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I have got two sets of E70/33/32 core of N87 material, one set with centered airgap of 8mm and another set is un-gapped core. I measured the inductance for N number of turns with centered gapped core and also I measured the inductance with 4mm airgap in the each leg (which gives effective airgap of 8mm). But I am getting different inductance value in both cases, for higher number of turns I am getting difference of 1 to 1.5uH. I am getting more inductance for distributed gapped core. I do not understand the reason behind it. I designed my own coil former(bobbin) just to give 4mm airgap in each leg and also tried putting a plastic of 4mm on each leg and measured it but the inductance is different in both the cases. I would want to go with distributed air gap because of its advantage. But I would like to know what is the reason to have different inductances in both the case. If anyone has experienced this please let me know. enter image description here

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    \$\begingroup\$ The difference value is not helpful. Please give absolute values of inductance - people here learned to do subtraction many years ago. Please also be absolutely clear about the two experiments. You have two cores and one with a centre ground down by 8 mm. You do one test then you use the same cores (because you only have two) and increase the gap all-round by 4mm - this is how it reads! \$\endgroup\$
    – Andy aka
    Jan 21, 2021 at 12:15
  • \$\begingroup\$ Bobbins generally (and specifically) do not create an air-gap - another anomaly that needs clearing up. Pictures/diagrams will really help. \$\endgroup\$
    – Andy aka
    Jan 21, 2021 at 12:22
  • \$\begingroup\$ well, The one with centred 8mm gap (4mm gap on each side) with 7 no of turn gives 8.5uH. The 4mm gap on each leg with 7 no of turns gives 10.4uH. Which picture do you need? Bobbins or Cores? \$\endgroup\$ Jan 21, 2021 at 13:05
  • \$\begingroup\$ The one with centred 8mm gap (4mm gap on each side) - what precisely does this mean??? And, how is that different to The 4mm gap on each leg. Please don't be ambiguous. \$\endgroup\$
    – Andy aka
    Jan 21, 2021 at 13:22
  • \$\begingroup\$ I attached a picture, g=4mm on both sides so total 8mm. Another Inductor which I tested has 4mm on each leg. Please do not say u did not understand \$\endgroup\$ Jan 21, 2021 at 13:41

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The E core is gapped in the centre which is series inductance to the much larger main inductance.

Making a gap to an ungapped E core reduces both significantly. Thus you cannot expect the same values for gapping 1 vs 3 legs.

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  • \$\begingroup\$ I did not completely understand it. Could you mention what might be the effect of this? Is there any particular reason? \$\endgroup\$ Jan 21, 2021 at 13:17
  • \$\begingroup\$ The total areas between the gaps are different for the centre vs all 3 interfaces. Thus you would have to reduce the gap to match \$\endgroup\$ Jan 21, 2021 at 13:30

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