# Archive for January 20th, 2013

### Hall Effect Current Sensor: More Toroid Numbers

After rummaging in the collection, it turns out those calculations for the FT50-43 toroid aren’t relevant: I only have a few of them. It turns out that the actual material doesn’t affect the result nearly as much as you’d think, because the air gap for the Hall sensor controls the net permeability, so I’ll start sawing toroids that I have in abundance…

The J ferrite mix has much higher permeability, at the cost of a lower Curie point. An FT50A toroid is slightly thinner and taller than an FT50, but I have good assortment of FT50A-J toroids:

• 0.50 inch OD = 1.27 cm
• 0.312 inch ID = 0.793 cm
• 0.250 inch height = 0.635 cm
• 0.152 cm2 area
• 0.558 cm3 volume
• 3.68 cm mean path length
• μ = 5000
• 4300 saturation flux (G) at 10 Oe
• AL = 2970 nH/turn2

For 1000 G flux in a 0.15 mm air gap:

`1000 = (0.4 π · 5000 · NI) / (3.68 + 5000 · 0.15) = 8.34 · NI`

So NI = 1000/8.34 = 120, essentially the same as NI = 122 for the FT50-43. Given that μ increased by nearly a factor of 6, that shows permeability doesn’t matter very much at all.

There’s a bag of F50-61 toroids that I assume are actually FT50-61:

• 0.50 inch OD = 1.27 cm
• 0.281 inch ID = 0.714 cm
• 0.188 inch height = 0.478 cm
• 0.133 cm2 area
• 3.02 cm mean path length
• 0.401 cm3 volume
• μ = 125
• 2350 saturation flux (G) at 10 Oe
• AL = 68.0 nH/turn2

Running those numbers for the same flux and gap:

`1000 = (0.4 π · 125 · NI) / (3.02 + 125 · 0.15) = 7.21 · NI`

Which gives NI = 1000/7.21 = 139. That’s larger, but still in the same inconvenient range.

I’ll start sawing a FT50-61 toroid…