Magnetic Reed Switches blow

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Hey freaks...

For the ones that has experience on Magnetic Reed Switches.
We are using a sensor that uses Magnetic Reed Switches. We put it on the doors in our app, but the place of our application has high humidity when it's raining.
We're having problem with this switches, the contact sometimes doesn't open anymore.

Our circuit is 12V -> reed switch -> 2k7 -> opto. My first thought was about the current, but 2k7 limit the circuit current.

Does anyone has experience with this problem?
Anyone knows a sensor that uses a digital hall effect IC? Using this IC the sensor are more robust (if the IC fails, it will open the contact).

Regards,

Bruno Muswieck

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You might try reducing the 12v or putting like 4k7 or 10k resistor.

I like hall effect sensors myself. Just got some, um, AH175-PL-B-B sensors from Mouser. They're tiny, with 3 leads. I use them for detecting the passing of magnets.

If you don't know my whole story, keep your mouth shut.

If you know my whole story, you're an accomplice. Keep your mouth shut. 

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The contacts should be sealed against humidity. More likely they are sticking from residual magnetism (magnets are too strong or close, or metal door) or the current is too high (what maximum does the data sheet allow).

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All of my reed switches are actually glass encapsulated, and therefore humidity isn't the issue.

See Dak's and Torby's replies.

JC

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Quote:
All of my reed switches are actually glass encapsulated, and therefore humidity isn't the issue

The same of our, glass encapsulated.
So I stay with the option that magnets are too strong. So maybe increasing the distance can be a solution, hum..

Regards,

Bruno Muswieck

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Magnetic reed switches are made of iron so that they will respond to a magnetic field. Further, as the reed gets close tot he contact, the magnetic field between the reed and the contact increases, helping to pull the reed firmly (and quickly) to the contact.

If the reed were to become magnetized, then it would never release. The stronger is the external field, the more likely this is to happen.

So, I like the idea that the magnet in the door is much stronger than it needs to be.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Dak wrote:

Quote:
More likely they are sticking from residual magnetism

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

A GIF is worth a thousend words   They are called Rosa, Sylvia, Tessa and Tina, You can find them https://www.linuxmint.com/

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Quote:
I need to de-magnetize the reed-contacts every few months.

Well, We think that is our problem :shock: . Our app most of the time the switches are closed.
How you do de-magnetize?

Fast solution is to put the reed contacts on the door (movement of the door to de-magnetize the contacts) but we still stay long time with the contacts closed.
Long term solution, made a sensor replacing the magnetic reed switches by digital hall effect IC.

Regards,

Bruno Muswieck

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With a de-magnetizer :lol:

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

A GIF is worth a thousend words   They are called Rosa, Sylvia, Tessa and Tina, You can find them https://www.linuxmint.com/

Dragon broken ? http://aplomb.nl/TechStuff/Dragon/Dragon.html for how-to-fix tips

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Quote:
I need to de-magnetize the reed-contacts every few months
It's probably your magnetic personality causing the problem. :)

Most of my alarm reed switches have been installed for about 20 years and never had any problems (and yes they still work well).

However as suggested above metal door frames (steel) or any other concealed magnetic material close to the switch could become magnetised and stop the switches from opening.

Another possibility is junk reed switches.

John Samperi

Ampertronics Pty. Ltd.

www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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Try rotating one side or the other to change the field direction. Then when the magnet pulls away the reeds will go through a period of zero flux which might be long enough for them to spring apart. Hopefully the remnant field won't be enough to pull them together again.

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my plastic-package cheapie has been running for years, with a dozen or more cycles every day. It has about 20mA at 6VDC flowing, to an LED within a 20 Amp solid state switch.

It's on an exterior door but not exposed to rainfall.

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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

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Quote:
my plastic-package cheapie has been running for years, with a dozen or more cycles every day.

Quote:
Most of my alarm reed switches have been installed for about 20 years and never had any problems (and yes they still work well).

I had just test our sensor that faild and some of the sensor doesn't close the contact anymore. I'm with the ideia that the sensor reed switch is a che... or the magnetic field is stronger than it should be.

Regards,

Bruno Muswieck

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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)

A GIF is worth a thousend words   They are called Rosa, Sylvia, Tessa and Tina, You can find them https://www.linuxmint.com/

Dragon broken ? http://aplomb.nl/TechStuff/Dragon/Dragon.html for how-to-fix tips

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As js has suggested there have been bad reed switches and I have experienced them. The contacts should be gold plated! If they are not, they grow tiny whiskers. The whiskers easily become permanently magnetized and hold it closed.
This is a similar mechanism that you can get in conventional relays where the hammering of the armature on the pole causes small areas of magnetization.

