I was trying to use a torroid core with a slot cut in it and a hall effect device placed in the slot to detect the current through a wire. (Image 1 & 2) I used a A3144 (On-Off) and an Honeywell SS495A (ratiometric). With the A3144, with the current one way, it would turn on and stay on, I had to reverse the current to get it to turn off, with the SS495A, it started with one voltage reading, I would add current, and remove it and then have a different voltage reading, add and remove current again and the reading would change some more. It finally dawned on me that the torroid core was magnetizing. My next attempt at a solution was to use a ferrite core induction (#3) and things worked like I thought they should, the A3144 turned on and off at about the same current level each time and the output of the SS495A returned to it's static level after current was applied and remove. I haven't tried inductor #4 yet.
Jeff
Try the test again, if the 0 reading is caused by remanence then the reading will be completely different next time.
Hi there,
I dont know where you got that drawing from but that's not the way to do it. It looks like a very basic way to construct a current measuring device, but only in the most basic sense. A practical design needs magnetic feedback.
In the more practical design, the op amp you see in that drawing is used with a driver to energize a SECOND winding on the same core. This second winding and current in it cancel the magnetic field effect from the current in the primary winding (current to be measured). Because of the turns ratio, when the current in the secondary is a certain level that exactly cancels the current in the primary, the secondary current is related by a constant to the current in the primary. So for an ideal example, if the primary has 1 turn and secondary 10 turns, if the secondary current is 0.1 ams then the primary current is 1 amp, when the "magnetic balance" is satisfied. This is again a take on the ancient dual pan weight balance, where when the weights match the unknown weight is related by a constant to the other weight and the constant is related to the placement of the fulcrum, and the other weight is the calibrated standard weight used to compare.
So to be practical, you need a second winding, driven by an appropriate circuit, and using the Hall Effect sensor to detect when the field has been brought up to the original intensity before any current was applied. To get a zero, you can use an adjustable offset circuit.
Note that once you do it that way the core will always be at the same level. If it does get magnetized however, you may have to use a degaussing circuit too or else just try adjusting the offset.
Even with the single winding though, as they say, "Kudos" for trying
Will try my test again, as soon as I get caught up.
Got the drawings from googling "hall effect current sensor" , there were a few that showed feed backView attachment 100170 but most did not, your suggested method and it's comparison has got me thinking, using feed back is like balanced scales, and the lack of is like measuring with scales using springs. That probably why the measure gold with balance scales.
Jeff
Yes that was repeatable, wasnt expecting that.
You used to be able to get high Remanence or square ferrite, it was used for data memory a long time ago, you can get low Remanence factor ferrite especially for current transformers, something I've not played with, yet.
Would your application lend itself to a degaussing circuit like in old Tv's?
View attachment 100202 Here's my test bed, One Fluke 87 for current, one for voltage, switch for measuring coil 1 or 2. the output lead on coil 2 looks like it contacts the coil, but it doesn't. The toroid has an a3144 in it and if I send current through the toroid and turn on the LED, the LED stays on until I reverse the current, I have something similar that came off the control board for a spa that did the same thing, but it wasn't a toroid, just an open loop of metal in a plastic case to house the hall device, that's the one that got me started on this project, I originally blamed the supplier for sending me latching halls, but they didn't latch when triggered with a magnet, I had to know why! It was remanence.
The whole idea, for me, behind this is to shut down the power to a 12 volt power steering controller when the steering reaches the end of travel and the current goes locked rotor, fried many a FET by holding it turned too sharp. The first unit I built had a chain drive and I could mount vanes on the links to break the magnetic field and shut it down, Then I got a more sophisticated drive train and had no where to put the vane and I've been trying to figure out a current sensor for some time. I think this may be it. I can use an air core winding and adjust the sensitivity by how many turns on the coil, I think. No remanence with air cores.
Jeff
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