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  1. Why is the reed switch sticking when the magnet is no longer nearby? Only when I tap the switch it releases, otherwise it just sticks for many minutes.

  2. Are there any best practices (do's or don'ts) with reed switches?

  3. Does it matter what magnet you use? Eg. regular versus rare earth.

schematic

simulate this circuit – Schematic created using CircuitLab

  • Did I manage to magnetize the contacts?
  • Is the current too large (approx 4mA)?

It is one of these and exact part number is unknown.

enter image description here

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  • \$\begingroup\$ Is this a new reed switch or one with an unknown history? \$\endgroup\$
    – JYelton
    Sep 1 '13 at 10:47
  • \$\begingroup\$ New reed switch (unmarked from China). \$\endgroup\$
    – jippie
    Sep 1 '13 at 11:21
  • \$\begingroup\$ Do you have another to swap in? I'd be curious if another one (same spec) does the same thing. \$\endgroup\$
    – JYelton
    Sep 1 '13 at 11:22
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    \$\begingroup\$ If it was swiped across a Nd or other high strength magnet, it might be magnetized. Maybe treatment with a de-gaussing coil could fix it. All pure speculation, though. \$\endgroup\$ Sep 1 '13 at 12:45
  • \$\begingroup\$ Where you are, are you bathed in a strong magnetic field that you don't know about? Do you have a compass, and does it point to the magnetic north pole? \$\endgroup\$
    – Kaz
    Sep 1 '13 at 15:39
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Reed switches can stick for the following reasons:

  • The metal of the contacts have been pitted or otherwise compromised, from arcing if switching with active loads
  • The contacts have become magnetized (or another magnetic field is unaccounted for)
  • The reed switch is physically damaged, such as through bending or cutting the leads improperly

Current Rating:

4mA doesn't sound like much current, even for switching. The lowest rated switching current value for reed switches I could find at Digikey is 50mA.

A reed switch has two values for current rating. Switching and carry. Switching is the amount of current that it is designed to switch on and off without harming the contacts. Carry is the amount of current for a switch that has already been engaged, and thus won't arc when switching.

Nickel and iron used for the contacts are soft metals, which means they more vulnerable to melting. So the contacts are generally plated with a harder metal (like rhodium or ruthenium) to increase the life of the switch.)

Residual Magnetism:

Reed switches are made of metal contacts that have been annealed, a process that leaves no or very little magnetic retentivity. This means after the reed switch is removed from a magnetic field, there should not be any residual magnetism in the contacts. (Meder)

It's possible the reed switch is defective and is not properly annealed, causing residual magnetism problems.

Physical Damage:

Reed switches are manufactured with fairly exacting tolerances. When cutting or shaping the leads, it's possible to damage the reed or glass. Meder has a whole document dedicated to handling precautions. In short, physical damage may be difficult to verify visually, but can affect performance. An excerpt from the linked document:

Internal damage can occur with no visible signs on the seal. In these instances, seal stress has occurred, leaving a torsional, lateral, or translational stress in the seal. This produces a net force on the contact area that can affect the operate characteristics (Pull-In and Drop-Out), contact resistance, and life characteristics.


Summary:

Did I manage to magnetize the contacts?

I would say unless the reed is defective (improper annealing) it is unlikely.

Is the current too large (approx 4mA)?

Without the datasheet specifics, unknown. But also unlikely.

Does it matter what magnet you use? Eg. regular versus rare earth.

No, it should not matter. The reed switch contacts simply react to a magnetic field, whatever its origin.

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  • \$\begingroup\$ When listing failure modes, you should add too low current (although it isn't the problem in this case). Quoting a vendor: "Every time the switch opens, a small needle of ruthenium is formed in the space between the blades. If you switch higher currents and voltages, these needles are vaporized. With currents below 1 mA, the needles remain there. If you have enough of these needles, they can fill up the gap." \$\endgroup\$
    – markrages
    Sep 3 '13 at 7:19
  • \$\begingroup\$ @Mark I seem to recall reading something similar to that once, but I wasn't able to find a reference. I use Meder reed switches occasionally, and they have a sizable selection of technical literature. I wasn't able to find a reference to this phenomenon among their documents. Perhaps you can provide a link and I'll incorporate it into the list? \$\endgroup\$
    – JYelton
    Sep 3 '13 at 7:58
  • \$\begingroup\$ I can't find the information stated publicly on their website. Maybe they were inventing excuses for their switches' poor performance? \$\endgroup\$
    – markrages
    Sep 5 '13 at 16:14
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    \$\begingroup\$ The seal was slightly chipped, maybe accidentally got the wires to bend too close to it. \$\endgroup\$
    – jippie
    Oct 15 '13 at 5:28
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I was using Madison float switches operating common ice cube relays at 24 VAC and had the same problem with the switch sticking after a few years of service. The website below explained that the contacts were being welded together and gave the solution. I've ordered varistors to limit the current.

http://madisonco.com/index.php/technical-resourcess/sensor-reference/electrical-considerations.html

Good luck

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This is one of our most common failure asked of float switch manufacturers. Spiking is most often the culprit. What most don't understand is that the spikes that occur are coming along the ground from the larger device as power is removed. So in the case of the relay, if the float switch is switching the relay that is controlling a larger device, a portion or all of that spike from the larger device will travel along the common ground and hit the reed switch, even if they are not tied close to each other.

The grounds must either be separated and it is always recommended that the larger device is tied directly to a hard ground with it own wires. Additionally, the Spike suppression circuits or an MOV (Metal Oxide Varistor) can be used to clamp the spikes from reaching the float switch as shown on the Madison Company technical report.

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