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Reed contacts sticking

T

Terry Pinnell

Jan 1, 1970
0
I spent some hours today trying unsuccessfully to find the cause of my
burglar alarm being falsely triggered. But in the course of that I
came across a serious (not to say alarming) flaw. Each of my windows
and doors is protected by a N/C reed relay enclosed in a small
cylinder of plastic, mounted in the moving part, so as to be flush
against a similarly mounted permanent magnet in the fixed frame. When
the window is open, resistance across the reed goes from close to zero
to effectively infinite.

But one window switch was remaining at zero ohms. After several
minutes opening, closing and banging it, it then returned to normal
operation. As mentioned, these are fully enclosed magnets and reeds.
Obviously, this makes me apprehensive about reliability. I'd assumed
reed relays *must* open when the magnet was removed.

Anyone have any thoughts on possible cause please?
 
R

Rene Tschaggelar

Jan 1, 1970
0
Terry said:
I spent some hours today trying unsuccessfully to find the cause of my
burglar alarm being falsely triggered. But in the course of that I
came across a serious (not to say alarming) flaw. Each of my windows
and doors is protected by a N/C reed relay enclosed in a small
cylinder of plastic, mounted in the moving part, so as to be flush
against a similarly mounted permanent magnet in the fixed frame. When
the window is open, resistance across the reed goes from close to zero
to effectively infinite.

But one window switch was remaining at zero ohms. After several
minutes opening, closing and banging it, it then returned to normal
operation. As mentioned, these are fully enclosed magnets and reeds.
Obviously, this makes me apprehensive about reliability. I'd assumed
reed relays *must* open when the magnet was removed.

Anyone have any thoughts on possible cause please?

Is the current and the inductivity of the load within
specifications?

Rene
 
I

Ian Stirling

Jan 1, 1970
0
Terry Pinnell said:
I spent some hours today trying unsuccessfully to find the cause of my
burglar alarm being falsely triggered. But in the course of that I
came across a serious (not to say alarming) flaw. Each of my windows
and doors is protected by a N/C reed relay enclosed in a small
cylinder of plastic, mounted in the moving part, so as to be flush
against a similarly mounted permanent magnet in the fixed frame. When
the window is open, resistance across the reed goes from close to zero
to effectively infinite.

But one window switch was remaining at zero ohms. After several
minutes opening, closing and banging it, it then returned to normal
operation. As mentioned, these are fully enclosed magnets and reeds.
Obviously, this makes me apprehensive about reliability. I'd assumed
reed relays *must* open when the magnet was removed.

Maybe the magnet is not being removed.
Have you checked for nails/screws nearby that may be magnetised?
 
J

Joe McElvenney

Jan 1, 1970
0
Hi,

Sticking reed relay contacts are an occasional fact of life
especially if there is a small DC current flowing through them.
This can magnetise the contacts (they may be iron plated) which
holds them in. You could try reversing the leads to the iffy one
to see if that helps.


Cheers - Joe
 
J

John Larkin

Jan 1, 1970
0
Hi,

Sticking reed relay contacts are an occasional fact of life
especially if there is a small DC current flowing through them.
This can magnetise the contacts (they may be iron plated) which
holds them in. You could try reversing the leads to the iffy one
to see if that helps.


Cheers - Joe

Reeds can also weld themselves shut through pure metal diffusion,
especially in applications where they stay closed for long periods of
time.

John
 
G

Guy Macon

Jan 1, 1970
0
Terry Pinnell said:
I spent some hours today trying unsuccessfully to find the cause of my
burglar alarm being falsely triggered. But in the course of that I
came across a serious (not to say alarming) flaw. Each of my windows
and doors is protected by a N/C reed relay enclosed in a small
cylinder of plastic, mounted in the moving part, so as to be flush
against a similarly mounted permanent magnet in the fixed frame. When
the window is open, resistance across the reed goes from close to zero
to effectively infinite.

But one window switch was remaining at zero ohms. After several
minutes opening, closing and banging it, it then returned to normal
operation. As mentioned, these are fully enclosed magnets and reeds.
Obviously, this makes me apprehensive about reliability. I'd assumed
reed relays *must* open when the magnet was removed.

Anyone have any thoughts on possible cause please?

Magnetic field (check to see if the replacement is stuck as well),
a defective part, or excess current welding the contacts shut.

Excessive current is unlikely to come from the alarm system.
Could it be from your testing methods? Another technician's
testing methods? A lightning bolt hitting nearby?

Now comes the interesting possibility; could it be that someone
who knows a bit about alarm systems wanted to come through that
window without setting off the alarm and without leaving behind
evidence of tampering? All it takes is a charged capacitor to
weld those contacts shut...
 
T

Terry Pinnell

Jan 1, 1970
0
Rene Tschaggelar said:
Is the current and the inductivity of the load within
specifications?

The alarm circuit current from this commercial unit is very low. I
wouldn't have thought the wiring and reeds to have any apprecaiable
inductance.
 
