Hall-effect Q's?

Discussion in 'General Electronics Chat' started by pntrbl, Aug 30, 2008.

  1. pntrbl

    Thread Starter Active Member

    Apr 21, 2008
    123
    0
    I'm trying to get an AH175 hall-effect switching with for my anemometer project with no joy to date. Digging around in the datasheet I see specific gauss ranges for Bop(operating point) and Brp(release point). I have no way of measuring those but I'm wondering if the 1/4" by 1/2" long neo cylinder I JB Welded it to is overwhelming it with way too much flux ..... because it sure seems to be stuck in the "on" state!

    Running on 12V. Got a 1.2K pullup on the OUT pin with a clipping diode to Gnd. I should see .7v from the diode in the "off" state but all I ever see with my DMM is <.2v. I believe it's sinking it's 10 mA from the pullup and staying that way.

    Realizing a DMM is not ideal I've got WinScope on my PC but that's not helping much either.:mad: As soon as I plug the power supply in the thing goes full scale with 60 cycle noise! What's with that? No volts/div so I can't quantify it, but turning the gain down to almost nothing seems to get it under control.

    It's probably safe to say there's a chance that it is in fact switching and I'm not using the WinScope good enough to see it, but right now I'm leaning towards a weaker magnet.

    Any and all opinions and/or advice is as always ......

    SP
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    I didn't realize you were planning on JB Weld-ing a big neodymium magnet on the Hall-effect sensor. Yes, you've definitely pegged the sensor. Can you get the magnet off without destroying the sensor? Or, do you have a couple of extra sensors?
    It probably is staying pegged. Unless you have an easy way to measure Gauss (I don't) you'll have to experiment with different distances and/or different size magnets. Basically, you want to find the half-way mark between always on and always off when you're spinning the anemometer reluctor (the 8-point thingie you scrounged).
    Sounds like you either have a floating ground, or you have a ground feedback loop. You DID tie the audio ground to your Hall-effect supply ground, right? That's what I was trying to infer by having grounds shown for the jack and the plug; all grounds needed to be tied together.
    I think it's safe to say that J-B Weld-ing the neodymium magnet to the sensor before doing some experimentation was likely not the best approach. :D

    You just need a very small magnet. You'll have to find out how far away it needs to be from the sensor by experimentation.

    See the attached. You'll need to keep fiddling with the A and B dimensions until you figure out what's optimal.
    As far as the distance from the Hall-effect sensor to the reluctor (dimension A) you basically want it as close as possible without hitting it, but allow for thermal expansion/contraction and bearing wear. 30 mils might be a good number.
     
    Last edited: Aug 31, 2008
  3. pntrbl

    Thread Starter Active Member

    Apr 21, 2008
    123
    0
    Thanx for the response Sarge. I've got some more AH175's so some experimentation is next up. Good thing too. I initially set up with an .020 air gap and my God ... what a Cog! :D Anemometers need to spin freely and that neo was suckin' on every pole. Eventually I had the air gap over .125 and it was still trying to grab every pole that went by.

    Looks like I've overdone it once again ..... lol.

    On the WinScope. For an input to my sound card I cut the wire off of a mic. It's a single wire but shielded so I tinned up and connected the shield to the common/blk/gnd post on the supply. Just ohmed it to be sure and it is connected.

    The ground feedback loop is not something I understand, but I'm interested. This particular ATX supply has a switch in the rear that I've been using since the green wire refused to co-operate, and even with that off I still get full screen 60hz noise. All I have to do is stick the AC cord in and boom, there it is.

    I wish I could measure the amplitude on it. It may just be natural low level noise. I do know a .01uf across my hall didn't phase it any cuz I tried one .....

    SP
     
  4. pntrbl

    Thread Starter Active Member

    Apr 21, 2008
    123
    0
    Just caught your edit Sarge. It would be very easy to use Alu between the hall and the neo. I'm thinking benchtest with a screwdriver to disturb the flux in the hall enough to switch. That way I'll know how deep to insert the neo into an Alu holder that will have the hall JB'd on.

    Yeah! Off to da shop .....

    SP
     
  5. pntrbl

    Thread Starter Active Member

    Apr 21, 2008
    123
    0
    More news! A hair dryer I use for shrinking tubing did the trick on loosening up the JB and I was able to verify the hall does shut off. With no magnet attached I see .7v just like I was supposed to. I get a North pole within 3/4" of the back of it and it switches on .....

    And then stays there! LOL! I guess I knew they latched, but now I'm wondering how on earth is that gonna work in my anemometer. I can flip the mag and bring a south pole in to unlatch it, but my anemometer can't do that.

