I recently built a wireless power transfer system using coils and capacitors to achieve maximum efficiency. I used a 120kHz frequency and measured the amplitudes and phase angle of my oscilloscope to determine the coil quality. To make life easier, I followed formulas from the Tektronix website to calculate the resistance and inductive reactance.

After inserting all the values, I got results that looked like this, which was mostly interesting for me because it allowed me to calculate the coil quality value at 120kHz. Because by simply dividing the one by the other, I finally got my long desired coil quality value. But don't worry, negative influences like proximity effect, skin effect or parasitic capacitances were all included in such a measurement and we basically now only have to repeat it with different frequencies to determine the coil quality sweet spot.

So after doing tons of tests and calculations, I was able to plot this graph which clearly shows us that the coil quality sweet spot can not be found at a higher frequency which was my expectation but between 50 to 60kHz. That is why I added a 330nF capacitor to the coil pairs which in combination with their known inductance should give us a new resonance frequency of around 57kHz.

After trying out this newly fine tuned system, I am happy to report that science did not let us down and I was not only able to achieve a bigger distance of 70cm but also a bigger short circuit current of more than double than before, awesome. But of course this was not enough for me yet because I still had lots of Litz wire lying around which should keep the skin effect losses to a minimum.

So I tried using it and was simply too dumb to think of simply putting 3 wires in parallel in order to decrease the copper resistance. So I did just that in a procedure that was almost 3 hours long and created two promising looking coils but honestly speaking, I was not in the mood of repeating all the long measurements and calculations from before.

And luckily I didn't have to because Elektor sent me one of their new 2MHz LCR meters whose test frequency you can fine adjust and it tells you all the important values right on its main screen. So after unpacking all the parts, assembling them with a couple of screws and wires and doing some calibrations, it was finally time to do some quicker measurements.

And after only 3 minutes I recorded all the coil quality values which to better visualize them I put in the same graph as the previous coil. It was interesting to see that their coil quality was about the same at higher frequencies while the Litz wire coil featured a higher quality once again at a sweet spot between 50 to 60kHz.

So I picked a fitting capacitor and build up my final test system which did once again exceed all the others with a distance of around 75cm and a short circuit current of 135mA. At the end I also tried calculating some efficiencies which varied quite a bit but were overall way more promising looking than before.

And with all my tests and measurements being completed so far, I think I made it clear why inductors or coils can be sometimes difficult to handle but I hope you understand them a bit better now. And I will see how I will use this LCR meter to improve my system even more in the future. If you want to see that then consider supporting me through Patreon so that I can keep producing videos.

Don't forget to like, share, subscribe and hit the notification bell. Stay creative and I will see you next time.

