Sticky magnetic contacts

[toasty]

Chaps,

I'm foxed.

I'm not an alarm fitter or anything, so go easy!

I have fitted the standard type of recessed magnetic reed switch ( http://www.tlc-direct.co.uk/Images/Products/size_3/TS011.JPG ) to may garage and shed door (wooden hinged normal door) I've also fitted a magnetic roller door reed to the roller door. ( http://www.tlc-direct.co.uk/Images/Products/size_3/TS013.JPG )

Now, these doors aren't opened very often (the shed door maybe only once a month if that) but they are used to trigger my home automation and turn on lights and trigger an actual alarm if necessary.

Because of this, they don't feed an alarm panel directly, they actually drive an opto isolated input which has a resistance of approx 1kohm, so at 12v this is approx 12mA. (There is a 1kohm resistor in series used to limit current to a 4N25 optocoupler, I'm using a supply of 12v)

What I'm finding after about 4 years of service, is that now both the roller door contact and the hinged door ones are occasionally sticking, so that when I open the door they don't actually operate.

If I give them a quick knock with a piece of wood or something then they operate, but I can hardly expect a burglar to do that for me!! And in any case it does cancel out the handiness of having the lights come on automatically as I have to scrabble about in the dark looking for a piece of wood!



My question is, do you think the 12mA is too much for the reed switch and they are effectively getting welded together a little?

Or are the reeds getting some kind of residual magentism that is causing them to stay stuck together?


Has anyone else experienced anything like this?

Cheers
-Dan

 

[Spark123]

12mA doesn't sound like much at all - probably the reed switches are getting old an tired.
Time for some new ones.

 

[toasty]

Really?

I agree 12mA isn't a lot, but I'm surprised that the contacts age so quickly.

Is there a longer lasting, possibly more expensive alternative?

 

[Lectrician]

Those cheapo plastic ones do fail like that after a while. I am suprised at the roller door one doing that though.

Look for grade 2 or 3 contacts.

 

[Europlex]

Quite normal, the contacts are designed to be operated. That's one reason for having a service contract with an alarm company.

Using the new "grade 2 or 3 contacts" will make NO DIFFERENCE, the little glass tube contact inside is the same, it will still need to be operated to stop it 'sticking'.

Stuck contacts are a fact of life, from the day they were first made if not operated, over time they would / will stick, regular maintenance is what is required.

Change them, a perfectly normal thing to do.

 

[toasty]

Thanks chaps, much appreciated.