[SI-LIST] Re: Latching relays in optical switches

  • From: Don Nelson <dhwn@xxxxxxx>
  • To: SI List <si-list@xxxxxxxxxxxxx>
  • Date: Sat, 30 May 2009 19:59:03 -0400

Hi Chris, and everyone else who was kind enough to offer their  
suggestions related to our latching relay issue.  We did find the  
problem and I'm almost--no, DEFINITELY--embarrassed to say what it  
turned out to be.
We discovered after much agony that our design was fundamentally  
sound.  The relay appeared to be operating as it should, but as it got  
hotter, or as the coil voltage was reduced, it took longer for the  
armature to swing from one position to the other (observed by watching  
the voltage across the un-engergized coil change as the armature  
magnet moved) until it wouldn't budge at all.  However, when wiring  
the relay in reverse (coils energized with reverse polarity) it worked  
just fine over a wide range of temperatures and voltages.  Uh oh...

The CAD designer (or whoever created the footprint) evidently made a  
mistake when he created the PCB shape and reversed the coil pin  
polarities, so that although the schematic was correct, the board was  
reversed.  Ugh.  Clearly, I feel like an idiot for not realizing this  
sooner!!  Even more insidious was the fact that the relay MOSTLY  
worked, and it was only a few that failed at high temperatures.  By  
the time I was brought in to look at this problem, the team was  
totally flummoxed, was screening parts, and the idea of a layout  
error--which typically results in a fatal error--was off everyone's  
radar.  Let this be a lesson to me NEVER to take anything for  
granted!  :-)

Thanks again to everyone who generously gave their time by giving us  
great suggestions.  This is a great community of engineers and I am  
now prepared to accept my deserved ridicule and derision!  :-)

Kind regards,
Don Nelson
Netronome Systems

>
>> From: Christopher.Jakubiec@xxxxxxxxxxxx
>> Date: May 29, 2009 9:37:21 AM EDT
>> To: dhwn@xxxxxxx
>> Subject: RE: [SI-LIST] Latching relays in optical switches
>>
>> Don,
>>
>> We are interested in your conclusion on this topic.
>>
>> Thanks,
>>
>> Chris
>>
>>
>> -----Original Message-----
>> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx 
>> ] On Behalf Of Don Nelson
>> Sent: Tuesday, May 19, 2009 2:46 PM
>> To: si-list@xxxxxxxxxxxxx
>> Subject: [SI-LIST] Latching relays in optical switches
>>
>> Hello,
>>
>>  I know that this question strays from SI, but it involves EM, and  
>> that's something that we in SI seem to know pretty well.  Well, I  
>> *thought* I knew it pretty well, but this has me stumped.  I was  
>> brought in to help solve a problem with an existing product.
>>
>>  We are using an optical switch that utilizes a dual-coil latching  
>> relay.  The relay will not reliably switch from one bistable state  
>> to the next at relatively high temperatures (still well within the  
>> environmental specification of the relay).  The company that makes  
>> the relay believes that the problem lies in the method we are using  
>> to drive it:
>>
>> We tie one leg of each coil to 5V and the other leg to a relay  
>> driver, which consists of an NPN transistor with an integrated  
>> diode to absorb the back-EMF created when the coil is de- 
>> energized.  This transistor switches the low-side of the coil to GND.
>>
>> The manufacturer recommends the opposite method: tying one leg of  
>> each coil to GND and switching the high side.  They contend that  
>> having the 5V potential on the coil permanently is "interfering  
>> with the magnetic field".  I find this hard to believe, but physics  
>> was a long time ago.
>>
>> My working hypothesis is that the coils are being energized and de- 
>> energized too quickly.  Because the relay coils share a core, the  
>> dI/dt in the coil being energized is inducing a current in the  
>> opposite coil: I can see a significant voltage spike generated  
>> across the opposite coil when I energize and de-energize the coil I  
>> intend to.  I am concerned that this spike in the opposite coil is  
>> preventing the relay from switching states by opposing the  
>> mechanical force.  So, I am reducing the edge rate at the base of  
>> the drive transistor to lower the dI/dt of the coil.  The spikes on  
>> the opposite coil are now reduced significantly.  I have not,  
>> however, gotten permission to test this modification on our only  
>> board that exhibits the problem reliably.  I, justifiably, need to  
>> make my case first...
>>
>> The problem is, the manufacturer disagrees and insists our circuit  
>> needs to be redesigned to permanently tie one leg of the coils to  
>> GND and switch the high side instead.  They will not tell me why,  
>> and cannot explain the physics behind this recommendation.  Since  
>> the coils have no reference, I don't understand why they would care  
>> what potential is on either leg--I thought that only the magnitude  
>> and direction of current through them was relevant.  I did check to  
>> see if the 5V rail was moving during the switching on and off of  
>> the coils, but it is stable.
>>
>> I am continuing to perform experiments in an attempt to isolate the  
>> root cause, but I am curious if anyone might have another  
>> hypothesis--in particular, why a coil might care if one leg was  
>> permanently tied high while the low side is switched?  Even if I  
>> accidentally try something that seems to fix the problem in the  
>> lab, I don't feel that I am truly understanding the root cause and  
>> am uncomfortable proposing a solution until I DO understand it.
>>
>> Thank you all kindly in advance for your assistance,
>>
>> regards,
>> Don Nelson
>> Netronome Systems
>> --
>> Don Nelson
>>
>> "The whole problem with the world is that fools and fanatics are so  
>> sure of themselves, and wiser people so full of doubt" --Bertrand  
>> Russell
>>
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