AVR Freaks

Dak wrote:

I can confirm that. I need to de-magnetize the reed-contacts every few months. They are built-in the wooden window- and door-frame, what makes it hard to replace them with Hall sensors.

Nard

With a de-magnetizer :lol:

Kidding aside: I use a de-magnetizer for de-magnetizing the heads in video- and audio-tape-recorders.

You should be able to visually inspect the reed if it is a glass capsule. 2x to 5x magnification should do it rotate the capsule axially so that you can look through the contact area (from side to side). Is there any air gap?

If not , it could be magnetized, as previously suggested. OR, the contacts could be welded. The latter can happen with a highly capacitive load (making a high inrush current) or an inductive load (arcing when it opens) or motor or incandescent light, both with possible high inrush, and the motor with possible inductive arc on open.

Reeds should do well with up to 25-50ma resistive load. Different reeds have different current ratings.

Jim

When testing the system when it was built, I found that I needed stronger magnets to prevent the alarm to go off when one of the kids (or me) slammed a door somewhere else in the house. I have had ONE false alarm in many years, and that was during lightning. Fixed that by adding a LPF.
Hehe, with sticky reed-switches, no false alarms :mrgreen:
(and no real alarm either, ... but that's just a detail)

And mercury wetted contacts ? Most likely no longer available because of RoHS, but IIRC those were the good reed contacts ?

I am having a similar issue with 3/8 inch magnet contacts that do a fan shutdown in a dorm. 5V DC and an oversized magnet. Contacts mostly stick when connected to the DC power. Anyone have similar experience? Thinking about using a plunger switch instead.

Welcome to AVR Freaks!

 

Is there any inductance in the circuit with the reed switch? Thinking of a relay, though long wires might be enough. If there is, the DC power and the inductance can cause arcing that will eventually cause the contact to fail.  That failure will often be with the contacts welded together. IF this is the case, you can add a series RC in parallel with the reed; I would try maybe 47 ohms and 1uf (non-polarized). 

 

Jim

Reeds will properly work with AC or DC. They typically have a lower voltage or current rating with DC. You will find that most reeds do not have a very large current rating (going on memory here, but maybe 50-100mA max, maybe less). Definitely lower rating in an inductive circuit. Wiring (especially a long two conductor cable) can be enough to do it. Adding the series RC may help. I think it is best placed AT the reed.

 

Jim

Good question. Is the reed directly in the power circuit of the fan? If so, it will be very inductive and prone to arcing, even with AC power.

 

Jim

Reed switches were very thoroughly vetted by the phone company & are regarded as about one of the most reliable parts known to mankind....the stats were very impressive....but you must use them properly.

 

Reed switches are a proven technology with a long track record. Their hermetically sealed contacts have contributed greatly to their use in many electronic applications. They provide reliable data without the need for physical contact, even in adverse environments containing dirt, vibration, moisture, gas, and oil. Their proven durability and reliability have made them a popular choice for several demanding applications including automotive safety products, medical equipment, security, utility metering, consumer electronics, and white goods/appliances.

Reed switches also offer an extended product life cycle. Because a reed switch only has internal components that bend and the electricity is completely contained in the reeds and lead wires, it does not have a failure mode related to mechanical wear. With some electrical loads, the operating life exceeds 50 years and one billion switching cycles. Worldwide demand for reed switch technology has continued to increase to over 1 billion each year. In response to this demand, manufacturers are making product improvements to reed switches to continue meeting the changing needs of the market.

 

interesting info about newer devices:

https://www.mouser.com/pdfdocs/RedRock-White-Paper-130417.pdf