OK, back to the actual research. Here is some of the experimenting I did yesterday. First, after tuning the L1 primary coil to resonance at roughly 500 KHz, I checked the capacitance of the C1 tuning capacitor to find the resonant value. Given the inductance of the coil I was expecting about 100 pF, and sure enough I measured 93 pF:
https://gateway.ipfs.io/ipfs/QmUu3QE...j/IMG_1533.JPG
Then I grabbed one of the many coils from my bin-o-previous-projects, an 80-turn coil on a 2: cardboard coil form. I can try using this as an L2 coil without having to wind a new one, at least for starters. It measures just under 80 uH:
https://gateway.ipfs.io/ipfs/QmUu3QE...j/IMG_1534.JPG
Now we will need a C2 tuning capacitor to bring the L2 coil into the same resonance as the L1 coil. Since the inductance of L2 is much less than L1, the capacitance has to be correspondingly more. The good news is that the voltage on the L2 side is much less, so we don't need a high-voltage tuning cap here with wide gaps between the plates. I had this one handy, it's one of the common "365 pF" radio tuning caps. It has two sections, and they can be paralleled for twice the capacitance, as seen in this picture. The maximum capacitance with the plates fully meshed is about 760 pF tested:
https://gateway.ipfs.io/ipfs/QmUu3QE...j/IMG_1536.JPG
And the minimum capacitance is about 40 pF. Note that this cheap L/C meter is probably not all that accurate, but it seems to do an adequate job for this purpose.
https://gateway.ipfs.io/ipfs/QmUu3QE...j/IMG_1537.JPG
Comparing the two variable capacitors, C1 and C2, you can see at a glance the difference in the spacing between the plates. The C1 cap looks somewhat odd because the one I bought off of ebay wasn't the model I was expecting and I had to rebuild it by doubling up the little spacers between the plates to get them wide enough. Hopefully the C1 cap should handle several kilovolts without arcing over between the plates now.
https://gateway.ipfs.io/ipfs/QmUu3QE...j/IMG_1539.JPG
Here's the arrangement for testing for resonance using a function generator. The L2 and C2 coils are in parallel, being driven by the function generator through a 10K ohm resistor. This produces a fairly sharp voltage peak right at resonance as you tune the function generator. This is because of the high impedance of the parallel LC tank circuit formed by L2 and C2. At exact resonance, the capacitive reactance and inductive reactance exactly cancel and it appears as a high resistance, proportional to the tuned circuit Q.
https://gateway.ipfs.io/ipfs/QmUu3QE...j/IMG_1540.JPG
Here is the scope and function generator being used to tune for resonance. With the C2 cap fully closed (maximum capacitance) the resonant frequency is right about 680 KHz. To tune down to 500 KHz to match the L1/C1 side, we will need to add a fixed capacitor in parallel with C2. I haven't yet calculated how much this needs to be, but probably around 500 pF as a first guess.
https://gateway.ipfs.io/ipfs/QmUu3QE...j/IMG_1541.JPG
Here is a short video of how to tune for resonance with a function generator. Watch as I change the cap holding the function generator frequency constant, and then change the frequency holding the capacitance constant. This is where a digital oscilloscope makes your life much easier because it will calculate and display the frequency in real time, fairly accurately. A separate frequency counter would also work for this. It would also be possible to connect the high voltage supply and spark gap to the L2/C2 combo like I did with the L1/C1 combo and read the ring frequency off of the scope, it should exactly match the results obtained this way with the function generator.
https://gateway.ipfs.io/ipfs/QmUu3QEhtVec3ADaXCvamrZEvijBLbwb9P34cYWKoWgacj/MVI_1544.MP4
Edit: I tried to include the pictures inline but they were too large to display correctly. I included them as links instead, you'll have to open each one to load and then you can see it either full size or scaled as you choose. Hopefully I got all these IPFS links correct, they just link directly to files on my computer. I apologize for the large file sizes, but I think about how much time I have spent studying pictures and videos by other researchers and I always wished the pictures were clearer so you could see every detail. I shoot them in full 10MP camera resolution for archival and now you can see them that way too. Using IPFS you can also download them to your computer easily, there are several different ways to do it. Learn more about IPFS.
