The New Hotness

For an upcoming Circuit Cellar column on Hall effect current sensing, I slit another pair of toroids:

Then wound them with grossly excessive amounts of wire (the up-armored core on the right appeared earlier):

The smaller toroid is an FT37-43 that barely covers the active area of an SS49-style Hall effect sensor, but experience with the FT50 toroid suggests that’ll be entirely enough:

Data on the uncut toroids:

Property	FT50-61	FT37-43
Outer diameter (OD) – inch	0.50	0.375
Inner diameter (ID) – inch	0.281	0.187
Length – inch	0.188	0.125
Cross section area – cm2	0.133	0.133
Mean path length (MPL) – cm	3.02	2.15
Volume – cm3	0.401	0.163
Relative Permeability (μr )	125	850
Saturation flux G @ 10 Oe	2350	2750
Inductance factor (AL) – nH/turn2	68.0	420

Those overstuffed windings improved the sensitivity, but increased the winding resistance far beyond what’s reasonable.

Data on the slit toroids:

Toroid ID	FT50-61	FT37-43	FT50-61
Measured air gap – cm	0.15	0.15	0.17
Winding data
Turns	120	80	25
Wire gauge – AWG	28	32	26
Winding resistance – mΩ	530	920	100
Predicted B field – G/A	872	660	163
Hall effect sensor @ 1.9 mV/G
Predicted output – mV/mA	1.7	1.3	0.31
Actual output – mV/mA	1.9	1.9	0.37
Actual/predicted ratio – %	+12	+46	+19

The last few lines in that table show the transimpedance (transresistance, really, but …) based on the winding current to Hall sensor output voltage ratio (in either mV/mA or V/A, both dimensionally equivalent to ohms), which is why the toroid’s internal magnetic flux doesn’t matter as long as it’s well below saturation.

Gnawing the 80 turn winding off the FT37-43 toroid and rewinding it with 15 turns of 24 AWG wire dropped the winding resistance to 23 mΩ and the transimpedance to 0.36 mV/mA:

However, applying a voltage gain of about 28 (after removing the sensor’s V_CC/2 bias) will produce a 0-to-5 V output from 500 mA input, which seems reasonable.