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

Coils throw electrons around with inductive charges . 
Caps throw electrons around with static charges.

Richard Jungert

> Subject: [SI-LIST] Re: Latching relays in optical switches
> Date: Sat, 30 May 2009 20:34:57 -0400
> From: RWeiss@xxxxxxxxxxxxxxxx
> To: dhwn@xxxxxxx; si-list@xxxxxxxxxxxxx
> 
> Let he without capacitance throw the first electron-------------
> 
> 
> 
> Roger E. Weiss, PhD
> Paricon Technologies
> 421 Currant Rd
> Fall River, MA 02720
> 
> 
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of Don Nelson
> Sent: Saturday, May 30, 2009 7:59 PM
> To: SI List
> Subject: [SI-LIST] Re: Latching relays in optical switches
> 
> 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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> 
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