https://gateway.ipfs.io/ipfs/QmUu3QE...j/IMG_1533.JPG
Then I grabbed one of the many coils from my bin-o-previous-projects, an 80-turn coil on a 2: cardboard coil form. I can try using this as an L2 coil without having to wind a new one, at least for starters. It measures just under 80 uH:
https://gateway.ipfs.io/ipfs/QmUu3QE...j/IMG_1534.JPG
Now we will need a C2 tuning capacitor to bring the L2 coil into the same resonance as the L1 coil. Since the inductance of L2 is much less than L1, the capacitance has to be correspondingly more. The good news is that the voltage on the L2 side is much less, so we don't need a high-voltage tuning cap here with wide gaps between the plates. I had this one handy, it's one of the common "365 pF" radio tuning caps. It has two sections, and they can be paralleled for twice the capacitance, as seen in this picture. The maximum capacitance with the plates fully meshed is about 760 pF tested:
https://gateway.ipfs.io/ipfs/QmUu3QE...j/IMG_1536.JPG
And the minimum capacitance is about 40 pF. Note that this cheap L/C meter is probably not all that accurate, but it seems to do an adequate job for this purpose.
https://gateway.ipfs.io/ipfs/QmUu3QE...j/IMG_1537.JPG
Comparing the two variable capacitors, C1 and C2, you can see at a glance the difference in the spacing between the plates. The C1 cap looks somewhat odd because the one I bought off of ebay wasn't the model I was expecting and I had to rebuild it by doubling up the little spacers between the plates to get them wide enough. Hopefully the C1 cap should handle several kilovolts without arcing over between the plates now.
https://gateway.ipfs.io/ipfs/QmUu3QE...j/IMG_1539.JPG
Here's the arrangement for testing for resonance using a function generator. The L2 and C2 coils are in parallel, being driven by the function generator through a 10K ohm resistor. This produces a fairly sharp voltage peak right at resonance as you tune the function generator. This is because of the high impedance of the parallel LC tank circuit formed by L2 and C2. At exact resonance, the capacitive reactance and inductive reactance exactly cancel and it appears as a high resistance, proportional to the tuned circuit Q.
https://gateway.ipfs.io/ipfs/QmUu3QE...j/IMG_1540.JPG
Here is the scope and function generator being used to tune for resonance. With the C2 cap fully closed (maximum capacitance) the resonant frequency is right about 680 KHz. To tune down to 500 KHz to match the L1/C1 side, we will need to add a fixed capacitor in parallel with C2. I haven't yet calculated how much this needs to be, but probably around 500 pF as a first guess.
https://gateway.ipfs.io/ipfs/QmUu3QE...j/IMG_1541.JPG
Here is a short video of how to tune for resonance with a function generator. Watch as I change the cap holding the function generator frequency constant, and then change the frequency holding the capacitance constant. This is where a digital oscilloscope makes your life much easier because it will calculate and display the frequency in real time, fairly accurately. A separate frequency counter would also work for this. It would also be possible to connect the high voltage supply and spark gap to the L2/C2 combo like I did with the L1/C1 combo and read the ring frequency off of the scope, it should exactly match the results obtained this way with the function generator.
https://gateway.ipfs.io/ipfs/QmUu3QEhtVec3ADaXCvamrZEvijBLbwb9P34cYWKoWgacj/MVI_1544.MP4
Edit: I tried to include the pictures inline but they were too large to display correctly. I included them as links instead, you'll have to open each one to load and then you can see it either full size or scaled as you choose. Hopefully I got all these IPFS links correct, they just link directly to files on my computer. I apologize for the large file sizes, but I think about how much time I have spent studying pictures and videos by other researchers and I always wished the pictures were clearer so you could see every detail. I shoot them in full 10MP camera resolution for archival and now you can see them that way too. Using IPFS you can also download them to your computer easily, there are several different ways to do it. Learn more about IPFS.
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