Hall Effect vs Reed switch

Discussion in 'General Electronics Chat' started by Elevatorinfo, Jul 12, 2015.

  1. Elevatorinfo

    Thread Starter New Member

    Jul 12, 2015
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    Hello Group,

    Attempting to cobble up a device that when placed near an energized relay will output an analog signal to say, pick up another relay with a transistor driver. Relay(s) to be monitored could be either AC or DC and run from 12 to 220 volts. All that needs to happen is for the output to go high when current is present and go low when it's not. Don't need to know how much just yes or no.

    So far tried Hall Effect sensors with little luck due to my lack of knowledge and they seem to have a love affair with DC. Same goes for reed switches.

    Thanks You Guys,

    Jim
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,564
    2,379
    You can't use an auxiliary contact on the relay? that would be the simplest by far, no matter if DC or AC relay coil.
    Explain a little more what you are trying to do.
    Max.
     
  3. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    There is no simple detector for both a steady-state and alternating magnetic field. hall effect comes close, but you have to process it with a circuit. An inductor can pick up an alternating field, but you need a circuit to turn it into a relay or transistor switch output. The same system might pick up the turn on transient of a DC relay coil, but you would need a latch to give you a steady-state output for as long as the coil is energized, plus a reverse-field-polarity circuit to detect turn-off and clear the latch. Doable, but maybe difficult depending on the range of relay coils you want to sense. Or something like that.

    ak
     
  4. Elevatorinfo

    Thread Starter New Member

    Jul 12, 2015
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    Thanks You Guys,

    Thanks MHR, Can't use a the coil voltage or aux contact. Relay(s) to be sensed are already in place. We need to make the "Device" (name from now on) non-evasive. Analog Kid, thank you as well. You are correct. Bought one of those pocket voltage/current detectors (Extech Magnetic detector) which does work on DC & AC but, as AC stated this DooDad simply uses a reed which is hunky dory for DC, after getting out my Dremmel preforming an Autopsy it does just what AC said by first sensing the magnetism when it goes plus then latching the out put on. Gets a little complicated and wont' work for what I'll trying to do here.

    Now that we have met let me introduce myself and explain what I'm trying to accomplish.

    I'm Jim a retired Elevator Man. I build some test equipment for the elevator/escalator industry. Nothing fancy just for relay and basic circuit testing. An elevator/escalator Inspector (You know the guy that signs the paper in the frame inside elevators that's always 2 years over due) came to me with his idea.

    When Inspectors do safety inspections on escalators they have to open or trip all the switches inside the escalator (there are about a zillion of them) to make sure the unit will stop. This can be a little difficult. They Safety switches are normally in series with a master safety relay or relays that shut down the unit when one or more of the switches open. He wanted to monitor the Master Safety Relay coil by using non evasive method such as Hall Effect or Reed. This sense output would provide an audio or visual signal. With this working he can move from safety switch to safety switch, trip it, hear or see a signal, close that switch and move on to the next one. Hope this gets for you. The device would be a Big Time labor savor for all escalators and especially the newer units where the controller(where the Master Safety Relays Live) are not always inside the escalator but can be found on occasion in the broom closet inside the unisex bathroom.

    Gotta be Portable as well, Appreciate your thoughts

    Thanks,

    Jim
     
  5. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,804
    1,105
    A simple magnetic compass could probably be used to indicate a change of relay state, but might not be able to tell whether a relay was in the energised or de-energised state.
     
  6. BillB3857

    Senior Member

    Feb 28, 2009
    2,400
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    I'm curious as to how monitoring the magnetic field of the relay coil is going to insure the proper opening/closing of the contacts. Relays have been known to have welded contacts that won't open and contaminated contacts that don't properly close.
     
    mcgyvr likes this.
  7. Elevatorinfo

    Thread Starter New Member

    Jul 12, 2015
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    Thanks All, Compass maybe, as I mentioned earlier. We have to monitor relay state remotely. Yes contacts do weld but in our applications not alot of juice flowing here.

    Thanks Again
     
  8. alfacliff

    Well-Known Member

    Dec 13, 2013
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    you might want to look up "Mag-Probe" a small handheld probe that lights up in the presance of magnetic fields, ac or dc. I have use one here to check relays, transformers, solenoid valves and more for quite a while. and while it will not tell you if the contacts are made, it will tell you if they are supposed to be, or if the current is flowing through a transformer. saves a lot of time, and when given to a plumber, it saves an electrician from having to test a solenoid valve for cureent.
     
  9. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    4,771
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    ^^This is the #1 issue the suggested method will NOT work and should not be used. You are only checking 1/2 of a safety critical part.
    The real reason the inspector is doing that task is to ensure the contact side of the relay is doing its job. Only looking at the coil is not sufficient IMO.

