While doodling about a bike taillight, I figured out the numbers for a 1 mH toroid on a ferrite FT50A-77 core. Even though I’m not going to use it, I may as well write it down so I remember what to do the next time around…
FT50A-77 Toroid data:
- 0.5 OD x 0.312 ID x 0.25 long, inch
- Mean path length: MPL = 3.68 cm
- Cross section: XC = 0.52 cm2
- Volume: V = 0.558 cm3
- AL = 1080 mH/1000 turn
- μ = 2000 (in gauss/oerstead, most likely)
- Saturation = 4500 gauss
- Tempco = 0.6%/°C
- Winding length = 0.688 inch/turn
Those mixed units barely make sense and then only in the USA; using cm for the magnetic dimensions gives CGS results. In some parallel universe, this would not be an issue.
The inductor:
- For 1 mH, turns = 1000 √(1 mH / (1080 mH/1000 turns)) = 30.4 turn
- Wire length = 31 turn × 0.668 inch/turn + 3 inch = 24 inch
Core saturation check for 100 mA:
- 100 mA x 30 turn = 3 ampere·turn
- Magnetomotive force = mmf = 0.4 π × 3 A·t = 3.8 gilbert
- Magnetizing force = H = mmf/MPL = 3.8 Gi / 3.68 cm = 1.0 oerstead
- Flux density = B = μ H = 2000 G/Oe × 1.0 Oe = 2000 G < 4500 G, so OK
I think the saturation check works, but the units always make my head hurt…
The Smell of Molten Projects in the Morning 
Have I already blathered at you about the insanity of Italian bicycles? An Italian bottom bracket thread is specified as 36mm x 24 threads per inch.
aieeeee.
A long time ago I got a copy of Sutherland’s and devoted some time to paging through it, which brought home the truth about standards: everybody should have one!
The really awful mismatches thread together well enough that you can make ’em fit with the BFW, but then they gall and never, ever come apart again…