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

Don.

I think the saturation current in the inductor is changing as temperature goes 
up. Have you considered this? I forget which direction it goes over temperature 
but last time I took a look at this circuit saturation current was an issue. 

Richard Jungert

> Date: Tue, 19 May 2009 14:46:10 -0400
> From: dhwn@xxxxxxx
> 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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