Author Topic: Reed Switch stuck closed  (Read 2060 times)

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Offline AlexMA

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Reed Switch stuck closed
« on: April 03, 2017, 03:18:29 am »
I use a "Thermalarm" to detect too high or too low temperatures in the greenhouse environment.  It uses reed switches (Hamlin/Littelfuse 59025) mounted on the high and low set temperature arms, activated by a small magnet which is mounted on the thermometer (current temp) arm.  Works great.  Reed switches are well suited to the greenhouse environment, which has high humidity and lots of bugs/spiders.

For years, we wired the Thermalarm through a relay to a siren to warn us of high/low temps, powering the circuit with a 12v automotive battery.  This year, I realized we have 120V outlet at same location and that we could use a 120VAC to 12VDC transformer wall plug.  I did so, but found a problem with the relay (spiders).  Then I thought maybe the relay was unnecessary.

I ran a 30 watt load through the Thermalarm which is rated for a 10 watt maximum load.  I ran this load for several seconds.  The switch now seems to be stuck closed.  Is this likely to be a permanent failure of the reed switch?  Or, is it repairable?

I have considered mechanically tapping the switch, applying a current of the opposite polarity to the switch, or waving a magnet in the vicinity of the switch.  Not sure whether any will work....  Any thoughts most welcome.  Thanks.

Al


 

Online edavid

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Re: Reed Switch stuck closed
« Reply #1 on: April 03, 2017, 04:54:29 am »
It's not uncommon... you welded the reeds together.  Replace it.
 

Offline CatalinaWOW

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Re: Reed Switch stuck closed
« Reply #2 on: April 03, 2017, 05:03:51 am »
Occasionally tapping the relay will break the weld and restore it for a while.  Low percentage success.  Contacts will be scarred, so future life is uncertain.  Replacement is the right option.  Unfortunately reed relays aren't as cheap as they once were.
 

Online edavid

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Re: Reed Switch stuck closed
« Reply #3 on: April 03, 2017, 05:32:22 am »
Occasionally tapping the relay will break the weld and restore it for a while.  Low percentage success.  Contacts will be scarred, so future life is uncertain.  Replacement is the right option.  Unfortunately reed relays aren't as cheap as they once were.

It's not a relay, it's just a packaged switch, of the type that was commonly used for burglar alarms in the past:

http://www.mouser.com/ProductDetail/Hamlin-Littelfuse/59025-030
 

Offline BurningTantalum

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Re: Reed Switch stuck closed
« Reply #4 on: April 03, 2017, 12:44:34 pm »
Reed switches don't like being tapped; they don't even like having their leads cut with cutters because the G-shock transmitted damages the hermetic seal. Full flush cutters are essential.
BT
 

Offline Nusa

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Re: Reed Switch stuck closed
« Reply #5 on: April 03, 2017, 02:28:50 pm »
30 Watts / 12 Volts = 2.5 Amps. That reed switch is rated to switch 0.5 Amps. No surprise it failed quickly. It's toast, so if you want to try tapping it, you got nothing to lose.
Even 10 Watts / 12 Volts is too much, but you may get away with that at the expense of switch life.
Replace the reed switch and use a relay circuit to drive your loads next time.
« Last Edit: April 03, 2017, 02:30:26 pm by Nusa »
 


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