Noninvasive current sensing.
Noninvasive current sensing.
(OP)
I have a single cable with 24vdc pos/neg within that switches a load of approx 200ma. Is it possible to determine when that line is active (switched on and running the load) by any noninvasive means currently available? Whether Hall Effect or other transducer/transformer?
Ideally, I'd like to be able to monitor such a line for on/off status without having to open the line and separate the conductors. I've been told that this is impossible - that any (noninvasive) attempt to detect a load will be cancelled out by the balanced lines.
Is this true?
Ideally, I'd like to be able to monitor such a line for on/off status without having to open the line and separate the conductors. I've been told that this is impossible - that any (noninvasive) attempt to detect a load will be cancelled out by the balanced lines.
Is this true?





RE: Noninvasive current sensing.
- You don't want to interrupt the current?
- You don't have physical access to the cable?
- You don't have legal access to the cable?
- You need to install the sensing in a remote location, where e.g. good power sources for the sensor are not available?
- You need to develop a technique and equipment to do this rapidly/ many installations/ with unskilled labor?
- You don't have any money?
I.e., I don't think it's impossible, but it may be difficult and/or expensive.
Mike Halloran
Pembroke Pines, FL, USA
RE: Noninvasive current sensing.
If it's a purely resistive load, that's much harder. There might be some detectable temperature change, but's probably pretty miniscule. The magnetic field ought to be neutral, but at very short distances, there might be some differences.
TTFN
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RE: Noninvasive current sensing.
A hall effect device probably will not work to detect the miniscule magnetic field. They also have temperature problems. A flux gate can be made very sensitive and has much less temperature problems. Use two of them and wire them so the cable's field adds but earth's field and other magnetic fields cancel.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Noninvasive current sensing.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Noninvasive current sensing.
RE: Noninvasive current sensing.
RE: Noninvasive current sensing.
For this to be ultimately useful, it would not require any alteration to the machine or it's accessories. My first approach was to insert an adapter with pigtail between the pedal/rheostat control connection on the welder. Unfortunately, there is no industry standard connection. Next I considered a clip on current probe for the control cable. So far this seems impractical given the requirement that it not require alteration to the unit or cables.
Next I consider a clip on current probe designed for the work or ground to torch lead. Here the probe must deal with 0-300amps AC and DC, pulsed current, and exotic wave forms. Probably a deal breaker by virtue of complexity.
One more current option would be to conceive an adapter at the power outlet to monitor current at idle/energize. This would be mechanically very expensive and again, there are a number of standard outlets, as well as two and three phase considerations.
Lastly, I could put a flow meter inline with the argon supply. Essentially this would monitor the internal gas solenoid of the welder and be relatively nonintrusive and actually quite reliable. Approx 22CFH at low pressure. Off the shelf switches of this type are anywhere from 300 to 800USD. Again, I'm frustrated with being unable to find an inexpensive (20-30USD) solution to this part of the project.
Any other ideas?
RE: Noninvasive current sensing.
If you care about the fact that the argon could run on for up to 20 seconds after welding ceases look a little harder for cheap low pressure sensors. You should be able to find ones in the sub $10 range that would work just fine.
Keep in mind that often the argon circuit is NOT used in some TIG welding as flooded work jigs are used instead.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Noninvasive current sensing.
No need to deal with mains voltage or high current.
Just an idea, don't know how feasible it may be for your setup.
-AK2DM
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"It's the questions that drive us"
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RE: Noninvasive current sensing.
Why is this a dealbreaker? It's just a simple thresholding requirement to detect current va. no current.
However, this seems to be a rather strange device. If the welder is running, I would think that only a legally-blind operator would fail to see the smoke and whatnot coming from the workpiece.
What exactly is the INTENT of this exercise?
TTFN
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RE: Noninvasive current sensing.
RE: Noninvasive current sensing.
RE: Noninvasive current sensing.
It's a $400 tool, with its relatively high accuracy (0.2%), not what the OP is asking for, but what technology does it use? Something that can be adapted for on-off detection at 10x the magnitude of the range of the Fluke?
