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Old 12th November 2008, 05:55 PM   #1
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Default Any Hall Effect gurus out there?

Can you help?

I'm looking for a very cost effective method of sensing low current ac and dc in a 12ga copper conductor which cannot be disconnected, tapped into, or looped. This requires the use of some form of split core ferrite/collector, and a very sensitive chip. The current in the cable is constantly moving, ranging from one amp to approx 250 amps. It is water cooled (explaining the small ga.) and the total size of the cable with water jacket is approx 8mm. This cable supplies a welding current to a torch. No metal shielding on the cable.

I'd like to apply a non-contact sensor to this cable to determing when there is current and when there is not. Threshold of approx 2 to 3 amps. Basically, enough to know if an arc is present at the torch or not.

There are many, many forms of HE sensors out there, and many of them bidirectional (to my understanding this indicates that they are capable of sensing ac and dc current). There are as well some magnetoresitive sensors out there which are more sensitive yet, but appear to be less straightforward in ease of use. Important since I'm not an engineer and must rely essentially on what I can glean from application notes accompanying the devices.

Here's one HE device I've identified as possible,
http://search.digikey.com/scripts/Dk...me=620-1213-ND

Here's one MR device I've found as well,
http://search.digikey.com/scripts/Dk...me=480-1997-ND

And finally, a solid core unit that seems to be capable and only 18USD, but alas, not split core accessible,
http://sensing.honeywell.com/index.c...1&pr_id=146381

I have eventually found versions of the above, completed for use, and split core. They are extremely expensive. For some reason, perhaps there is only a very small demand in the market, there are no budget variety ac/dc split core sensors on the market. It looks like I could fab one pretty easily with the first two devices I've linked by epoxying them into a gap in a split ferrite ring.

It's difficult for me, with limited electronics language skills to ascertain from the data sheets if these are actually capable of meeting my needs.

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?
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Old 12th November 2008, 07:17 PM   #2
albin is offline albin  United Kingdom
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have you searched Rogowsgi
probably not cheap tho
regards
max
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Old 12th November 2008, 07:52 PM   #3
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Rogowski is ac only. There are other very simple (relatively) methods of detecting just ac. Flux Gate devices can be whipped up with various bobbins and lacquered wire.

The HE chips like the first one I linked to have all the bells and whistles contained within for ac and dc detection with just a little signal conditioning necessary afterward to take advantage of both sensitivities. Datasheet..
http://www.allegromicro.com/en/Produ.../3245/3245.pdf

I'm just hoping that someone out there has actually fiddled with one of these chips in a similar application. I get, essentially, the method of physical assembly. The chip is cemented into a gap that is ground into the face of one side of a split core, or it is inserted into a gap cut out of a solid ring. Then is it a matter of just an opamp or transistor, and one or two resistors and caps to turn it into an ac/dc current detector.

If anyone knows of a schematic for such a circuit, I'd be ever so grateful.
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Old 12th November 2008, 11:46 PM   #4
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Can't remember the number, but you should try to find/download a manual for the old HP AC/DC clamp-on current meter. Lots of good info on flux gates and the circuit could probably be done with far fewer parts today.
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Old 13th November 2008, 12:27 AM   #5
infinia is offline infinia  United States
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Hi BB
Just some thoughts...
The DC capabilty is the real driver for using a HE sensor. If AC current sensing is the only reqirement then your task becomes much easier. I don't know much about welding but perhaps the DC has some AC component that could be taken advatage of. Maybe a dual current sensor could be devised to take this in consideration. I think if you don't care about absolute current accuracy, might free up the requirement of a HE?
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Old 13th November 2008, 03:22 AM   #6
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There's a good possiblity that the DC welding current will be impure enough to do so. Even the arc itself probably would contribute to that. But with such low amperage, I'd just have to do a lot of experimenting to see about that. I'd rather aim for something that is dedicated to the task. The spec sheets on some of these devices indicate this ability.
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Old 13th November 2008, 03:24 AM   #7
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EDN had a sensor circuit based upon an Allegro chip a few months ago -- search on their site.
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Old 13th November 2008, 05:28 AM   #8
poobah is offline poobah  United States
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F. W. Bell and/or Sypriss has sensors that will do the job. There WILL be some signal processing and circuitry to build.

Wherever split cores are involved... price will be an issue. It is simply because of the precise machining that is involved to provide any decent level of interchangability/accuracy.

Fluke makes reasonably priced current clamps. Assuming that you see the 2 Amp reading on a meter (easy)... what would you then DO with that signal?

I ask because your biggest stumbling block here is not the sensor... but rather how you need to interface that sensor to something else...

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Old 13th November 2008, 01:22 PM   #9
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I found one of the articles in my files: "Magnetic Field Measurements hold the key to reducing dc/dc emi" by William Bowhers of Merrimack College -- June 20, 2006. If you put Bowhers in the search engine the article should come up.
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Old 13th November 2008, 04:05 PM   #10
jcx is offline jcx  United States
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GMR sensors may be worth looking at

nve has analog sensors and current measurement app notes, poke arround the site

a summary white paper:

http://www.nve.com/Downloads/lowfield.pdf


there are also more sensitive hall materials than Si (InAs):

http://www.asahi-kasei.co.jp/ake/en/product/index.html

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