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Forum Index : Electronics : Toroidal resonance question
| Author | Message | ||||
Murphy's friend Guru  Joined: 04/10/2019 Location: AustraliaPosts: 648 |
Back in the days, Warpspeed recommended that we fit a capacitor to the secondary to resonate this at 75Hz. This was to suppress harmonic oscillations. OK, the practical suggestion to do that was to use a sine wave oscillator and experiment with parallel capacitors to find the resonance point. I find that very difficult to do as the resonant 'hump' is very shallow. So it occurred to me there might be an easier way. I have a LC meter which lets me measure the secondary inductance (primary open). This measured at 50 milli Henry's. Now, using this onlinecalculator I get a capacitor value of 90 uF. Would this be correct, or did I do something wrong? That value is much higher than the value I thought I got by the sine oscillator method. Thanks. |
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| Murphy's friend Guru Joined: 04/10/2019 Location: AustraliaPosts: 648 |
This forum does no let me edit so an update: I made the mistake of not zeroing my LC meter so the toroid's secondary actually measures 80 milli Henry. The calculator gives me 56 uF for resonance at 75Hz. Still higher than I thought. The LC meter is a JUNTEC Model LC-200A. I find it very useful as it measures bigger values than the usual combo meters. |
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KeepIS Guru Joined: 13/10/2014 Location: AustraliaPosts: 1679 |
Not trying to answer your question, but I get very sharpest and high resonance on the DSO. Just a 10k in series with gen output and a known cap value across the inductor or transformer, in may case 4.03uf mains cap, it is a really noticeable peak on the toriod transformers. I know you understand all of that, and have tried it many times, but I'm wondering if the Generator output is being pulled down or something strange, I have never found a Toroid or choke that isn't as sharp as anything with a big peak at resonance. I've got an inductance meter and will try with the calculator when I get a chance tomorrow, but LC meters are often not very accurate on big toriods that you are trying to resonate at 50 to 75 Hz. . Just noticed your last post, that meter may be better, I agree seems a bit high, but not by much. There was a marked shift with values as little as 1uf as I parallel adjusted caps for 75Hz. . Edited 2023-05-12 16:28 by KeepIS It's all too hard. Mike. |
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| Murphy's friend Guru Joined: 04/10/2019 Location: AustraliaPosts: 648 |
Thanks Mike. I originally tested this with a home built Wien bridge oscillator and a 35 year old CRO. I just realised that my tiny, handheld, battery operated digital scope has a signal generator function, might try that too. Sadly, I do not have all the testing gear I would like to have  . |
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| KeepIS Guru Joined: 13/10/2014 Location: AustraliaPosts: 1679 |
I have a silicon chip digital sine / square wave LF / HF signal generator with digital display. It was built by an electronics friend who passed away, you are welcome to it for the cost of postage, just thought I'd offer that in case your are interested. . It's all too hard. Mike. |
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| Murphy's friend Guru Joined: 04/10/2019 Location: AustraliaPosts: 648 |
Thanks Mike, that would be very useful and a lot better than that ancient thing I built 50 years ago. My little hand held digital scope does not have an adjustable frequency output, just fixed steps too far apart. I'll send you a PM with my email address so we can arrange postage. |
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| KeepIS Guru Joined: 13/10/2014 Location: AustraliaPosts: 1679 |
 It's all too hard. Mike. |
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InPhase Senior Member  Joined: 15/12/2020 Location: United StatesPosts: 178 |
I've been wondering about this as well. I don't think the inductance is a fixed value. Doesn't it depend on the frequency we are checking it at? I mean, the inductance at 75 Hz will be different than the inductance at 10 kHz, will it not? I suggest that you can find the inductance by actually powering it with 50 Hz and measuring the current draw. The DC resistance of the wire can likely be ignored. The current through the coil will be V/2piFL. If you know V and F=50, then do some algebra to determine L. But then again, I'm quite dumb and could be waaaayyy off. |
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| pd-- Senior Member Joined: 11/12/2020 Location: AustraliaPosts: 122 |
Ye but with the core materials we are using and the relatively small differences between our operating frequencies and testing frequencies it isent going to make much of a difference if you look at the graph on page 16 https://publications.lib.chalmers.se/records/fulltext/183920/183920.pdf if we are using an inductance tester that operates at 10khz and our inductor is going to operate at 20khz then we are in a very flat section of the curve and our inductance would drop by less than 1% |
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| rustyrotors Newbie  Joined: 07/01/2023 Location: United StatesPosts: 36 |
i just use headphone output from my laptop, volume cranked, with a 16 ohm resistor inline. connect it and an oscope across the primary. sweep the freq with a web based tone generator. output will be low, in the 10-100mV range |
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Madness Guru Joined: 08/10/2011 Location: AustraliaPosts: 2498 |
If you google pc function generator app there are many free apps that will do the job. I tried with a frequency meter it showed exactly the same hz as the app. If you want more voltage just use a audio amplifier. There are only 10 types of people in the world: those who understand binary, and those who don't. |
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| analog8484 Senior Member Joined: 11/11/2021 Location: United StatesPosts: 108 |
That seems to be off by an order of magnitude. Was it measured at 100Hz? Edited 2023-05-25 05:09 by analog8484 |
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| pd-- Senior Member Joined: 11/12/2020 Location: AustraliaPosts: 122 |
I tried a couple of different LCR meters and could not get a sensible reading from them. I then connected up my trusty little DSO-TC3 as a signal generator connected to the toroid via a 15k resisted " 10k or 15k or sumthing there abouts " with my old cro connected directly across the toroid secondary with the capacitor the peek is quite well defined i think the trick is the 10k resister. i ended up at 4.4uf using a 0.22uf cap for the final couple of hertz trim. Mi old mostly analog signal generator wondered around the place at low frequencies and was not at all suitable . but the DSO-TC3 was rock stable and you can increment/decrement the HZ by 1 making it easy to spot the peek even on a 35 year old cro. |
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