Archive for February 3rd, 2012

1 mH Choke on FT50A-77 Toroid

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…

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