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

There is a good chance that the problem is the protection diodes for 
your drive transistors are defeating the magnetic circuit.  This thing 
probably works as a center tapped transformer.  When one side switches, 
the other side wants to swing the same voltage about the common terminal 
voltage, but your protection diodes limit the swing to about a volt.

Using low side drivers, add a diode in parallel to your low side driver 
to protect against negative spikes when the other side turns off.  For 
local flyback, connect the cathodes of the flyback diodes to 10V zener 
in parallel with an RC snubber. Now when one side switches low the other 
will be able to jump up to 10V without canceling flux in the magnetic 
circuit.

Before you do all this, you can connect the device to a signal generator 
to determine the coupling between coils.

The odd comment recommending 5V probably comes from a drive circuit that 
happened to work, and now 5V has become a witchcraft belief.

Steve.

Don Nelson wrote:
> 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
>
> ------------------------------------------------------------------
> To unsubscribe from si-list:
> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
> or to administer your membership from a web page, go to:
> http://www.freelists.org/webpage/si-list
>
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
>
> List technical documents are available at:
>                 http://www.si-list.net
>
> List archives are viewable at:     
>               http://www.freelists.org/archives/si-list
> or at our remote archives:
>               http://groups.yahoo.com/group/si-list/messages
> Old (prior to June 6, 2001) list archives are viewable at:
>               http://www.qsl.net/wb6tpu
>   
>
>
>   


-- 
Steve Weir
Teraspeed Consulting Group LLC 
121 North River Drive 
Narragansett, RI 02882 

California office
(866) 675-4630 Business
(707) 780-1951 Fax

Main office
(401) 284-1827 Business 
(401) 284-1840 Fax 

Oregon office
(503) 430-1065 Business
(503) 430-1285 Fax

http://www.teraspeed.com
This e-mail contains proprietary and confidential intellectual property of 
Teraspeed Consulting Group LLC
------------------------------------------------------------------------------------------------------
Teraspeed(R) is the registered service mark of Teraspeed Consulting Group LLC

------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
http://www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field


List technical documents are available at:
                http://www.si-list.net

List archives are viewable at:     
                http://www.freelists.org/archives/si-list
or at our remote archives:
                http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu

Other related posts: