7:56 am Post subject: latching relays

At 05:59 AM 3/13/2011, you wrote:




May I offer a thought in support of latching relays.



How likely is it that the fault that causes smoke in the cockpit 

coincides with the failure of the relay to open?

Wouldn't they be generally independent failures?



    Very low risk.



I do agree that if you have plenty of amps it is hard to find a 

compelling reason to use them.

On the other hand - Tyco must see a market for automotive power saving.



    I suspect that 'power saving' is way down

    on the list of design goals. Increased longevity

    due to low values of internally generated heat

    might be a big driver. The mechanically latched

    device is not going to aggravate the effects of

    a transient event. It wouldn't drop out and re-close

    during a severe brown-out . . . and the folks who

    brought these to market may not have considered

    either . . .



For the minimalist system with PM alternator I think the notion of 

using latching relays can be played with.



    Absolutely. I should remind readers that my

    background calls for sifting the sands of

    risk for the purposes of keeping folks-who-know-

    more-about-airplanes-than-we-do happy. I'm

    not implying that regulatory mandates do not

    mesh well with elegant solutions. But they DO

    tend to discourage new solutions from emerging

    and evolving.



    Let's do our own sand-sifting.

   Bob . . .

JLuckey(at)pacbell.net
Guest

Posted: Mon Mar 14, 2011 1:59 pm Post subject: latching relays

Circuits like these do have their places but I don't think controlling the

coil of a master solenoid is one of them.



This sort of smells like a solution looking for a problem.  Keep in mind

that when you place more components in the critical path, the reliability of

the system goes down - no matter how reliable those components are.



In the case of master solenoids, (as Bob & others point out) they have

proven to be pretty darn reliable. Therefore so I would be reluctant to

"over-engineer" a solution to a problem that doesn't exist.



--

user9253

Joined: 28 Mar 2008
Posts: 1921
Location: Riley TWP Michigan

Posted: Wed Mar 16, 2011 6:07 am Post subject: a non-critical component

 	  Quote: 	
		  Circuits like these do have their places but I don't think controlling the coil of a master solenoid is one of them.

This sort of smells like a solution looking for a problem. Keep in mind that when you place more components in the critical path, the reliability of the system goes down - no matter how reliable those components are.

In the case of master solenoids, (as Bob & others point out) they have proven to be pretty darn reliable. Therefore so I would be reluctant to "over-engineer" a solution to a problem that doesn't exist. 	


I agree with what you are saying except for the "critical path".  There should always be a backup plan like an alternate feed path.  A backup plan will turn a critical component into a non-critical one.  I am not recommending reduced current to the master contactor for most installations.  But it will be of benefit to systems that have limited generating power, or to prolong battery life when the alternator fails.  A contactor coil operating at reduced voltage will generate less heat and thus be even more unlikely to fail.  

Joe

_________________
Joe Gores

JLuckey(at)pacbell.net
Guest

Posted: Wed Mar 16, 2011 7:45 am Post subject: latching relays

WARNING - STANDBY FOR LEVEL 4 NIT-PICKING. IF YOUR'RE NOT REALLY INTO THIS,

DO NOT WASTE YOUR TIME

I'm gonna provide MY definitions for some of the terms we are throwing

around (just 'cause it's a slow morning around here)



Let's talk about Critical Path (CP 'cause I'm a lazy typist) and Redundant

Circuitry (RC).



Critical Path:

The chain of components required to make a circuit operate as designed.



Example:

Circuit breaker -> switch -> contactor coil and the wiring that connects

them. All those things are in the CP because if any one fails, the circuit

stops working. [Let's add an LED in parallel w/ the coil (to give the

operator a visual indication) - I would say that the LED is not in the CP]



Every circuit has a CP. (regardless of other circuits that may be added for

redundancy)



If every circuit has a CP and adding components to the CP reduces

reliability, we can still increase reliability by adding a Redundant

Circuit.  The RC still has its own CP, but the reliability of the SYSTEM

goes up.



----------



I agree that reduced coil current may add to coil longevity but these

devices have proven to be so reliable that I don't think it's worth the

effort.

-Jeff

--

nuckolls.bob(at)aeroelect
Guest

Posted: Wed Mar 16, 2011 7:00 pm Post subject: latching relays

 	  Quote: 	
		  If every circuit has a CP and adding components to the CP reduces

reliability, we can still increase reliability by adding a Redundant

Circuit.  The RC still has its own CP, but the reliability of the SYSTEM

goes up.


	


    Generally speaking, the adjective "critical" is reserved

    for conditions that have a potential for (1) single

    points of failure in (2) systems needed for comfortable

    termination of flight where (3) no effort has been

    made craft a failure tolerant system. When failure

    tolerance has been achieved, no single failure is

    "critical". In little airplanes maintained by the

    competent observer/owner/operator, probability

    of two potentially critical failures in any single

    flight cycle (typically 4 hours max) is vanishingly

    small.



    Once a degree of failure tolerance is achieved, the

    system integrator can begin to massage cost/performance

    and parts reduction issues.



    The successful program starts with design goals.

    Goals that add/upgrade performance features (A-N

    versus VOR versus TACAN versus GPS). To reduce cost

    (less expensive parts with perhaps shorter service

    lives are attractive). No doubt there are other

    goals.



    When I was heavily involved in new products design

    and certification at Electro-Mech, one could look

    forward to a pretty steady stream of hopeful sales

    folks with a new electro-whizzy to show off. Some

    were really amazing advances of the state of the

    art . . . and would be considered for a new design.

    But the $time$ required to modify and old design

    made it unlikely that anything new gets a field

    retrofit. In 40+ years, I've only worked three

    programs that produced a fleet wide change-out

    of hardware and two of those were under safety of

    flight duress.



    Even when I could tell the bright-eyed salesman

    that he had a really nifty product, I had to

    remind him that we'd beat him up on price, delivery,

    cert documentation, and maybe . . . just maybe

    we'd bless him with an order for 1000 parts spread

    out over 10 years. Of course, that order would

    get perpetually modified/canceled as our line rates

    moved up and down with the stock market.



    Aviation is one of the most demanding customers

    yet the numbers of airplanes built every year

    makes them a ho-hum target for the electro-

    whizzy guys. Now, if you're selling rivets

    or aluminum . . .

    Okay, you've got this nifty latching contactor. I think

    it's fairly obvious how it might be used in our

    airplanes. The questions to be asked and answered

    are (1) does this product 'fix' some nuisance of

    maintenance, costs or sense of less-than-elegant

    functionality?



    Since (repeat after me) we DESIGN, CRAFT AND MAINTAIN

    FAILURE TOLERANT SYSTEMS, reliability and criticality

    are not part of the considerations. Only cost of

    ownership and reasonably seamless integration

    into the-best-we-know-how-to-do-today are the go/no-go

    drivers.

   Bob . . .

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