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Author Topic: Question about a magnetoresistive sensor  (Read 1137 times)
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TucoRamirez
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« on: April 23, 2013, 11:59:23 11:59 »

hi,

I have a question to ask ...

i have a kmz51 magnetoresistive weak field sensor... i make it work with the attached schematic to polarise it and have an output..


As long as i understand , weak field sensor means that its sensible to weak fields like earth field, so if i approach a mobile to it , it must degenerate then restitute to its value ... and it happens!

But,  one of my students approached  a fridge magnet to it and the sensor stayed blocked on one single level ...  Cry

and the only way to keep it working now is by helding the magnet near to it  Angry

the question is ... so i am right and he killed the sensor by polarising in a "evil" way the internals or i still have a way to save the sensor's life ?  Is it possible to reverse the effects of 5mT (fride magnet field) under a 0.2KA/m ( 250uT ) rated  sensor Huh  



many thanks
« Last Edit: April 23, 2013, 11:19:31 23:19 by TucoRamirez » Logged

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Unhappy
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« Reply #1 on: April 23, 2013, 09:50:34 21:50 »

Dear Tuco
As much as I could understand from your description and picture attachment, your sensor cannot be saved now because of exposure to very strong magnetic field and Sensor having very good response to + BH curve of magnetic field

On other thought you may use a tape demagnetizer to demagnetize it, I am not sure whether it will work or not.
« Last Edit: April 23, 2013, 09:54:40 21:54 by Unhappy » Logged
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« Reply #2 on: April 23, 2013, 10:04:28 22:04 »

I agree with unhappy, did you try to recalibrate the device, if yes, then use a tape demagnetizer or as your in a school, maybe you have a tool demagnetizer on site.
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TucoRamirez
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« Reply #3 on: April 23, 2013, 11:14:46 23:14 »

excuse my ignorance but i never had a desmagnetiser  ^^


Anyway a kick in the face of my student will help to demagnetise him ^^   ... and and another kick in the face of the other teacher who suggested him that "yes my circuit is better than Tuco's  ...  just try to put a magnet and you will see mine dont even cry ..."

Whatever!!!  i didnt used any feedback for offset cancel because i used correctly my flip circuit and i changed the preamp gain to put the minimal output value closer as possible of ground (i think with an instrumentation amp i'll obtain something really precious but the application doesnt demand that complexity), then any desired output will need or an attenuation or a gain (thats why i put the non inverter )  anyway i can power @ 3V3 quietly and have a pretty confortable output with enough range to be measured and exploited Smiley

my output is a clean out with a simple circuit Smiley
--------------------------------------------------
For the interested people following this story ..

what  my "colleague" did ...


he read the aplication note but he misunderstood one fact ...

in the AN  flip circuit and filters are suited in that way that the incoming flip causes an alternative output that changes its amplitude with the variations of the angle ... then its fair to use a controlled rectifier ... like
( http://www.geofex.com/article_folders/polarity_reverser/polarity_reverser.htm )

but when your signal is a clean line dc-like, the effect of that block is nooothing...  even worst it adds switching peaks to the output that become worst if a post amp is added and adds a response time against external soft fields veeery slow...


i'll add the sketch of the circuit (just for info)

in the attached figure   replaced the cd4053 (that he put) (cuz multisim doesnt have it ^^ ...  but in real world it doesnt work it only add spikes to the signel and slow the response ...




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« Reply #4 on: April 24, 2013, 12:54:04 00:54 »

A high frequency reversing magnetic field is good for demagnetizer.

A coil on a high freq AC source is good enough for something small.
I think you can even use a function generator and a couple of push pull transistor to drive it..
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« Reply #5 on: April 25, 2013, 09:57:53 09:57 »

With demagnetizing you have a chance to save the sensor. You could try buying a demagnetizer for a hardware store, which is usually used to to demagnetize screwdrivers.
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TucoRamirez
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« Reply #6 on: April 25, 2013, 09:59:39 09:59 »

oki, i'll check it next week cuz the girl that buy the components and hw for the students is in holidays ^^ 
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« Reply #7 on: April 25, 2013, 12:47:52 12:47 »

oki, i'll check it next week cuz the girl that buy the components and hw for the students is in holidays ^^ 
Tell us about the results +ve/-ve to update our knowledge
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« Reply #8 on: April 25, 2013, 03:30:21 15:30 »

Tell us about the results +ve/-ve to update our knowledge
?

you mean the output behavior? 

i posted a clip

http://www.youtube.com/watch?v=Y0abbYK0DIg

anyway, with a very low load, the impulsions cant be viewed with a voltage scope (but they exist trust me ^^)  and the output becames a smooth line ...so no complicated conditionning needed after...

( ah, and btw the bench table has a poor grounding, so i put some caps on the breadboard to mitigate any mains noise incoming
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