T

Terry Pinnell

Jan 1, 1970
0
Ian Stirling said:
Maybe the magnet is not being removed.
Have you checked for nails/screws nearby that may be magnetised?

The magnet itself is certainly being removed well away. And, although
I'll re-check, I'm pretty sure there are no ferrous parts close enough
to the reed to hold it closed.
 
T

Terry Pinnell

Jan 1, 1970
0
Joe McElvenney said:
Hi,

Sticking reed relay contacts are an occasional fact of life
especially if there is a small DC current flowing through them.
This can magnetise the contacts (they may be iron plated) which
holds them in. You could try reversing the leads to the iffy one
to see if that helps.

Thanks, I'll try that reversal tip, although it's astonishing to me
that directivity of current flow across a pair of reed contacts could
be relevant!

Of course, the worst aspect of this is that I discovered it only
accidentally, and it was transient. Other windows and doors in the
full loop use identical components, and some are opened very
infrequently, like this one.
 
T

Terry Pinnell

Jan 1, 1970
0
John Larkin said:
Reeds can also weld themselves shut through pure metal diffusion,
especially in applications where they stay closed for long periods of
time.

In that case, that seems the most likely explanation so far. (I guess
this window hasn't been opened for sveral weeks.) But that's quite an
issue then, given their widespread use in alarm systems!
 
T

Terry Pinnell

Jan 1, 1970
0
Magnetic field (check to see if the replacement is stuck as well),


a defective part,

Haven't replaced it yet, and was rather hoping not to have to.
or excess current welding the contacts shut.

As per other reply, don't think so. Not the alarm circuit anyway.
Excessive current is unlikely to come from the alarm system.
Could it be from your testing methods?

Fair point, but I'd be amazed if the current passed by my DMM on its
200 ohms range would be sufficient to weld the reed contacts together?
Another technician's testing methods?

No, only myself and the component are fair targets for blame.
A lightning bolt hitting nearby?

Finest day here this year. Not a lightning bolt to be seen within
hundred yards of this end of the road.
Now comes the interesting possibility; could it be that someone
who knows a bit about alarm systems wanted to come through that
window without setting off the alarm and without leaving behind
evidence of tampering? All it takes is a charged capacitor to
weld those contacts shut...

Interesting - but up there above lightning in the Unlikely Stakes!
 
N

N. Thornton

Jan 1, 1970
0
Terry Pinnell said:
I spent some hours today trying unsuccessfully to find the cause of my
burglar alarm being falsely triggered. But in the course of that I
came across a serious (not to say alarming) flaw. Each of my windows
and doors is protected by a N/C reed relay enclosed in a small
cylinder of plastic, mounted in the moving part, so as to be flush
against a similarly mounted permanent magnet in the fixed frame. When
the window is open, resistance across the reed goes from close to zero
to effectively infinite.

But one window switch was remaining at zero ohms. After several
minutes opening, closing and banging it, it then returned to normal
operation. As mentioned, these are fully enclosed magnets and reeds.
Obviously, this makes me apprehensive about reliability. I'd assumed
reed relays *must* open when the magnet was removed.

Anyone have any thoughts on possible cause please?

Reeds are known to have reliability issues. A simple improvement is to
use 2 reeds, or better 4. Or just a better type of sensor.

Regards, NT
 
T

Terry Pinnell

Jan 1, 1970
0
Reeds are known to have reliability issues. A simple improvement is to
use 2 reeds, or better 4.

Thanks, that sounds a good idea. For this particular window, I may now
just cut and rewire another reed in series. But quite a chore if I
apply that to *all* of them.

Or just a better type of sensor.

Such as?

I use a microswitch on a couple of doors, but only because drilling
holes for the magnet and reed is impracticable. I assume reliability
of a mechanical microswitch is much less. Or rather I did assume that,
until this apparent issue of reds reliability arose.
 
T

Terry Given

Jan 1, 1970
0
Terry Pinnell said:
Haven't replaced it yet, and was rather hoping not to have to.


As per other reply, don't think so. Not the alarm circuit anyway.


Fair point, but I'd be amazed if the current passed by my DMM on its
200 ohms range would be sufficient to weld the reed contacts together?


No, only myself and the component are fair targets for blame.


Finest day here this year. Not a lightning bolt to be seen within
hundred yards of this end of the road.

several km can be close enough....
Interesting - but up there above lightning in the Unlikely Stakes!

Terry Pinnell
Hobbyist, West Sussex, UK

Hi Terry,

one possibility not mentioned is of course random failure - ALL parts have a
statistical likelihood of failing randomly during use. In reliability
engineering one uses the "bathtub" curve - a "U" shape with a flat bottom.
failure rate is the vertical axis, time is the horizontal. The Left-hand
edge of the bathtub curve represents infant mortality - the design/process
arent working properly, and defective product is produced. The right-hand
side is wear-out - eg electrolytics drying up, IGBT modules failing due to
thermal cycling etc. In-between is the (hopefully very) low "random" failure
portion. These "random" failures are often not random at all, but represent
things the user has no control over. cheap shitty parts have a high "random"
failure rate, good quality parts will be (sometimes a LOT) lower.