    It is MUCH better to simply add a device in parallel on the contact side of the relay in question..
     
  10. Elevatorinfo

    Thread Starter New Member

    Jul 12, 2015
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    This is getting good..........they (inspectors) before they do anything make sure the "Safe" relay(s) stop the unit. At this point it doesn't look like a probe will work any way. Our original intent was to add a device parallel to the safe relay but we started looking for a non evasive method. The Mag probes I have checked will turn on the output on DC just fine but it seems they have to latch the out put for AC.
    After listening to you guys we are going back to directly monitoring the safe relay. If you guys are interested I attached a diagram of a modern escalator.

    Thanks for yo
     
  11. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    I respectfully disagree. If the job is to test a bunch of things wired together, modifying the wiring doesn't seem to me to be a good first step. And, as stated in posts #1 and #4, the intent of the device is not to test the relay contacts. The intent is to test the switches that control the relay coil by giving an indication when it is energized, and without electrically touching the native wiring in any way. Interpreting that indication is all wetware, relieving the device of all but the most simplistic determination.

    It's more money, but I like the voltmeter clamp-on current probe because it takes physical, close-proximity access to the relay coil out of the question. Clamp-on Hall Effect current probe, preamp, (adjustable?) level detector, missing pulse detector, LED and beeper. A 358, a 555 (got Got GOT to have me one of them), and a 9V battery should do it in a project box the size of a cigarette pack. Ish.

    ak
     
  12. Elevatorinfo

    Thread Starter New Member

    Jul 12, 2015
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    Thanks Again AK and thanks for coming to my defense. Until I get my mitts on Bartal Mag probe we are proceeding with the parallel hook-up.

    To further explain the whole deal..........Inspectors inspect escalators and elevators to make sure they meet code and don't cut off limbs. Elevator Mechanics (now called techs) Service and repair the equipment. To help with this idea the inspectors will ask the Mechanics to attach a couple of test leads to the safe relay(s) coil. With voltages running from 24 to 220 ac/dc the output to the visual/audio signal has to be isolated from any external test gear. So we will use ss relays with an input managed by voltage dividers or resistor network to apply a working voltage to the testing device. Noticed a couple of down loads of the modern escalator. The new attachment goes completely the other way. Old escalator that ran on what we call City DC. Power that ran the street cars (250-500 VDC) used to run escalators and elevators.

    Thanks and Regards,

    Jim
     
  13. Elevatorinfo

    Thread Starter New Member

    Jul 12, 2015
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    Good Morning All You Wizards,

    The Bartol Mag Probe (http://bartolmagprobe.com/) arrived and works as advertised. It would be the perfect solution for our project.

    Senses DC and AC relay coils with no latching for AC. Attached is a pic of it's innards. When I took it apart I expected all kinds of mystery electronic gizmos within the case. Not so, Reed switch. resistor, 6V battery and an LED. It has one and only one serious rub. "Drop it and it's gone forever." After visiting their site and checking out the comments, it happens to everyone. Should have picked up on this because it came packaged as if they were sending "Nitro" in the mail. My best guess it that the Bartol people somehow rearrange all the little Neutrons and Electrons within the reed switch at some ultra secret facility under the South Pole. When dropped all these little guys quit holding hands and bingo all you have a a little blue and red thing that could pass for a rectal thermometer.

    Your Thoughts All
     
  14. Elevatorinfo

    Thread Starter New Member

    Jul 12, 2015
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    Please don't run off,

    Need your help. Please check out the attachment. If I could use this one it might fill the bill.

    You Thoughts Please,

    Jim
     
  15. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    This is a conducted current sensor, and has features specifically to reject the influences of ambient magnetic field, exactly what you are trying to sense. I've used other MR sensors, and they always seemed a little weird.

    ak
     
  16. Elevatorinfo

    Thread Starter New Member

    Jul 12, 2015
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    Thanks for your reply..........."The Magnetic field produced by AC and DC relay coils.

    Jim
     
  17. MrAl

    Well-Known Member

    Jun 17, 2014
    2,439
    492
    Hello,

    You should also do a test of the relay coil(s) you intend to use this with. Many coils have a core that has a wide gap until the relay is energized. Some short time after it is energized it will show a magnetic field that radiates out from the gap, but as the gap closes the field will be reduced dramatically. There will still be some gap, but it could be very small, which means most of the field is contained within the core material.

    So you will have to test the different relays to see how it works with each one, and if you can place it closer to the gap it will be the most sensitive place.

    Just to note, when you try to rewrite tests that have been done for years and have been proven to be effective, you take the life of your career into your own hands. If you overlook even one small problem, it could cost you everything. At the very least you should hire an engineering company to research the new test procedure and new test equipment used for that procedure.
     
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