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

Don,
Correct me if I'm wrong, but that 2V spike on the RESET coil would be
referred to the other end of that same coil, which is held at 5V - so
the open end gets to 7V. I would probably expect it to go higher than
that, and if that end is still connected to the driver, it may be that
the 6.6V zener is limiting the voltage. 
I haven't used latching relays before but my experience with driving
relays is that limiting the turn off voltage spike has the effect of
slowing down the turn off time of the relay rather than stopping it
switching. So you have the relay in the SET condition for example. It's
latched so neither the SET or RESET coil are energized. To reset the
relay you pull the RESET coil to ground, placing 5V across it and the
coil current increases until it reaches the point at which it will
switch the contacts, after which you can turn the SET coil off. 
But the increasing SET coil current induces a voltage in the RESET coil
which exceeds the zener voltage and therefore starts to conduct,
resulting in RESET coil current which opposes the original SET coil
current and slows the operation down. Increasing the length of the pulse
should eventually result in a switching action, but only if you wait
long enough. 
When you say the relays aren't reliable, do you mean they do not switch
at all, or just not within the time you want?




> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of Don Nelson
> Sent: Wednesday, 20 May 2009 9:52 AM
> To: steve weir
> Cc: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: Latching relays in optical switches
> 
> Thank you all very much for your comments and suggestions.  I'm
currently
> spending some quality time in the lab characterizing the circuit; I'll
be
> back soon with what I found.
> 
> Steve: By "protection diodes", do you mean flyback diodes across the
> coils?  If so, the existing circuit does not have them.  The relay
driver
> (an MDC3105) has a 6.6V Zener across the collector/emitter for its
> protection.  The relay two coils appear to be wound on the same core,
in
> opposing directions (we ripped one apart).  The bistable armature that
> actuates the optical relay lays parallel to the core.  As expected,
the
> applied voltage across the SET coil (measured at a rate of 0-to-5V in
<10
> ns!)  results in opposing spikes on the (open) RESET coil of ~2V,
below
> the breakdown of the protection zener in the RESET coil driver.  So,
> unfortunately, it doesn't appear (at first blush) that the magnetic
flux
> is being canceled.  I haven't gotten far enough into characterizing
this
> yet to know if the 2V observed on the un-driven RESET coil is
reasonable.
> 
> Thanks again,
> -don
> 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
> 
> 
> On Tuesday, May 19, 2009, at 04:12PM, "steve weir" <weirsi@xxxxxxxxxx>
> wrote:
> >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
> >>
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> >
> >
> >--
> >Steve Weir
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