High Voltage Forum
Tesla coils => Dual Resonant Solid State Tesla coils (DRSSTC) => Topic started by: Nijin on August 03, 2018, 10:34:21 AM
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Hello coilers,
Last week I finished my DRSSTC and the first test gave me 240cm of spark. My goal is 3m and I have many tracks of improvement. I wondering how can I choose the Primary Impedance for a given frequency and the impact of it.
I use GTO capacitors rated at 3,5uF 1500Vdc per caps.
With my primary coil, I can change my primary caps for 388nF (9 GTO in series), 435nF (8 GTO in series) or 500nF (7 GTO in series) to stay between [52-60]kHz depending how much I want to detune my coil. I've done some simulation with different L C and Fres values to calcultate Z surge, it's between 5,64 and 7 depending F res chosen.
(https://highvoltageforum.net/proxy.php?request=http%3A%2F%2F%5Battachment%3D1%5D%5B%2Fattachment%5D&hash=0c82d6fed55acc164f4acdd9cd8f1251675d9e42)
Ocd is set at 1350 but I can push it at 1700A easily with SKM400GB125D IGBT.
Based on your experience wich primary capacitance is the best to perform the biggest streamer ?
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Congratulations on finishing your DRSSTC!
Lovely video :)
Longer sparks are a product of a higher energy transfer, the biggest effect from a larger MMC is first and foremost much brighter sparks. Look at coils like Kizmo's BiggerDR and Roy Piepers DRSSTC, both have a 1 uF or 1.5 uF MMC, but their sparks are not getting that much longer as the short transfer time limits the system that way.
For a coil you size I would not go below 500 nF, the smaller MMC capacitance will only result in longer on-time needed to produce just as/longer sparks.
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Thank you Mads for your answer :D
I think the best way is to test both 438nF and 500nF. Futhermore, it will reduce the maximum number of turns of my primary and the price ahah
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I enjoyed your video :)
A larger MMC will cause a faster rampup of primary energy, so the higher notes usually limited by short bursts will be louder. On the other hand you might not be able to run long bursts at all since the OCD will limit the length. For biggest arcs, given that you won't cut off the burst by the interruptor, the primary Z should be chosen, so that the OCD will barely not be triggered.
Can you tell us, where your peak primary currents are at and whether your OCD is triggered regularly? If the latter is the case, you should reduce the MMC. If the peak current always stays well below the OCD limit, you should increase the MMC capacitance or detune a bit more.
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I will give you more details about primary current when I will return at school in september, I just tested the coil last week but without any measures and tuning (I think the primary is too detuned for the secondary).
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I found that I got similar spark lengths, but different behaviour, when tuning primary below (i.e. comes into tune with longer streamers) or near/above the secondary. The streamers were "crazier" when tuned below secondary, but MIDI worked better when tuned near/above, so the choice may also depend on how you plan to run the coil. It sounds like you have the flexibility to do either, and those IGBTs are nice and fast, easily able to do 60kHz or more (where did you get them?).
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The coil was design to represent my engineering school during events, we want to make long spark and play music so I think the tuning will change depending where I present it ( I don't think I'm allowed to make 3m spark in a classroom ;D ). I order IGBT on rs-online, they was paid by my school with the DC bus caps but recently I saw on semikron website that you can order samples IGBT brick. I didn't tried yet.