RE: Noninvasive current sensing.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Noninvasive current sensing.
Similar arcs used to be used for radio communications...
Using AC techniques would eliminate many of the issues mentioned above.
RE: Noninvasive current sensing.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Noninvasive current sensing.
RE: Noninvasive current sensing.
I need to tell (trigger with a signal level relay) a separate process control when there is arc established, and when it is stopped. This needs to function with any tig welding machine and be non contact (not require any modification of the machine/process). I only need to recognize/sense the first couple of amps through the cable. When that threshold is reached and lost.
The HF start would not be an issue. The trigger does not require strict accuracy in this regard. It can activate upon application of HF for arc starting since the arc will follow anyway in short order.
Ideally I could clamp on the pedal/rheostat control cable (pedal on.. pedal off) but here I don't have wires separated out as with the torch current supply, and I'm dealing with only milliamps to boot. Perhaps the fluxgate as was mentioned can be investigated. I'll read that link.
1 - 300 amps range is mentioned only to point out what the process is capable of. I am only concerned with the first few amps. In need to sense arc starting and arc stopping. I need to identify it at AC and DC, but as has been pointed out, in this application I may be able to use AC sensing and still get by.
I've spoken with a few manufacturers now and generally have been told that one or two amps is going to be extremely difficult to sense. The current supply to torch is approx 12ga copper with a water jacket, the whole being approx the diameter of a bic pen. No telling what the water jacket may effect on sensing. The return is commonly a 2ga copper ground wire. Non-contact sensing two amps through this is, I'm told, pretty impossible.
MR sensing technology has been suggested a few times, but noone seems to be very up to speed on it. Finally, it was suggested that I simply get this
http:/
and just start experimenting!
I had no idea this was going to be such a can of worms.
RE: Noninvasive current sensing.
I have mentioned two times that a flux gate can detect the flux unbalance from the 200 mA DC current going in two wires with opposite current direction. A flux gate does not have temperature issues (like Hall elements and MR sensors). It just works. You may have to shield it from the Earth's magnetic field, but not always.
Again: have a look at http://www.speakesensors.com/ I have used their products in custom RCDs for DC. Trip level between 20 and 30 mA achievable with no sweat.
Is my english so bad that it can't be understood? Or is it the flux gate technology that can't be understood?
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Noninvasive current sensing.
and the sales guy said that 300amps would certainly break it.
I'm finding diy sheets on how to physically split round iron cores and jimmy in hall effect sensors to do this stuff. Am I really ruduced to that? Is this such a theoretically esoteric thing to do that I can't find a <20USD fixed point low amperage sensor/gaussmeter/etc to do this?
I've had a few engineers tell me that it's impossible to measure low current (by magnetic field) in a #2 copper cable. I've had a few others tell me this is nonsense... that one amp is one amp through any sized cable and will eminate the same measurable field around that cable.
AAAAAARRRRRGGGGGG!
RE: Noninvasive current sensing.
No, I did actually mention that in the previous post. I went to that website. There are no international/U.S. contacts. Prices look right. The circuit descriptions give little clue (to me anyway) as to how the devices might be used for cable current sensing.
I googled "flux gate" and "flux gate switch" and was unable to find any manufacturers/suppliers other than the link you provided. Is it a question of nomenclature? Are these available under varying descriptions? Perhaps Digikey even stocks this animal, but under a different description.
Can you cite any more links that might describe the use of these in the sort of application I'm involved with?
RE: Noninvasive current sensing.
OK. I will try and find some more stuff. Stand by a couple of hours
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Noninvasive current sensing.
Experiment: wire loop and oscilloscope.
RE: Noninvasive current sensing.
But, you are right. There are not many products available. I think that is beacause it is so simple to roll one's own. There are many practical and theoretical papers in the 149 hits and I think that a winder could make you the magnetic circuit (copper wire and high-permeability iron). The electronics is simple.
Not exactly what you may want to pursue. But a way out if nothing else works.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Noninvasive current sensing.
I believe that for the time being I'm going to order this
http:/
and epoxy it into a gap in a split iron core. I'll clamp that around the output lead to the torch and start experimenting.