FT82-43 Slit Toroid: Calibration

I’d have trouble faking this with a straight face:

FT82-43 - 56 turns - 24 AWG

FT82-43 – 56 turns – 24 AWG

That’s measured with the 56 turn winding connected directly to a bench power supply, cranking up the current, taking the reading, and turning the current back down again, so as to avoid cooking the poor thing inside its PLA armor:

FT82-43 toroid - mounted

FT82-43 toroid – mounted

The “49E” sensor came from one of the bags of eBay fallout. They saturate around 4.25 V; the outputs above 4 V lose their linearity due to the sensor, not ferrite saturation.

The original calculations guesstimates suggested 25 turns would produce full scale at 5 A, so 56 turns should top out at 2.2 A. Frankly, given all the imponderables in this lashup, a factor of two seems pretty close.

Offsetting the output by -1 A would yield a 2 A range that’s just about exactly right. Unfortunately, some fiddling about with neodymium magnets suggests that you (well, I) can’t stuff enough opposing field into the slit without saturating (some part of) the ferrite core, reducing the permeability, and blowing all the assumptions.

So that suggests a buck winding, obviously with more turns to allow less current for the same magnetizing force. Wrapping 110 turns reduces the buck current to 500 mA and assuming a bit over an inch/turn requires 10 feet, which is nearly 1 Ω of 30 AWG wire: the buck current dumps another 250 mW into (a somewhat larger version of) that PLA armor.

Or just throw away half of the Hall effect sensor range and use an op amp along the lines of the LED current sensor.

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  1. #1 by hexley on 2014-08-26 - 12:58

    Wrapping 110 turns…
    That sounds like a lot of work. Could you use the existing coil with a parallel current-sharing resistor of 0.1 ohms? Seems like that would split the current in roughly equal parts since the resistance of the 56-turn winding was [reported] (http://softsolder.com/2014/08/21/ft82-43-slit-toroid-construction/) as 128 milliohms.

    • #2 by Ed on 2014-08-26 - 13:24

      That sounds like a lot of work.

      Winding turns goes much faster through the slit, but, yeah, that does seem like a nuisance. Plus, it need a calibrated current source and, because the buck winding goes over the main coil, the current source should be on the line-voltage side Just In Case.

      Being a sissy, I want to minimize the number of hot parts: I’ll toss half the sensor’s dynamic range with an op amp to remove the offset and expand the scale.

  2. #3 by Jim R on 2014-08-26 - 14:49

    Would removing half the turns not be a reasonable solution?

    • #4 by Ed on 2014-08-26 - 15:54

      That’s pretty much The Plan: 25-ish turns for a 2 A range from 2.5-4.0 V, then an op amp to get 0-5 V for the ADC. The answer obviously depends on which 49E sensor I’ve got in there, so I should check a few others to make sure I haven’t randomly picked the only super-sensitive one in the bag.

      I really wanted to gimmick the magnetic field to make the answer come out right without all the circuitry… [sigh]