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Switches and Relays

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hotwheels

Joined: 01 Jun 2007
Posts: 240

Posted: Thu Jan 29, 2009 9:33 am Post subject: Switches and Relays

Is there a general rule of thumb regarding max current load through a switch before it is prudent to add a relay to handle switching higher current loads (similar to S704-1 12v/20A)? Maybe this is spelled out in an AEC doc, but if so I couldn't find it.



A couple of components which might require relay switch protection come to mind including: pitot heat, landing lights, standard nav lights, and maybe fuel pump... each appear to draw 5A or more in steady state. 



The AN5812 heated pitot might be a strong relay usage contender as it has a hefty initial current spike followed by a sizable current draw (see AEC article "Gauging Pitot Heater Performance" 15 Feb 2005).



1N5400 (3A/50v) diodes are commonly used on Z-diagram contactors across the low current coil side to suppress current spikes. Similar best practice would seem to follow for S704-1 type relays too? 



The 1N5400 diode seems like it would be good candidate. Are mil-spec components advised or is the local Radio Shack sufficient?



I'm all for sweating through the load analysis and thinking things out in advance so as to not end up with arc welded switches later.



Thanks,

Jay

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nuckolls.bob(at)aeroelect
Guest

Posted: Thu Jan 29, 2009 1:24 pm Post subject: Switches and Relays

At 11:33 AM 1/29/2009, you wrote:

 	  Quote: 	
		  



Is there a general rule of thumb regarding max current load through 

a switch before it is prudent to add a relay to handle switching 

higher current loads (similar to S704-1 12v/20A)? Maybe this is 

spelled out in an AEC doc, but if so I couldn't find it.


	


   What particular situation drives this consideration?

   Aside from pitot heaters, toe heaters, and klieg lights

   for landing and taxi, you're unlikely to overload the

   plain vanilla switches offered by most suppliers. See:



http://aeroelectric.com/articles/Switch_Ratings.pdf

 	  Quote: 	
		  A couple of components which might require relay switch protection 

come to mind including: pitot heat, landing lights, standard nav 

lights, and maybe fuel pump... each appear to draw 5A or more in steady state.



The AN5812 heated pitot might be a strong relay usage contender as 

it has a hefty initial current spike followed by a sizable current 

draw (see AEC article "Gauging Pitot Heater Performance" 15 Feb 2005).

    Yes, this is an excellent application for a relay . . .
	


    especially a solid state relay.

 	  Quote: 	
		  1N5400 (3A/50v) diodes are commonly used on Z-diagram contactors 

across the low current coil side to suppress current spikes. Similar 

best practice would seem to follow for S704-1 type relays too?


	


   Yeah, except that the lead wires on a 5400 series

   device are pretty hefty and may be difficult to

   install as shown in . . .



http://www.aeroelectric.com/Pictures/Relays/s704inst.jpg



 	  Quote: 	
		  The 1N5400 diode seems like it would be good candidate. Are mil-spec 

components advised or is the local Radio Shack sufficient?


	


   The lowly silicon diode rectifier is the oldest

   semiconductor device in the marketplace. It's so easy

   to make a really good diode that nobody even bothers

   to offer parts screened to military specs. Finally

   the electrical stresses on spike catcher diodes is

   measured in MILLIJOULES . . . a sneeze in a thunderstorm.



   Anything from this list of parts offered by RadioShack

   and many others can be considered to your tasks.



http://www.aeroelectric.com/Pictures/Misc/RS_Diodes.jpg

 	  Quote: 	
		  I'm all for sweating through the load analysis and thinking things 

out in advance so as to not end up with arc welded switches later.


	


   I've never seen an arc welded switch that's been

   "overloaded". Switches fail open after they burn

   for a host of reasons not the least of which is

   mechanical inadequacies at riveted joints.



   What are the current ratings for systems you're

   considering?

        Bob . . .



        ----------------------------------------)

        ( . . .  a long habit of not thinking   )

        ( a thing wrong, gives it a superficial )

        ( appearance of being right . . .       )

        (                                       )

        (                  -Thomas Paine 1776-  )

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

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hotwheels

Joined: 01 Jun 2007
Posts: 240

Posted: Thu Jan 29, 2009 4:02 pm Post subject: Re: Switches and Relays

Bob,



Thanks for the reply. 



