DIY inductors and common mode chokes - Page 1
Forum section: electronics / projects

author: okw
author_rank: Frequent Contributor
timestamp: January 22, 2025, 06:44:20 pm
content: I want to wind my own power inductors (470uH) for a line filter (7A 240V).Most articles I've seen are focused around ham-radio / RF.Any good articles for this?I'd like to order the toroids (ring cores) and wire from LCSC:https://www.lcsc.com/products/Accessories-Inductors-Transformers_11244.htmlAny hint on which ones to order for my specs (470uH 7A 240V)?I'd also like to try to wind my own common mode chokes (3mH and 33mH, 7A 240V), but I suspect this is more difficult to calculate?

author: Terry Bites
author_rank: Super Contributor
timestamp: January 23, 2025, 06:30:55 pm
content: The same principles apply.https://www.nessengr.com/technical-data/toroid-inductor-formulas-and-calculator/What you have to be wary of with power inductors is core saturation."Iron powder" cores can handle high flux densities than ferrites.Given the nuisance factor of rolling your own, see.https://www.aliexpress.com/item/1005006196819772.html?spm=a2g0o.productlist.main.21.a8a95a11dHXKhn&algo_pvid=7035b5e8-3b33-4eab-8c5d-9a30dd02293d&algo_exp_id=7035b5e8-3b33-4eab-8c5d-9a30dd02293d-10&pdp_npi=4%40dis%21GBP%213.00%213.00%21%21%213.59%213.59%21%40211b628117376569090366961ef7fa%2112000036226643056%21sea%21UK%213100120370%21X&curPageLogUid=HsUDZfxMnAzm&utparam-url=scene%3Asearch%7Cquery_from%3A

author: okw
author_rank: Frequent Contributor
timestamp: January 23, 2025, 06:46:44 pm
content: Thanks. I guess I can use the Ali 470uH 10A chokes you linked to as a single inductor (not connect the other two pins)? Cause I can't join two lines into the choke and get "inductor" behaviour? I need 6 inductors, so that would make it 3 chokes.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 23, 2025, 08:21:07 pm
content: The common mode chokes / balanced inductors need to be used with both coil balanced. Otherwise they will saturate the core well before the rated current. One can use just one coil as an inductor, but only for low current (e.g. 10 mA, maybe 100 mA). There inductors are also not independent but more coupled as transformers.

author: TimNJ
author_rank: Super Contributor
timestamp: January 24, 2025, 01:35:57 am
content: In my experience, the design of these kinds of chokes is an exercise is optimization. Usually, there are constraints like size and temperature rise which govern what the final design needs to be.Differential-mode (DM) chokes: Typically wound with alloy powder / iron powder core, low permeability (typically between 60u and 160u). "Sendust" and iron powder are perhaps the most common(?) / cheapest. But, something like "high flux" will yield a smaller inductor due to greater saturation flux density. The inductance of powder cores rolls of relatively gently with applied MMF (i.e. current). You might design this choke to have 30% roll-off in inductance at the peak current you expect. Though, whether that is acceptable really depends.Core size and temperature rise are related because the core ID dictates the wire size you can use. You might typically fill ~30% of the ID of a toroid with wire. (total cross section of wire / πr^2 < 30%). I would not go higher than 50% fill.If, to achieve 30-50% fill, you determine that you need wire which is too small for the rated current, then you should consider a larger core with a larger ID. Judging what size wire is big enough depends on the thermal resistance (to ambient) of the wound choke. This will vary depending on construction, size, and if there is any airflow. With static air, you might guess around 50-75°C/W for this size. So, you probably don't want too much more than a watt of dissipation.Common-mode (CM) chokes: The process is similar. You need to pick a core size that allows you to fit the number of turns you require with wire of sufficiently large diameter. Because of the equal and opposite flux, you can use a much, much higher permeability core. Most commercial EMI filters will use between 7,000u - 12,000u ferrite cores for low frequency common-mode chokes.CM chokes typically require more turns of wire to achieve adequate attenuation compared to a DM choke in the same filter. (At least that's my experience.) CM chokes may have comparatively more interwinding capacitance, or "self capacitance". You might need to consider the self-resonant frequency of the choke and whether it makes sense for the EMI you are trying to attenuate. You can measure this with an inexpensive nanoVNA.For example, if your 33mH choke has a self-resonant frequency of 50KHz, and the EMI frequency is 500KHz, it won't do a very good job. This is to say: Inductance is an OK parameter to design with, but be wary that it is just a single point on the impedance vs. frequency plot.

author: Simmed
author_rank: Frequent Contributor
timestamp: January 24, 2025, 05:33:40 am
content: https://www.dxzone.com/dx33538/mini-ring-core-calculator-calculate-windings-on-ferrite-cores.html

author: Picuino
author_rank: Super Contributor
timestamp: January 24, 2025, 12:59:47 pm
content: First of all, you have to check that the core is able to store the necessary energy.Some formulas:ℜ = λ / (μ · Ae)ℜ = 1 / A_LL = N^2 / ℜB_max < N · I_max / (A_min · ℜ)B_max < L · I_max / (A_e · N)---ℜ = Reluctance [H^-1]A_L = Inductance factor A = Magnetic section area [m^2]I = Electric current N = Coil turns B = Magnetic Field λ = Magnetic circuit length ---A_L (inverse of reluctance ℜ) must be large enough so the magnetic field B_max is less than the maximum allowed.