Wire size for 100 Watt LPF - Page 1
Forum section: electronics / rf-microwave

author: whalphen
author_rank: Regular Contributor
timestamp: July 19, 2019, 01:39:06 pm
content: I'm designing a set of low pass filters for 1.8 - 50 MHz and would like a recommendation on wire size for the inductors. Power rating is 100 watts. Inductors will be toroids and air core. Toroids will be size T68. Can someone point me to reference tables with the wire sizes for specified currents at RF?

author: radiolistener
author_rank: Super Contributor
timestamp: July 20, 2019, 01:20:20 am
content: I think you can estimate it by required Q factor for your inductor.There is online calculator to calculate wire resistance at high frequency due to skin effect:https://chemandy.com/calculators/round-wire-ac-resistance-calculator.htmYou can use it to calculate active resistance of indictor and then calculate Q factor of your inductor:Q = 2 * pi * f * L / Rwheref - frequencyL - inductanceR - inductor resistance (calculated with online calculator above)in such way you can calculate Q factor for specified wire and estimate if it is acceptable for you.In order to calculate inducatance of air coil, you can use this method:http://electronbunker.ca/eb/CalcMethods3b.htmlFor metal powder toroids you can try to use this online calculator, but I don't have experience with that:http://toroids.info/T68-2.php

author: D Straney
author_rank: Frequent Contributor
timestamp: July 23, 2019, 01:58:20 am
content: The skin-depth calculator is a good start, but unfortunately it doesn't tell the whole story about your wire's AC resistance. Notice that it says it's for "a straight wire" - once you start looping the wire around, you get these interesting current-crowding effects where the current is no longer evenly distributed around the whole circumference of the wire. I really wish I had some good pics on hand to show you directly, but the EM sims I did on this were for a previous job so no longer have access to them. If you want to see for yourself (before I get around to reproducing those) at some point, make a solenoid coil in the surprisingly-good free FEMM simulator (http://www.femm.info/wiki/HomePage) with an axisymmetric setup, then plot the current distribution.Anyways, the reason this happens from a physics point of view involves a few different effects: one part involves the edge of one winding that's near the edge of the adjacent winding - the current flowing in one of these "side edges" (not the inside or outside edge of the winding) induces eddy currents in the adjacent "side edge" which cancel out a lot of the current on that "side edge" of the adjacent winding, and vice versa. I don't remember the exact reason why the outside-edge currents go away, but the upshot is that the currents in the windings end up hugging the inside edge pretty closely, concentrating over maybe a third or a quarter of the original circumference instead of using the full thing. The effects get worse with more tightly-spaced windings, or with a higher-mu core.Unfortunately I don't have a good estimate to give you of exactly what your AC resistances will be; when I was last dealing with these sorts of magnetics, it was small-scale and simple enough that "wind it and see" was by far the easiest option. There's probably someone out there though who's derived some sort of decent estimate formula from either the empirical-curve-fitting end of things or the basic-EM-equations end of things. So really all I can tell you for sure is that the skin-depth-calculator's result will be very much a lower bound on your AC resistance.