    Hmm. The more I learn the less I know. :D

    SP
     
  6. pntrbl

    Thread Starter Active Member

    Apr 21, 2008
    123
    0
    Got the fine tune goin' on now. With .900 of Alu between the North pole of that stout little neo and the back of the hall it does NOT switch on ... until I pass a screwdriver nearby! :D

    I'm still not understanding the latch tho. Obviously I have zero experience with these devices but the datasheet says it's useful for rpm detection. If it latches low on the 1st pole I'd like to know how .....

    SP
     
  7. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    That Neodymium magnet is so strong you'll probably have to get it quite a distance from the Hall-effect sensor before it'll turn off.

    N45 material is about 12,000 Gauss. Your Hall-effect sensor turns on at somewhere between 5 and 15 Gauss. You have indeed gone for maximum overkill. ;)
     
  8. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    OK, we cross-posted. :)
    Well, get that anemometer reluctor in there and see how it's switching with that.

    Remember, the Hall-effect sensor wants to see a certain polarity of magnetism for best results. Experimentation is the key here.
     
  9. KMoffett

    AAC Fanatic!

    Dec 19, 2007
    2,574
    230
    neo cylinder...JB Weld :rolleyes:

    I tried that once. Seems that JB Weld is epoxy "filled" with a ferris metal. T'was interesting to watch the liquid epoxy creep "up" the lines of force surrounding the magnet, before it cured. :)

    Ken
     
  10. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Now that's something I hadn't even considered! :eek: Very useful tidbit of information. Sounds like our OP is not going to be able to get JB Weld anywhere near the Hall-effect sensor without changing the magnetic lines of flux, thus throwing off any pre-J-B Weld "tweaks".

    Well, red silicone RTV would work pretty well, I'll bet - perhaps a bit on the flexible side, but that's OK.
     
  11. KMoffett

    AAC Fanatic!

    Dec 19, 2007
    2,574
    230
    Plan old garden variety epoxy should be fine.

    Ken
     
  12. pntrbl

    Thread Starter Active Member

    Apr 21, 2008
    123
    0
    As KMoffet surmised it was very interesting dropping JBWeld on a neo. Talk about flow! But it actually "welded" up real nice. Had a good looking bead all around it that held just fine. I had to use a heat gun to separate them.

    I did mess up the end of the neo a little bit on a grinder and even got a small hole in it with a carbide drill to give the JB some traction. And that was funny too in that I couldn't get the JB to flow into the hole. It was like it was polarity challenged in there and it would not go!

    One tube is labeled Steel and the other Hardener. Apparently there's some truth to that ....

    SP
     
  13. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Heat is not good for high-strength neodymium magnets. The really high-grade magnets can be destroyed at 175°C. Lower grades might be good up to 200°C or higher.

    I bought some really small neodymium magnets for Hall-effect sensor testing, but I've never taken them out of their packaging. They're about 1/8" x 1/8" cylindrical magnets. Even those tiny things are probably far too strong to be directly attached to the back of the sensor.

    If you want to try something interesting, drop one of your neo magnets down through a piece of copper tubing (onto a soft surface, of course). It'll take much more time to fall through the copper tube than you would expect. ;) This is due to the induced eddy currents in the conductive copper tube; it acts like a brake.
     
  14. pntrbl

    Thread Starter Active Member

    Apr 21, 2008
    123
    0
    I think I've learned hall effects and neodymium magnets don't play well together. The ones I'm using are only 1/2" long but they'll flip a hall from almost an inch away.

    Because I inadvertently selected a latching hall sensor I've given up on the neo's. I've also recently learned how fingers can be wrapped around a magnet to create alternating poles around the circumference, and then realized that's exactly what the GM rotor looks like before I rape it for the 8 pointed plate.

    So I ran off to the junkyard for another rotor, tried it out in it's entire state, and viola! It works. Here's a pic.

    [​IMG]

    I've been holding the hall close by hand and it's switching. Rotating it slowly it'll show up on the DMM and I'm seeing high speed pulses in the middle of all the 60cycle noise on WinScope too.

    We got this one covered and Thank You very much gentlemen.

    SP
     
  15. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Very interesting! What motor did you pull that rotor from? I've not seen one that looked like that - but then again I haven't ever seen a GM V8 distributor that was made in the last 20 years or so, either.

    You just might have a winner there ;)
     
  16. pntrbl

    Thread Starter Active Member

    Apr 21, 2008
    123
    0
    I believe the rotor is out of GM's 2nd generation of electronic ignitions. 1st we had the HEI where the coil was mounted in that giant distributor cap, but somewhere around the late 80's, and in conjunction with fuel injection, this system showed up. Any GM car, van, or pickup with a 350 V8 should have it.

    SP
     
Loading...