Close packing of relays

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haklesup
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Close packing of relays

Post by haklesup »

When designing a high density switch matrix board one sometimes needs to be concerned with placing the relays so close together that the magnetic field from one relay can effect another right next to it. Not so much that the other realy will close unintentionally but that it can effect the current needed to operate the other relay.

Is this a concern mainly with reed type relays or does it also apply to electromechanical (lever) types. I only see notes about this in reed relay datasheets. Can anyone shed a little more detailed light on this subject for me.
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MrAl
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Re: Close packing of relays

Post by MrAl »

Hi there,


I've never seen this happen in any real life application, even ones that used some 10 double acting relays
fairly close together, but i would think that is has to do with the construction and exactly how close
they are mounted together. If they can and are mounted very close, they might start to exibit
transformer action, and although this may not actually be enough to activate the non energized
relay it may be enough to cause a higher than normal voltage across the open terminals which
could blow out the drive circuit for that other relay.

I would say that for any setup where they have to be close together that at least a little
testing should be done by at least measuring the coil voltage of a non energized relay when
the one right next to it is energized and deenergized. That's probably the only way to be sure
as i doubt many manufacturers test for this and certainly wont spec anything for this behavior.
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Rodney
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Re: Close packing of relays

Post by Rodney »

I've never found this to be a problem as the relays themselves have a flux path built into them in order to be efficient. No exterior path has much if any effect.
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MrAl
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Re: Close packing of relays

Post by MrAl »

Hi Rodney,


I believe that is true when the relay is closed, but when the relay is open there is more flux leakage.
Thus, i believe a test would be to drive one relay alternately closed and open while monitoring
the other relay.
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Lenp
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Re: Close packing of relays

Post by Lenp »

Years ago we used a design with 36 rather inexpensive Omron GL series relays. They were packed side by side, back to back on a board with just enough space between to allow insertion. The design was generic so AC and DC relays were both used on the same board foil pattern. Never was there a problem observed, or reported.

I also ran into an RC modeler that used very small relays, with plastic cement holding the plastic dust covers together so he could pack them into available space.

The above applications were using conventional armature relays. With reed relays I feel that transformer action is rather unlikely, since most all reed relays, by design and application, use DC coils! I think an AC coil reed relay, if they exist, may impart unwanted noise on any signal carried by the reed, since it looks somewhat like a 1 turn secondary. There are shielded relays but that may suggest that they are for low level audio and RF applications.

Len
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zotdoc
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Re: Close packing of relays

Post by zotdoc »

I made a matrix of 12 volt auto relays 12 by 12 relays glued together side by side and hand wired for a fireworks control board. We fired the fireworks with a small coil of nichrome wire using 2 12 volt auto batteries in series. It was enough current to destroy the nichrome coils for each firework fuse, and the relays worked just fine.
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