author: whalphen
author_rank: Regular Contributor
timestamp: July 24, 2019, 03:32:05 pm
content: Thanks toradiolistenerandD Straneyfor the replies to my question. This turned out to be more complex than I anticipated. I'm surprised that hams haven't published any recommendations on wire sizes for LPF inductors that I could find. Lot's of HF power amps with LPF banks have been built!Anyway, here's the approach I took. I designed the filters using the Elsie software. I then simulated them in LTSpice to determine the current flow through the inductors (and the capacitors) using loads to represent SWRs of 1 and 2. (For 100 Watts of RF input, I got inductor currents typically around 2 Amps rms when SWR was 1. The largest current was 3.6 A rms when SWR of 1.) Higher SWR resulted in higher currents.I then used the AC resistance calculator (for straight wire) as recommended byradiolistener. With the AC resistance and current results, I determined the power dissipation for each inductor. (I doubled the current numbers to allow for increased SWR and/or the current crowding effects indicated byD Straney.) I did this for different wire gauges: 22, 20, and 18 AWG. Power dissipation results came out to be under 1 watt for 20 AWG wire in all cases. For 22 AWG it slightly exceeded 1 watt for only one inductor.I also ran some simulations using the femm software recommended byD Straney. I'm not sure if I was using it correctly. The calculated inductance and impedance came out different, but in the same neighborhood of other online inductor design calculators. But the calculated power was higher by a factor of about 2 to 3. I doubt that it would really be that much higher due to the current crowding effects for only a few turns of wire in a single layer coil. Anyway, even if the power dissipation ends up being 2 to 3 times higher than the straight wire results, my design can handle the resulting heat.So I'll proceed with using 22 AWG wire (and 20 AWG for a few of the inductors.)I've also found that getting SMD capacitors rated at the calculated currents is not easy. I found only one manufacturer (Johanson Technologies) that publishes the current ratings for their SMD caps. And, there is a limited set of values to select from. Furthermore, at low capacitance values, the current ratings are very low. So I had to design the filters to use specific capacitors or combinations of capacitors to accommodate the small set of available caps at the current levels required.I know that there are power amp designs out there using SMD capacitors and at much higher power levels than my design. I wonder how they source their capacitors. If anyone has any suggestions, please let me know.

author: D Straney
author_rank: Frequent Contributor
timestamp: July 25, 2019, 12:59:48 am
content: Glad that helped! I'd be curious to see how close the FEMM sims came out compared to the real thing...if you get a chance to measure the AC resistance (at your frequency of interest) of some real coils you wind, let us know how the numbers stack up side-by-side.As far as capacitors, I remember having similar problems a while back finding any sorts of solid info on AC current for ceramic caps - you could calculate the power dissipation from the ESR data, and some manufacturers would give you a vague recommendation on upper limit for temperature rise, but the equivalent thermal resistance (to find the inside's temperature rise from the power dissipation) was the missing link. I think AVX's SpiCalci software (http://www.avx.com/design-tools/ceramic-capacitor-models/at bottom) shows an estimated temperature rise; I also remember American Technical Ceramics (ATC), who make a bunch of RF caps, having some application notes or other semi-solid data on how much current you can put through their caps.

author: Odysseus
author_rank: Regular Contributor
timestamp: July 25, 2019, 01:01:11 am
content: http://www.atceramics.com/makes some of the best RF caps on the market. Not exactly cheap.

author: bob91343
author_rank: Super Contributor
timestamp: July 25, 2019, 01:06:07 am
content: Perhaps pick up an old Drake or Johnson filter and reverse engineer it. I have one of each.Of course they are rated for 1 kW but it should be easy to scale it down.I stopped using filters a while back. I don't know if they would serve a purpose any more, now that TV has other means of reception.

author: D Straney
author_rank: Frequent Contributor
timestamp: July 25, 2019, 01:13:48 am
content: Yep, ATC is pretty top-of-the-line: just checked because I was curious and found the notes I was thinking about:http://atceramics.com/Userfiles/thermal_resist.pdfThey give maximum dissipation for 100A and 100B series caps on pages 9-12, if you want to skip all the derivation, and then you can use the ESR data they publish to figure out the dissipation from the current. These ones are afew dollars each in small quantities at Digikey, so weigh the tradeoffs I guess.They also have current ratings for their "RF Power Capacitor Assemblies" (http://atceramics.com/Capacitors/18/RF-Power-Capacitor-Assemblies) but those are specialized, expensive, and likely overkill for what you're doing.

author: whalphen
author_rank: Regular Contributor
timestamp: July 25, 2019, 08:25:17 pm
content: The ATC document is helpful. Unfortunately, the price is several times greater than the Johanson Tech components. So I'll use them only if I can't find the desired value at the lower cost.Thanks.