Toroid transformer distributed vs close winding - Page 1
Forum section: electronics / beginners

author: matthuszagh
author_rank: Frequent Contributor
timestamp: February 26, 2023, 05:17:08 pm
content: I have to build a small (12.7mm OD) three-winding transformer on a toroid. The windings have 28, 14, and 7 turns. I'd previously done this by first winding the 28-turn winding as evenly as possible (in regard to wire pitch) around the whole toroid. Then the same thing for the 14-turn winding on top of it. And finally the 7-turn winding. I never really got a great result. The windings were a bit looser than I would have liked, not super evenly distributed. I also think I had a few inadvertent wire cross-overs.I think it will be easier to constrain each winding to a small section of the the toroid so they're spatially separated. I'll keep the wire pitch at a minimum. What are the benefits and drawbacks of this relative to the distributed approach?One benefit (apart from it potentially being easier to perform) is that it should have lower inter-winding capacitance since the individual windings are spatially separated. On the other hand, the intra-winding (am I using these terms correctly?) capacitance should be higher since each loop is closer together.I would also expect the coupling factor between windings to be a little lower, but I don't know. Will less of the magnetic field be confined to the toroid? Anything else?

author: jonpaul
author_rank: Super Contributor
timestamp: February 26, 2023, 05:21:56 pm
content: frequency band? application? Balun? Z match?Normally wound bifillar or,trifilar to reduce Llkgj

author: matthuszagh
author_rank: Frequent Contributor
timestamp: February 26, 2023, 05:27:39 pm
content: Quote from: jonpaul on February 26, 2023, 05:21:56 pm10 MHz. It's part of a buffer for a low phase noise oscillator. Not a balun and doesn't require impedance matching.Quote from: jonpaul on February 26, 2023, 05:21:56 pmI'd seen this frequently recommended when the same number of turns is used for each winding, but never once seen someone discuss the case of unequal numbers of turns (maybe I'm looking in the wrong places). Anyway, I could wind them trifilar and have the 28-turn wire go all the way around the toroid, the 14-turn go halfway around the toroid and the 7-turn go a quarter of the way around the toroid. Is that acceptable? What is Llkg?

author: Kleinstein
author_rank: Super Contributor
timestamp: February 26, 2023, 05:53:28 pm
content: The close winding case will have more stray field, especially if the core is low µ or even approaches saturation.With the ditributed winding one may easily have that one hidden parasitic winding parallel to the core and this can give a stray field in the unexpected direction. With the localized windings one is more likely to go back the short way.Chances are the close winding case can get less capacitive coupling between the windings.

author: matthuszagh
author_rank: Frequent Contributor
timestamp: February 26, 2023, 07:40:00 pm
content: It was a bit tougher than I had previously thought to do the close winding technique. I couldn't get the winding tight enough for the wire to stay at a tight pitch. 28 turns ended up taking about half of the toroid (with 30 AWG). That's probably still ok but it was a little more difficult to do than I would have hoped.On the other hand, using a trifilar winding technique made it quite easy (image attached). I first twisted the longest and second longest wire. I then twisted that pair with the shortest wire. The interwinding capacitance isn't great: I measured 16p, 50p and 75p for the various combinations, but I simulated it in my design and it seems ok.

author: jonpaul
author_rank: Super Contributor
timestamp: February 26, 2023, 08:07:12 pm
content: most house/ distribution clock amps use stock 1:1 isolation transformers designed for 50 ...75 Ohm impedance.please post the schematic or photo of the unittrifilar, quadfilar etc uses wires of equal legnth and turns.Ratios are achieved by connecting several windings in seriesjon

author: matthuszagh
author_rank: Frequent Contributor
timestamp: February 26, 2023, 08:31:51 pm
content: Quote from: jonpaul on February 26, 2023, 08:07:12 pmI could do that. Does each series element go around the full toroid (and each subsequent series element would overlap the previous), or each series part winds around part of the toroid and then all series elements together go around the full toroid? If it's the latter, I don't really see the benefit of that method over mine. If it's the former, then all windings make it around the full toroid and I suppose there will potentially be less stray magnetic field? Your method will be a bit messier than the one I used because there will be intermediate wires poking out. And, of course, they need to be insulated to prevent shorting together with other intermediate wire endings.Quote from: jonpaul on February 26, 2023, 08:07:12 pmMy choice not to share this was somewhat deliberate. I'd rather the conversation focus on how to wind >2 winding transformers than the best way to build a buffer amplifier for a clean clock. The optimal way to wind toroids is probably context-dependent - that's fine. But, even aside from this application, winding toroids is a skill I'd like to develop and I'll be perfectly happy to know the various methods of constructing them and the benefits and drawbacks of each approach.But, since you asked, the buffer amplifier I'm building is the first one here:http://www.ko4bb.com/~bruce/OCXOBufferAmplifiers.html. I'm using 2 stages of that in series to increase reverse isolation and placing two amplifiers in parallel to get two buffered copies of the clock. This is designed to slightly boost and buffer an HP 10811A. The 10811A gives +7dBm and this will boost the level to +13dBm in addition to providing reverse isolation.

author: jonpaul
author_rank: Super Contributor
timestamp: February 26, 2023, 09:24:12 pm
content: Rebonjour, cher monsieur1/ most toroids are wound with windings distributed over,the circumrerence.2/ Advantag ,of multifilar is easier to wind three or four in hand, we use different wire colors to identify.3/ the reduced space between adjacent windings reduces the Llkg = leakage inductance, for wider BW, lower pulse aberrations.4/ No need for fancy buffers as most HP, GenRad, Tek RÉF osc in (5 or 10 MHz) very loose for the levels and waveshape.5/ Using Leo Bodnar GPSDSO for 10.0000 clk, +32 max out drives 4..8 50 Ohm refs via 1:1 Mini-Circuits splitters, 1>2,4,6,8 way.Bon chanceJon