The biggest current users on my panel are AN5812 pitot heat (~8A steady state after the initial inrush spike is done), landing lights (~4A each), Whelen strobes (~7A total), NAV lights (~6A total) and boost fuel pump (~5A). 



My take away from reading the AEC Switch article is that there's no need to protect panel switches except those used to operate the high current starter and battery contactors. 



The entire topic really boils down to this paragraph:



"The heaviest currents handled by panel switches are landing/

taxi lights (which have their own special inrush values

-see "lamp" ratings in table above), and pitot heat. For most

14-volt airplanes this is about 8 amps. Everything else drops

rapidly from there. I can also tell you that switching an

8-amp landing light with a 4-amp "lamp" rated switch is not

an automatic formula for welding. The 200% "overload" will

indeed reduce the life of the switch. However, let us suppose

the switch was originally rated for 10,000 cycles (a low

estimate) and the reduction was to 10% of rated life(also

very low) . . . How long will it take you to put 1000 cycles

on your landing light switch? "



Does anyone have the definitions of an "Electrical Code Rating" as referenced in the Microswitch Catalog table? The table would probably make better sense then.



There's a reference to switch cycle life above, but switches aren't typically sold with that information easily obtained up front. Maybe someone has a link to a good website as that info could be useful.



I must be missing something. People must be buying S704-1 relays as B&C sells them and you reference them in several docs... but what are they using them for if there's really no utility in it? 



Jay

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rv-9a-online(at)telus.net
Guest

Posted: Thu Jan 29, 2009 5:49 pm Post subject: Switches and Relays

I've looked at this in some detail.  Since I've had multiple switch failures 

(long Carling switch thread), I have looked at relays for switching heavy 

loads.



In my new A/C (Harmon Rocket), this thinking has led me to a bank of relays 

controlled by grounded switches.  I need a bunch of relays for the trim 

systems and other stick-grip switches anyway, so I went all the way with all 

of my electrical loads switched with relays.  While not strictly necessary, 

it allowed me to place the relays and breakers off-panel where the heavy 

gauge wiring runs were shorter.  This saves many dozens of feet of heavy 

wiring running back and forth to the panel.



Most of the switches on the panel are grounded to activate the corresponding 

relay.  This allows me to use a single 22 AWG wiring for each relay control. 

I can also use lighter weight toggle switches on the panel, which also 

happen to be cheaper.  The lighter weight switches and shorter heavy gauge 

wiring runs should roughly balance out the relay weights.



Another benefit of this approach is that rotary switches can be used instead 

of toggles.  For example, I plan one rotary switch for my main lights: 

OFF-WIGWAG-PULSE-LDG-TAXI-BOTH.  This takes one position on my panel and is 

configured to drive my two lamp relays as necessary (left and right wingtip 

lamps).  Also, I plan on connectorizing the panel... it's a lot simpler with 

fewer light gauge wires.



So an architecture based on relays has other benfits other than protecting 

switches.  These benefits must be weighed against the complexity and 

potential failure modes of relays compared to switches.  Sometimes, you just 

want to experiment!



Vern Little

---

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fvalarm(at)rapidnet.net
Guest

Posted: Thu Jan 29, 2009 8:49 pm Post subject: Switches and Relays

Bob,



Speaking of switch failures,



I think you had mentioned that you would test the practicality and function

of adding JB weld to switch rivets to see if it would prevent them from

loosening without a downside.  Any progress there?



Bevan

RV7A wiring

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Peter Laurence

Joined: 10 Jan 2006
Posts: 50

Posted: Fri Jan 30, 2009 5:51 am Post subject: Switches and Relays

Vern,



About 4 years and 500 hundred hours ago, I wired a velocity for a friend as

per Bob's Z drawings. I too was hesitant about running large wires up the

door post to the overhead switch gang.



I made a box with post terminals in the box  and a gang of 13 relays on

top. This was mounted on the forward bulkhead close to the battery. 



Have not had to replace a relay yet. However he does keep a couple of spares

in the plane.

Peter Laurence

RV9A N60PL



I've looked at this in some detail.  Since I've had multiple switch failures



(long Carling switch thread), I have looked at relays for switching heavy 

loads.



