low current hall effect sensing

[wsemajb]

Hello.

I have a 12ga copper wire with two amps current. Voltage is approx 12v. Current is sometimes ac, sometimes dc. Rubber cladding around the wire increases the total cable diameter to approx 8cm.

I'd like to find a noncontact method of switching a relay on when the current is present in this wire, and switch off when current is not. I've so far only identified the following,
https://www.electro-tech-online.com/custompdfs/2008/11/3245.pdf
and in application notes at the Allegro website they describe the use of a flux concentrating ring to use these devices for low amperage. I need this sensor to be clamp on, so I'm looking at split core rings. I'll probably just have to saw one in half.

It only needs to operate a switch/relay and further accuracy is unimportant. All I need is signal to switch a relay when this approx 2amp +/-1.0 amp threshold is made (current running through the measured wire). The unit above has an onboard transistor cabable of switching a signal level relay. That works for me.

Basic idea looks simple enough in the spec sheets and application notes. Does this need any further signal conditioning to make use of its ability to sense ac or dc? Should I just order one of these hall sensors, a ferrite ring with an 8mm hole, and just start experimenting? I'd have to cut the ring in half, shave a gap sufficient to epoxy in the sensor, and then clamp the thing onto my cable. Is bigger better with the ferrite ring? How do I set the switch point and release point of this IC?

I need to make this from scratch with parts out of digikey, mouser, et al. This isn't a homework assignment. It's just that I need to keep the total parts cost below about 15USD or this won't be practical.

Advice would be heavenly!

 

[crutschow]

The hall effect device you chose will switch on and off at a 120Hz rate when the current threshold is reached. Is that acceptable?

And the tolerance of the device is quite wide (the operate point varies from 15 gauss to 55 gauss) so you would have to be able to adjust the gap or adjust the magnetic field by some other means to calibrate the trip point for a particular IC.


The ferrite ring need be no thicker than the diameter of the hall effect IC and with a diameter just sufficient to go around the wire. Incidently you said the wire was 8cm in diameter, I assume you mean 8mm?

Considering the caliabration issues, it might work better to use a linear hall effect voltage out device with an external comparator to determine the current trip point. That way you can adjust the current trip sensitivity with a simple pot to set the comparator voltage trip level.

 

[wsemajb]

Any sensor has to be bidirectional (ac/dc) for this to work.

As far as set point, I really only need this to switch when ANY ac/dc current is detected, and the smaller the better (barring, obviously, anything weakly extraeneous like the earths field or speaker magnets in the same room).

There are two aspects of these sensors that I'm unable to find unequivocal answers to. First is the specifice range. If a sensor specifies 1 amp to 60 amp range, does that mean measurement or tolerance? Ive had some engineers tell me that it means measurement, and that exposure to additional current in the line will have no effect on the device. Others have told me just as certainly that this spec means that currrents over 60amps will produce enough field to destroy the device. None of the technical sheets on these devices shed light on this.

Another question is whether I can use the 2ga copper ground wire for current detection. Some engineers have said that 2amps in such a conductor will not produce sufficient gauss to be measurable. Others have told me this is nonsense... that gauss is measured from the center of the conductor and mass has nothing to do with the existance of a measurable field except that the sensor will be necessarily farther from that center with such a fat conductor. Which is true?

If I could use the ground conductor It would offer some advantages.

 

[MrAl]

There are two basic types of magnetic hall sensors. One is a switch type like what
you looked at, and the other is a linear type which puts out a voltage proportional
to the magnetic field strength.
The switch type have set switch points, and you have to have a signal and
core combination that can take advantage of the switch points.
The linear type have a continously variable output, so you can use an external
ic comparator to trip when needed, which makes it a bit easier to adjust.

The range of a device say for 1 amp to 60 amps could mean for one device
that it works within that range and doesnt work outside that range,
but for another device works within that range and blows out above that
range. It depends on the actual part being used. If you dont know, use
the 60 amps as an absolute max and dont go above that.

