How to identify toroidal cores? - Page 1
Forum section: electronics / rf-microwave

author: RoGeorge
author_rank: Super Contributor
timestamp: July 10, 2019, 03:49:00 pm
content: Is there any way to find out the recommended usage for any of these cores?I need 2 of them to make a directional coupler that should work up to 100 MHz. The black ones are most probably from RF chokes on DC wires, so I guess they will have too big losses at 100 MHz. The only one marked have 2 colors, blue-green and grey-green with a number 4 written, but I couldn't find them online.Anybody recognizes the ones marked with a 4, please?Are the white ones for RF, or just normal ferrite with fancy color?

author: D Straney
author_rank: Frequent Contributor
timestamp: July 10, 2019, 10:57:10 pm
content: The blue-green could be some Micrometals -52 iron-powder material:https://www.micrometals.com/materialsAccording to the chart they top out around 590 kHz - I don't know exactly what criteria they're using, but still a few orders of magnitude off from what you're looking for unfortunately.Don't know about the others, although for what it's worth I have seen a white-coated ferrite toroid before (Ferroxcube, I think).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 11, 2019, 02:36:41 am
content: The ferrites are probably your best bet, but a #43 or #61 would be preferable. Maybe one of them is. You'd have to measure the dimensions (OD, ID, thickness) and inductivity (uH/t^2) to have a good guess at that.Powdered iron for power conversion purposes (all the inductors pictured, I would think) is trash for power conversion already, let alone at higher frequencies.Go figure.Tim

author: RoGeorge
author_rank: Super Contributor
timestamp: July 12, 2019, 05:20:54 pm
content: After spending hours searching for specs, I realized that in fact I don't need the specs, but to know how well these cores will perform as a transformer between the whole range of 0.1-100 MHz, so I just probed them.Made this measuring setup of a 1:1 transformer:Then fed the primary wire with a linear sweep between 0.1-100 MHz (10 MHz/div) 1.5Vpp sinusoidal, from a frequency generator. The oscilloscope trace was probing the secondary voltage on the secondary 1 turn wire with a 50 ohm load. The results were mixed.For the smallest black cores, the induced voltage in the secondary was the biggest:The white cores (not painted white, they look like they were made of a white ceramic), the voltage was lower, especially at higher frequencies:The biggest black core has almost the same response as the white ones:As a side test, if both of the big cores are inserted simultaneous, the response almost doubles (note the next plot is with 200 mV/div):The green+blue marked with a 4 are indeed for low frequencies:Just out of curiosity, this is the induced voltage by the wires alone, without any ferrite core in between (note the scale for this plot is 2 mV/div):By comparison, the cores used initially in the directional coupler were a really bad luck choice, they have the worst response:Those were replaced with the two medium size black cores, which have a response almost identical with the first plot, and now the directional coupler works much better but it is still not as good as I would have like it to be.Anyway, it seems my assumption that the RF suppressing cores should have big losses at high frequency was wrong.The next plot is for the long ferrite tubes (3 of them can be seen in the left side of the pic from the first post, in the same bag with the green+blue ones). Those beads have great response at high frequency. In fact, the thin long ferrite tubes made the best transformers out of all of the tested ferrites here (note that the sensitivity for the next plot is 200mV/div, while the previous plots were 100mV/div):I am puzzled. Judging by its shape, about 1.5 cm (~0.5 inch) long and 3 mm (~0.1 inch) external diameter, this was obviously made to be used as a ferrite bead to stop the RF. I thought a ferrite bead was suppose to have big losses at high frequency in order to turn the RF energy into heat, but that doesn't seem to be the case.What am I losing here? Are the RF beads acting in fact as RF stoppers or reflectors, and not as RF absorbers? Or it is some other reason for why the response is so flat for this tube, the flattest of all measured here?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 12, 2019, 10:10:21 pm
content: They're still quite lossy, you're just using a lower impedance in parallel. The core might present 30 to 100 ohms (per turn squared) at 100MHz say, but if you're looking at the parallel equivalent with 50 ohms, and using several turns, that's not all that much insertion loss -- a few dB tops.Core loss acts in parallel with a transformer winding, so it can be simply swamped by system impedance.If you tried running any real power through it, you'll find it gets nice and toasty.Now, leakage acts in series between windings. It is usually made to be low in a transformer. As the name suggests, leakage doesn't participate in the core*, but rather depends on the proximity of the windings to each other, and their wire lengths.*For most winding geometries. Though your side-by-side windings test actually accentuates the core's effect on it. This contributes to the insertion loss at high frequencies (>20MHz?), in this case probably about doubling the leakage due to the windings alone.Tim