In my new A/C (Harmon Rocket), this thinking has led me to a bank of relays 

controlled by grounded switches.  I need a bunch of relays for the trim 

systems and other stick-grip switches anyway, so I went all the way with all



of my electrical loads switched with relays.  While not strictly necessary, 

it allowed me to place the relays and breakers off-panel where the heavy 

gauge wiring runs were shorter.  This saves many dozens of feet of heavy 

wiring running back and forth to the panel.



Most of the switches on the panel are grounded to activate the corresponding



relay.  This allows me to use a single 22 AWG wiring for each relay control.



I can also use lighter weight toggle switches on the panel, which also 

happen to be cheaper.  The lighter weight switches and shorter heavy gauge 

wiring runs should roughly balance out the relay weights.



Another benefit of this approach is that rotary switches can be used instead



of toggles.  For example, I plan one rotary switch for my main lights: 

OFF-WIGWAG-PULSE-LDG-TAXI-BOTH.  This takes one position on my panel and is 

configured to drive my two lamp relays as necessary (left and right wingtip 

lamps).  Also, I plan on connectorizing the panel... it's a lot simpler with



fewer light gauge wires.



So an architecture based on relays has other benfits other than protecting 

switches.  These benefits must be weighed against the complexity and 

potential failure modes of relays compared to switches.  Sometimes, you just



want to experiment!



Vern Little

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nuckolls.bob(at)aeroelect
Guest

Posted: Fri Jan 30, 2009 4:29 pm Post subject: Switches and Relays

At 10:13 PM 1/29/2009, you wrote:

  	  Quote: 	
		  --> AeroElectric-List message posted by: "B Tomm" <fvalarm(at)rapidnet.net>



 Bob,



 Speaking of switch failures,



 I think you had mentioned that you would test the practicality and function

 of adding JB weld to switch rivets to see if it would prevent them from

 loosening without a downside.  Any progress there? 	


    Some.



    I have looked at every riveted Carling switch in

    the shop (about 25 pcs) and found none with observably

    loose joints.



    I did do some tests to determine that the contact

    material (which includes the formed rivet head)

    is solderable - as are the brass fast-on tabs. 

    The conditions are favorable soldering the rivets

    to the tabs and then covering the joint with a dab

    of JB Weld.



    This still begs the question as to return on investment

    for this effort.



    We've identified no changes to process or design

    that would account for a rash of failures in these

    switches. The design has been essentially unchanged

    for decades and what must be many millions of devices.

    I'll suggest it's pretty clear that the failures

    were not a function of failure to observe ratings.



    It's clear that the strongest potential for failure

    happens under the formed head of the rivet. If it's

    too loose, corrosion can begin and failure is

    a sure thing given sufficient time.



    The soldering process is not a quick-n-easy thing.

    I was able to get good flow to the two materials

    but it was a bit fussy. Couldn't be sure that I

    wasn't degrading the joint quality at the base

    of the saddle inside the switch while I was making

    them "better" on the outside.



    If you've got switches that are not yet installed,

    it wouldn't hurt to beef them up with some JB Weld.

    Use the slow setting stuff so you've got plenty of

    time to work carefully. You can also mix up a larger

    quantity of the slow stuff and do a batch of switches.

    



 [img]cid:.0[/img]

 

     Take care to avoid contaminating the tabs.



     I'm not confident that soldering the contact to the tabs

     is an "improvement". The epoxy beef-up can't hurt anything

     and it may be helpful.



 

        Bob . . .



        ----------------------------------------)

        ( . . .  a long habit of not thinking   )

        ( a thing wrong, gives it a superficial )

        ( appearance of being right . . .       )

        (                                       )

        (                  -Thomas Paine 1776-  )

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

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rv-9a-online(at)telus.net
Guest

Posted: Fri Jan 30, 2009 7:18 pm Post subject: Switches and Relays

Thanks for the reference Peter.



I too plan to have a spare relay or too handy (or I could swap out a 

non-critical one such as Pitot Heat or Nav Lights) for emergencies.

I have relays with interlocking sockets that connect together.  This should 

make for a stable mounting system.



Vern

---

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