WEBVTTKind: captionsLanguage: enThis is my wireless power transfer system that I built during a previous video. And as you can see it still works perfectly fine; at least according to its capabilities. Because even though the coils were able to transmit a bit of power,  the overall power losses were huge and thus the efficiency pretty horrible. Back then I said that the loss factor who is partly responsible for the power losses  is dependent on the coupling factor multiplied by the coil quality. And while we can not do much about the coupling factor, I tried increasing the coil quality by  playing around with different amount of windings, coil diameters and even copper wire types. And the experiment results clearly showcased a noticeable difference but sadly I never  got to determine a numerical value for the coil quality which definitely would have been helpful. But that is about to change in this video because being able to properly measure the most important  characteristics of a coil is not only super important for such wireless power systems  but in general for every application where inductors are being used with a higher frequency. So let's find out all about it and possibly create my best wireless power transfer coil so far. Let's get started! This video is sponsored by Elektor who not only publish the Elektor magazine  which comes with awesome articles about everything related to electronics  but they also recently launched a new LCR Meter  which will help me later during the video and save me a lot of time while experimenting.  So feel free to have a look at it on their website and while you are it you might be interested in  checking out their projects archives with over 1000 DIY electronics projects. All you need is a  membership which you can get quite a bit cheaper by clicking the link in the video description. First off you might be asking yourself why I am making such a big deal of coil measurements. I mean if you want to design such an awesomely efficient switched mode power supply,  then all you would have to do is to follow the given formulas to calculate the required  inductance, get yourself a fitting inductor with this value, solder it in place and it just works. And yes; because the power supply uses a rather low switching frequency  of only 80kHz the inductor still behaves just like an inductor, *cough... foreshadowing. But if we look at other power supplies we can easily determine that they are utilizing way  higher frequencies, in this example around 1.4MHz which the inductor apparently does not mind though  because it's still doing its job just fine, right?To find that out I desoldered it from its PCB  and hooked it up to my function generator and oscilloscope in combination with a shunt  resistor to measure not only the voltage across it but also the the current flowing through it. And by adjusting the frequency to the previously utilized 80kHz, we can see that the current  waveform is lagging with a phase angle of around 89 degrees in comparison to the voltage which is  a completely normal behavior for inductors.But if you are super confused right now then  I would encourage you to watch my video about impedance to truly understand what is going here. Because now I am slowly cranking up the frequency to the higher 1.4Mhz  and as you can see on the oscilloscope our phase angle decreased just a tiny bit. The reason is that there exists no ideal inductor which only comes with inductance. Every inductor also comes with a parasitic resistance which is partly the resistance  of the wire and also a parasitic stray capacitance which represents mostly the  capacitance between the coil windings.This value is of course super tiny  but it is still there which means there exists a frequency at which the inductive  reactance equals the capacitance reactance.This frequency is the so called self resonant  frequency and above it the inductor will actually start acting more like a capacitor  than an inductor while below this frequency the inductive characteristic over weights. But don't worry commercial power inductors usually have a high self resonance frequency  which in the case of the coil we had a look at so far seems to be around 12MHz. And it was actually kind of funny seeing the phase angle suddenly jumping around near this frequency. But getting back to the point because if you start winding coils by yourself for whatever reason  then you can quickly create a coil that reaches this self resonance faster  and thus it becomes pretty much useless which could have been avoided by properly measuring  the inductor characteristics for its different application frequencies. And needless to say by gathering all these values; we can easily convert  them into the coil quality that I am after.To do that we can use an LCR meter which spits  out the impedance, phase angle, inductance, resistance and even the coil quality itself. But sadly most LCR meters only do that for certain frequencies  which is of course not the frequency range I am interested in for my wireless power transmission. So we have to do our own measurements for which I can use almost the same setup as before  except that this time I am using two passive probes and a 200ohm shunt resistor which I hooked  up to the coil and oscilloscope/frequency generator according to this schematic. The used method is called I-V aka current voltage method in which we have to measure  the current and voltage amplitude and the phase angle between them. So as a first example I went with a frequency of 120kHz  and used the functions of my oscilloscope to measure the amplitudes and phase angle. Now you might be asking yourself how we can turn these values into those and for that we  can make our life easier and skip the complicated derivations by simply following these formulas  from the tektronix website where you can also find a guide about my used measuring method. And after inserting all the values we get results that look like this  of which we are mostly interested in the resistance and inductive reactance. Because by simply dividing the one by the other we finally get my long desired coil quality value for  120kHz which by the way is the frequency I have been using so far for my wireless power setup. And do not worry; negative influences like the proximity effect, skin effect or the  parasitic capacitances are all included in such a measurement and we basically now  only have to repeat it with different frequencies in order to determine the coil quality sweet spot. So after doing tons of tests and calculations I was able to plot this graph which clearly shows  us that the coil quality sweet spot can not be found at a higher frequency  which was my expectation but between 50 to 60kHz. That is why I added a 330nF capacitor to the coil pairs which in combination with  their known inductance should give us a new resonance frequency of around 57kHzAnd after trying out this newly fine tuned system I am happy to report that science did not let us  down and I was not only able to achieve a bigger distance of 70cm but also a bigger short circuit  current of more than double than before, awesome.But of course this was not enough for me yet  because I still had lots of litz wire lying around which should keep the skin  effect losses to a minimum and last time I tried using it I was simply to dumb to  think of simply putting 3 wires in parallel in order to decrease the copper resistance. So I did just that in a procedure that was almost 3 hours long  and created two promising looking coils but honestly speaking  I was not in the mood of repeating all the long measurements and calculations from before. And luckily I didn't have to because Elektor sent me one of their new 2MHz LCR meters whose test  frequency you can fine adjust and it tells you all the important values right on its main screen. So after unpacking all the parts, assembling them with a couple of screws and wires  and doing some calibrations, it was finally time to do some quicker measurements. And after only 3 minutes I recorded all the coil quality values  which to better visualize them I put in the same graph as the previous coil. It was interesting to see that their coil quality was about the same at higher frequencies while  the litz wire coil featured a higher quality once again at a sweet spot between 50 to 60kHz. So I picked a fitting capacitor and build up my final test system which did once again exceed  all the others with a distance of around 75cm and a short circuit current of 135mA. At the end I also tried calculating some efficiencies which varied quite a bit but were  overall way more promising looking than before.And with all my test and measurements being  completed so far I think I made it clear why inductors or coils can be sometimes  difficult to handle but I hope you understand them a bit better now. And I will see how I will use this LCR meter to improve my system even more in the future. If you want to see that then consider supporting me through Patreon  so that I can keep producing videos.Don't forget to like, share,  subscribe and hit the notification bell.Stay creative and I will see you next time.