[SI-LIST] Re: Latching relays in optical switches
- From: Richard Jungert <r_jungert@xxxxxxxxxxx>
- To: <rweiss@xxxxxxxxxxxxxxxx>, si list freelist <si-list@xxxxxxxxxxxxx>
- Date: Sun, 31 May 2009 06:34:44 -0700
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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