Remember science at school. When an unmagnetized steel rod is longitudinally aligned in a North-South direction and then hit with a hammer in a longitudinal direction, you get an instant permanent magnet!

Charles Darwin, Lord Kelvin & Murphy are always lurking about!
Lee -.-
Riddle me this...How did the serpent move around before the fall?

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Yes, frequent jarring in a strong magnetic field tends to magnetize the contacts (e.g. a slamming door), as does elevated temperatures. A cushion gasket under the reed switch might help. I have no long term statistics.

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I tried to go back to some of my paper notes that I had on reed relay interfacing & use in telephone exchanges especially the Belgian 10C system, which was 100% reed, but I suspect they were thrown out.
IIRC, whiskers grow due to metal migration due to complex conditions Ie. uni-directional current, heating, mechanical jarring etc. After a while, the contact resistance increases exacerbating the problem. After a while "stiction" occurs, where they stick to to both magnetic & mechanical forces. This is a bigger problem with non-gold plated contacts.

Charles Darwin, Lord Kelvin & Murphy are always lurking about!
Lee -.-
Riddle me this...How did the serpent move around before the fall?

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

A GIF is worth a thousend words   They are called Rosa, Sylvia, Tessa and Tina, You can find them https://www.linuxmint.com/

Dragon broken ? http://aplomb.nl/TechStuff/Dragon/Dragon.html for how-to-fix tips

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I am just trying to recall where I came across mercury wetted contacts. IIRC it was a Clare relay in some part of the LME crossbar telephone system. But I can't quite remember it's exact properties (it must have been reliable, because it never failed).
I can't recall mercury wetted reed contacts? Most voice switching contacts were wetted with some DC current.

Charles Darwin, Lord Kelvin & Murphy are always lurking about!
Lee -.-
Riddle me this...How did the serpent move around before the fall?

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Dear Forum members,

Firstly my apologizes for re-opening a one year old post, but it is the only site where I have found any similar to my problem.

I am using a reed magnetic sensor in order to know when the door is opened in a farm installation. I have been testing it in laboratory for months with no problem. I use a reed sensor instead a open colector one because of the consumption as the electronic device is supplied by batteries.

My problem is that after two days the contacts seem to be sticky. After having read your responses it can be due to be screwed in to a steel frame of a sliding door. In fact, both parts of the sensor are on a steel frame, so it is very likely that the contacts are magnetized, arent they? Because after disassembling the sensor from the door and some minutes later it starts to work fine again.

I dont know why exactly!

The sensor model is CELDUC PLA41201. Normally open but it remains almost all the time closed (with the magnetic part).

Its very confusing so I ask for your help.

- Is not possible to use this kind of sensors in metal/steel doors?
- Is there any solution to my problem?
- Do exist any sensor better to avoid contact stick?

Thanks and regards,

Asier

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Quote:
a steel frame of a sliding door

If the door frame is steel then perhaps it is becoming magnetized.

Holding a compass near the door is an easy way to check for this.

JC

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Commercial product lines usually have matching spacers for use with steel doors.

Extends magnet and switch away from the metal by equal amounts. Problems arise if either is too close.

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Hello,

Thanks for your responses.

Quote:

Holding a compass near the door is an easy way to check for this.

I am going to buy a compass to know that. But a big frame of steel can be magnetized by a small magnet?

Quote:

Extends magnet and switch away from the metal by equal amounts. Problems arise if either is too close.

I had done that with homemade plastic spacers but may be that the screws used to fix the sensor to the door frame, which are made of steel, could be the problem as I have realized that after having been in contact with magnet they are magnetized.I have changed them by some screws made of nylon to test and I will tell you.

Quote:

Commercial product lines usually have matching spacers for use with steel doors.

Would you mind recommending me some one, I havent found it. I dont mind the prize. It would be great!!

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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.

NGerke

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I should include that there are no steel parts to this window all PVC, aluminum or vinyl.

NGerke

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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

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

Last Edited: Tue. Dec 19, 2017 - 06:32 PM
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So, figured out on the same pair of conductors I have 24ac and between 1-10dc. The DC is the induced voltage best I can tell. I think it probable that the contacts used are not rated for AC and are getting corroded internally. That's why it's possibly getting stuck.

NGerke

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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

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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do a fan shutdown in a dorm

How? Do they the power to the fans directly? How much current does the fan use?

John Samperi

Ampertronics Pty. Ltd.

www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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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

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Reed contacts are quite fine & delicate - they are not designed for switching any significant load.

 

A problem with putting too much current through a reed is that the contacts can weld together.

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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

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

Last Edited: Thu. Dec 21, 2017 - 03:23 AM
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avrcandies wrote:
vetted by the phone company

exactly.

 

As I said in #32, they are designed as small-signal switches - not for switching any significant of power.

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