I once repaired a Genrad ICT that had a stuck 0V relay (cant recall if they
were reed). The huge circuit card had 32 of these things in parallel. We
didnt know which one was dodgy, so I got staff in the factory to shout out
"random" numbers and replaced that relay then tested the unit. Kim guessed
the right relay on try number 4, which was pretty good, and she was chuffed.

Cheers
Terry
 
T

Terry Given

Jan 1, 1970
0
Terry Pinnell said:
Thanks, that sounds a good idea. For this particular window, I may now
just cut and rewire another reed in series. But quite a chore if I
apply that to *all* of them.



Such as?

I use a microswitch on a couple of doors, but only because drilling
holes for the magnet and reed is impracticable. I assume reliability
of a mechanical microswitch is much less. Or rather I did assume that,
until this apparent issue of reds reliability arose.

Terry Pinnell
Hobbyist, West Sussex, UK

switch reliability is a strong function of manufacturer, followed by
application circuit. lots of people screw-up switch & relay designs - power
relays often need quite high minimum currents to clean the contacts, and
stick quite readily if used for signal currents. Lots of people stick 100nF
caps across switches with low current ratings, then wonder why they weld :).
i have a book on power relays published by schrack, you shoudl see if they
have it in PDF form - its a fascinating read. Or if you wanna go crazy, read
"electric contacts" by Ragnar Holm

cheers
Terry
 
G

Guy Macon

Jan 1, 1970
0
Terry Pinnell said:
In that case, that seems the most likely explanation so far. (I guess
this window hasn't been opened for sveral weeks.) But that's quite an
issue then, given their widespread use in alarm systems!

Several weeks isn't long enough for the metal diffusion effect.
that takes years.
I assume reliability
of a mechanical microswitch is much less. Or rather I did assume that,
until this apparent issue of reeds reliability arose.

I don't like microswitches for alarm applications. Too many problems
with dirt and with alignment.

Let's back up for a moment and look at the big picture here.
there are hundreds of thousands of alarm systems using magnetic
reed relays, and most are trouble-free in the area of contacts
sticking. What is different about this one?

If I were the technician working on this, I would replace the part
(if it stuck onece it is likely to do it again) and would open up
the old one and a new one. I suspect that the old one will have an
obvious mechanical problem such as the spring not being attacked
properly.
 
T

Terry Given

Jan 1, 1970
0
Guy Macon said:
Several weeks isn't long enough for the metal diffusion effect.
that takes years.


I don't like microswitches for alarm applications. Too many problems
with dirt and with alignment.

Let's back up for a moment and look at the big picture here.
there are hundreds of thousands of alarm systems using magnetic
reed relays, and most are trouble-free in the area of contacts
sticking. What is different about this one?

If I were the technician working on this, I would replace the part
(if it stuck onece it is likely to do it again) and would open up
the old one and a new one. I suspect that the old one will have an
obvious mechanical problem such as the spring not being attacked
properly.

I presume failure to properly attack the spring ultimately results in
feelings of superiority, leading to stop-work meetings. Cheap sandwiches are
often used for such meetings, leading to the possibility of contacts
becoming "jammed" :). A correctly administered thrashing during manufacture
should prevent such behaviour in the field.

cheers
Terry the pedantic sod
 
S

Spehro Pefhany

Jan 1, 1970
0
Several weeks isn't long enough for the metal diffusion effect.
that takes years.


I don't like microswitches for alarm applications. Too many problems
with dirt and with alignment.

Let's back up for a moment and look at the big picture here.
there are hundreds of thousands of alarm systems using magnetic
reed relays, and most are trouble-free in the area of contacts
sticking. What is different about this one?

If I were the technician working on this, I would replace the part
(if it stuck onece it is likely to do it again) and would open up
the old one and a new one. I suspect that the old one will have an
obvious mechanical problem such as the spring not being attacked
properly.

Spring? Reed capsules are completely sealed and the spring action is
an inherent part of the strucuture. Unless the glass is broken (say
they were dumb enough to pot the thing in epoxy...)

http://megachem.co.jp/Photo-2/Hako357.JPG


Best regards,
Spehro Pefhany
 
J

Julie

Jan 1, 1970
0
Terry said:
Haven't replaced it yet, and was rather hoping not to have to.

Not replacing it is a bad solution --

Replace the defective switch. Then take the old one, cut it open/disassemble
it or otherwise try to determine the source of the problem. Use that knowledge
to determine if this was an anomaly or is an indication of a systemic failure
and then take the appropriate action.

Check your alarm system *monthly*. It probably says this several times in the
alarm system manual.
 
J

JeffM

Jan 1, 1970
0
given their [reed switches] widespread use in alarm systems!
Terry Pinnell

Indeed. The sealed nature is a big selling point.
Imagine an upward-facing actuator on a standard switch
with crud collecting around the plunger.
You just know that eventually that switch is going to stick
--and Murphy says in a way that will let the burglars in without any alarm.

....not to mention the old putty knife trick to defeat plunger switches.
 
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