[SI-LIST] Latching relays in optical switches]

Don - 
It's probably worth checking (if you haven't already) that the relay doesn't 
have any self-contained reverse-biased diodes. If the relays do have these 
diodes, and you then connect in the 'non-approved' manner, then I can see 
circuit configurations where they become forward biased when you try to 
energise the coil. This would shunt away some of the coil current and degrade 
performance.

Just a possibility. 

Mike.



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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