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Old 13th November 2008, 05:22 PM   #11
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Here's another article: "Magnetic-field probe requires few components" -- by Rama Sarma in Hyderabad. EDN Dec 15, 2006 -- uses an Allegro A1323.
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Old 13th November 2008, 06:47 PM   #12
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Default Re: Any Hall Effect gurus out there?

Quote:
Originally posted by bluebeard
Can anyone comment on whether I will be successful at this --given that I am not capable of much in the way of signal conditioning once I've put these two items together?
If you're lucky and the threshold is fairly well defined then you may get away with something as simple as a comparator to drive an LED, but until you get some idea of what kind of output you can generate then there's no way of knowing.

These Allegro components are pretty cheap, and there's quite a lot of conditioning built in. I'd buy one, cobble up some kind of clamp (maybe out of some regular [iron] plumbing fitting or maybe you can cut a ferrite with a diamond disk), power it up in situ and put a scope across it and see if anything is visible in the conditions you are interested in. You might need to change the device orientation to get the best (any) result. Give it a local supply with a 7805 and a couple caps and you won't blow it up.

If the trace is lumpy then you may have to think some more (maybe a unity gain buffer and an RC filter), but if there is a reasonable offset well distinguished from the noise then you're home and dry other than checking that it's stable with temperature...

w
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Old 13th November 2008, 07:37 PM   #13
jcx is offline jcx  United States
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flux concentraors may require a little care in material selection - you need hysterisis < 1% to detect 2A followng some time at 250A

a quick look at NVE sensor chips suggests that they might work without flux concentration - or with a low concentration factor making the shape and tolerances easy
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Old 13th November 2008, 08:31 PM   #14
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There are many flux sensors out there intended for current measurement and with integrated stabilisation and amplification, Have a look at Honeywell and LEM. But they are all quite noisy. A range of 1A 250A is on the edge.
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Old 13th November 2008, 08:44 PM   #15
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What is often overlooked or unstated with these sensors is that:

A sensor with a stated range of 1 to 50 Amps... may be perfectly capable of carrying 300 or more amps. The reading will, of course, be erroneous. But, no damage will be sustained. You must check with the manufacturer.

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Old 14th November 2008, 02:38 AM   #16
anatech is offline anatech  Canada
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Hi bluebeard, Conrad,
I think you are looking for the old HP 428B DC amp meter. The aux output will respond accurately to an AC signal. I think it was limited to 200 Hz or something in that area. I have one I'm going to restore and use.

These are vacuum tube type instruments, but I think it's the principle you are after. The service manual explains that in detail. Basically, it's an AC excited coil (chopper amplifier) with a type of discriminator. You aren't looking for extreme accuracy or repeatability. Therefore this might answer your questions. You can easily scale the range to what you need.

Have a look on the HP web site HP archive site under equipment. My link dumps you there. I see there are two versions for this meter.

Good luck with your project!

-Chris

Edit: I forgot to mention how accurate this meter still is. It surprised me, so much so that I feel it's a keeper. Yep, I intend to rebuild and continue using it.
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Old 14th November 2008, 06:20 PM   #17
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Thanks so much for all of the input.
The HP articles are very interesting reading for some of the physical considerations of this thing. Poobah, that really is an overlooked aspect of these things. The application sheets seem to take for granted this understanding, but it is in no way universal. I had more than a couple of engineers tell me that amperage in excess of the rating would destroy the sensor. Most, however, told me that the device merely 'saturates' harmlessly and since I'm not concerned with accuracy above my nominal threshold, not to worry.

I've got some samples of the Allegro Hall sensor A3245LUA-T due in the mail as well as a few samples of an asahi-kasei sensor
http://www.asahi-kasei.co.jp/ake/en/.../eq-711l_e.pdf

I've got enough ferrite rings laying about the shop, but I ought to look around for a well made split core of the proper end profile to fit the sensor best. Absolute machined tolerance is not that important since real accuracy is not essential -- only sensitivity. The asahi distributor even faxed me a little circuit necessary to get me up and running. There's a trim pot identified that will allow me to adjust my operating threshold after I get everything clamped together, and then I can simply replace that pot with the right resistor value.

I'll drop back in here in a week or so when I've had some success (I'm optomistic). I never fails to impress me how deep and wide the range of electronics skills are in diyaudio. Thanks so much.
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Old 14th November 2008, 06:43 PM   #18
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Keep in mind that all other things aside, your reading will be roughly proportional to the width of the gap in the ring. So... while absolute accuracy in machining is not required... a stable mechanical setup is. You will want to keep that gap as stable (and then some) as the measurement you seek. You'll also want that gap as small as possible to get useable flux.
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Old 14th November 2008, 07:08 PM   #19
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Good point. This thing will be taken on/off occasionally, so I'll pay close attention to the hinge and snap accuracy. I may even be able to source a well made snap-on ferrite choke with nylon housing of the right dimensions and grind a gap into it for the sensor.
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Old 14th November 2008, 11:58 PM   #20
anatech is offline anatech  Canada
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Hi bluebeard,
That big hairy fella knows more about this subject than I do - for sure.

What you could do is cut the ring in half, then cut one of the halves again to mount your sensor. Then align it and use epoxy to make a solid half assembly again. After that, it's only a matter of maybe more epoxy to mount and align a hinge and snap assembly for the other side. That should be stable enough to give repeatable measurements. If you use a test loop and pass an amp through it, you can calibrate the unit every time to make sure. If you loop twice, one ampere will read as two amperes.

Finally, your results will be better if you polish the mating ends very smooth and parallel so they come together with minimal gap. Should work fine until it's dropped.

Hi Poobah,
Really good to see you!

-Chris
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