[SI-LIST] Re: Latching relays in optical switches
- From: Richard Jungert <r_jungert@xxxxxxxxxxx>
- To: <dhwn@xxxxxxx>, si list freelist <si-list@xxxxxxxxxxxxx>
- Date: Tue, 19 May 2009 15:59:18 -0700
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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