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'[EE] Reed relay sticking'
2006\05\26@100329 by Steven Howes

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Hi, I was wondering if anyone here could work out why a 1A reed relay
would 'stick' when carrying only 700ma.. It is only a low voltage (ie 2
or 3 v) so it is unlikely to be a power issue. The reed relay in
question is an RS Electronics 291-9710. They appear to stick randomly
then sometimes unstuck which seems a little odd. Is it possible that
they were damaged during soldering? I have noticed most images of reed
relays online have them in sockets. Excuse my general stupidity but I
have had little to do with these little bastar.... Components in the
past. Thanks in advance for any help.



Steve Howes

Poundbury Systems Ltd.

2006\05\26@104756 by Bill & Pookie

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I seem to remember something about magnetic reed switches in security
systems welding or platting closed after a long period of continues current.
And I would think that the current was more like less than 10 ma.  The fix
was to open the contacts occasionally or reverse the current through the
contacts.  Also, what opens the contacts, the coil or the reeds themselves?

Bill

{Original Message removed}

2006\05\26@110129 by Paul Hutchinson

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> -----Original Message-----
> From: spam_OUTpiclist-bouncesTakeThisOuTspammit.edu On Behalf Of Steven Howes
> Sent: Friday, May 26, 2006 10:03 AM
>
> Hi, I was wondering if anyone here could work out why a 1A reed relay
> would 'stick' when carrying only 700ma.. It is only a low voltage (ie 2

Could be caused by transients created by the load switching on or off.
Transients from the load switching could easily exceed the voltage and/or
current rating of the contacts. Test the switching reliability with a very
low power load like an ohmmeter.

> or 3 v) so it is unlikely to be a power issue. The reed relay in
> question is an RS Electronics 291-9710. They appear to stick randomly
> then sometimes unstuck which seems a little odd. Is it possible that
> they were damaged during soldering? I have noticed most images of reed

Because these are completely encapsulated, soldering damage is much less
likely than with non-encapsulated types where you are directly soldering to
the leads that pass through the hermetically sealed glass. While it is
possible that they where damaged by heat it would have to be a extremely bad
soldering job. To test this, simply change the relay for a socket and test
the operation with new relays.

Paul

> relays online have them in sockets. Excuse my general stupidity but I
> have had little to do with these little bastar.... Components in the
> past. Thanks in advance for any help.
>
> Steve Howes
> Poundbury Systems Ltd.

2006\05\26@111359 by Steven Howes

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They spring open by themselves. And they are being used to flash a
pattern so its not closed for more than a second if that. Reversing the
current isn't really possible.. hence my confusion :P

Steve

{Original Message removed}

2006\05\26@112021 by Harold Hallikainen

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Is it a resistive load?

Harold


{Quote hidden}

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2006\05\26@115631 by David VanHorn

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I'd bet that you're welding the contacts.

They have very small thin contacts, and something as simple as a small
capacitor on the output side could cause enough inrush current to weld.

The only other reason would be an external magnetic field, but it would have
to be pretty significant, probably 50-500 gauss.

2006\05\26@120242 by Geo

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face
On 26 May 2006, at 15:03, Steven Howes wrote:

> Hi, I was wondering if anyone here could work out why a 1A reed relay
> would 'stick' when carrying only 700ma.. It is only a low voltage (ie 2
> or 3 v) so it is unlikely to be a power issue. The reed relay in
> question is an RS Electronics 291-9710. They appear to stick randomly
> then sometimes unstuck which seems a little odd. Is it possible that
> they were damaged during soldering?
No.
They always have stuck - a well-known company produced data-loggers
incorporating reed switches for switching very low voltages (from
thermocouples etc). The reeds were always sticking randomly after a few
thousand operations (not very long at up to 100 channels per second). This
caused all sorts of problems as thermocouples would get jpined to others at
different "chassis" potentials. There was no cure so we (the service guys) just
had to carry a large stock of the damned things and replace all of them ( 4 per
channel and up to 500 channels) at once. If we just replaced the faulty one
then we could expect another call a few days later...
These were high quality reeds. Cold welding was the given answer.

George

2006\05\26@121705 by ardhuru

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face



> I'd bet that you're welding the contacts.
>
> They have very small thin contacts, and something as simple as a small
> capacitor on the output side could cause enough inrush current to
> weld.

Is there a solution to this?

I have added a sugar cube to my PC SMPS, powered from the 12 volts. This drives all the peripherals (monitor, speakers, scanner, printer). When I hibernate (or shut down), everything gets switched off completely.

I have also observed that at times (once a couple of weeks) the reed does get latched.

Regards,

Anand

2006\05\26@151227 by Peter

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On Fri, 26 May 2006, Geo wrote:

The capacitance of a few meters of cable under voltage carries enough
microjoules to weld a reed relay. The real real fix is to add small
resistors in series with the reeds directly at the reeds, and use gas
filled reeds (not vacuum ones). Think about it, there used to be phone
exchanges with millions of these in them.

Peter

2006\05\26@153247 by David VanHorn

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On 5/26/06, Peter <.....plpKILLspamspam.....actcom.co.il> wrote:
>
>
> On Fri, 26 May 2006, Geo wrote:
>
> The capacitance of a few meters of cable under voltage carries enough
> microjoules to weld a reed relay. The real real fix is to add small
> resistors in series with the reeds directly at the reeds, and use gas
> filled reeds (not vacuum ones). Think about it, there used to be phone
> exchanges with millions of these in them.


Yup, mercury wetted ones work nicely too, but I suspect they are becoming
hard to find.
Hard to weld mercury.

2006\05\26@164815 by Steven Howes

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part 1 1118 bytes content-type:text/plain; (decoded quoted-printable)


Yea we were looking into mercury wetted ones but getting them as soon as we needed them was going to be tricky. We have since adjusted the circuit to switch it in a different manner. Perhaps i should look into the downsides of components before using them next time ;). Thanks to you all for your help.

Steve

{Original Message removed}

2006\05\26@170027 by David VanHorn

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On 5/26/06, Steven Howes <EraseMEstevespam_OUTspamTakeThisOuTpoundbury.com> wrote:
>
>
> Yea we were looking into mercury wetted ones but getting them as soon as
> we needed them was going to be tricky. We have since adjusted the circuit to
> switch it in a different manner. Perhaps i should look into the downsides of
> components before using them next time ;). Thanks to you all for your help.


Wetting current is another one that will get you. For any normal mechanical
switch, there is a minimum current to assure the contacts actually close.
Otherwise, they may from time to time go to high resistance values or even
open up entirely.

Mercury wetted relays were used a lot with audio circuits where there wasn't
any significant current flowing, for this reason.

2006\05\30@044641 by Alan B. Pearce

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>Think about it, there used to be phone
>exchanges with millions of these in them.

And how many are there now? I used to hear horrendous stories of reed relays
sticking in telephone exchanges.

2006\05\30@190322 by Peter

picon face

On Tue, 30 May 2006, Alan B. Pearce wrote:

>> Think about it, there used to be phone
>> exchanges with millions of these in them.
>
> And how many are there now? I used to hear horrendous stories of reed relays
> sticking in telephone exchanges.

Not on the phone I guess ;-)

They were not so bad but normally one thinks of relays as rugged devices
that withstand even moderate ligtning. Reeds are not like that at all.
The .ni.com link I posted explains it very well.

Peter

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