2 amps can be detected in a conductor using a magnetic core like you seem to
be considering. It's also possible to wind the wire around the core several
times (rather than just once through the center) to increase the apparent
current level because this increases the magnetic excitation and thus
produces a more intense magnetic field. The wire goes in one side, out the
other, and then back in the other side and out the other again, and this
constitutes two turns (instead of just one). The field increases and so
the hall detector will be able to detect the presence of current more
easily provided the core does not saturate. If you had a magnetomotive
force of 2 amps with 2 amps and one pass through the center, then
you end up with 4 amps with 2 amps and two passes through the center,
and 6 amps with 2 amps and three passes (three turns) through the
center, etc., so the more turns the more field as long as the core
doesnt saturate. With an air gap to fit the sensor it probably wont
saturate anyway, but if it does by then there should be enough field
to be detected by the hall device.

2 amps also isnt that high, and 12 gauge wire is an overkill. You could
for example wind the core with 16 gauge, then connect the 12 gauge to
the 16 gauge. In other words, you make a little inductor with an air
gap with several turns of 16 ga wire on it and then connect it to
the 12 ga circuit wire.

Experimentation depends on what device you you choose...HE switch or
HE linear. With the switch device you have to keep adding turns until
you get the device to switch with the desired dc current level.
With the linear, you have to get a decent change in output voltage
with the desired current level. In both cases you measure dc
current and adjust something until you get a switch transition to
occur.

You also have to consider your desired current level, is it really
2 amps or does it also have to work down to 1.5 amps?

With AC instead of DC, the output will be pulsing as someone mentioned,
so you'll have to use a detector of some sort like a peak detector
to average the output of the hall sensor or the comparator.

The permeability of the core affects the operation too, and usually the
higher the better, but sometimes even a small metal object like one
of those black paper clips (binder clips) with the wire run through
the opening and the hall effect device placed between the bind faces
of the clip. The metal of the clip amplifies the field and the
field is concentrated in the metal and follows the metal path, and
the hall sensor, being in the magnetic path, detects the field.

A small coil alone could work too. The hall sensor is placed
in the middle of the coils, as if to separate the coil into
two coils with the sensor in the center of the two. The more
turns the higher the magnetic field and the easier it is for
the sensor to pick this up.
Two steel nails, one inserted inside each coil, would help to
amplify the field also.
The hall sensor has to be oriented correctly however, with
the poles aligned with the axis of the coils.

For cutting the core in half, it is possible, but you probably need
a Dremel drill and a cut off wheel. A diamond wheel works best.
The cores are very hard. You can also wrap the core in a rag
and clamp it in a vice and turn the vice until it just cracks. You'll
get around 4 pieces that you can glue back together to make two
halves. This is a little messy but works. Using super glue the
gaps dont amount to very much once glued back together,
but then you'll be inserting the sensor anyway.
If you intend to experiment this way it's best to purchase
a few extra cores.

They do make very sensitive HE sensors too, but they may
not fit your budget. They might be sensitive enough to
simply tape to the wire and measure the output.

 

[wsemajb]

Thank you very much for that.

multiple passes in either case, the 12ga lead or the 2ga lead is not going to be possible. This is actually a process control on a tig welder that I'm working on. I would like to be able to use it on any tig welder in the field.

The 12ga stated is the approx size of the copper inside of the water jacket of the lead on a water cooled tig torch. With jacketing, it comes to about 8mm dia. This realistically cannot be looped inside of a sensor ring. Nor can the 2ga copper ground wire.

Connecting the 12ga to a 16ga, if I understand your suggestion correctly, violates the non contact, non alteration, requirement. Pulsed output on these machines should not be an issue since in every real case of tig welding there is a background current setting that the current never dips below, so as never to actually extinguish the arc. The current would be pulsing, but well beyond the 2amp range I'm hoping to work with.

The extremely sensitive HE devices out there actually might work and are not beyond my budget. The ones I found are listed on honeywells SSEC, solid state electronics center, webpages and used for cell phone embedded solutions, geographic survey equipment, etc. The technician there explained however that these are considerably less "stand alone" in their intended design and require pretty extensive engineering skills to implement in final designs.

Seems like I'm in a no mans land between industrial and more sensitive sensing applications. Maybe I can't do this. At least without hiring an engineer.

As a postscript..
I did find this
**broken link removed**
and it would appear to suit my needs precisely. 18USD at Digikey. There is bound to be some devil in the details/specs that disqualifies it, but I cannot see it yet. All I need is to find a split core version, or purchase this one and attempt to halve it myself, or identify a version with a big enough through hole to accept my 2ga ground wire (since these ground wires are very simple to separate from their dinse connections and I could actually apply one of these in the field. Remove dinse connector, put the cable through the sensor, and reconnect dinse connector)