mini tesla (hopefully not a fail)

[bogdan]

Hello everyone, after a fail with my attempt to make a big tesla coil, i decided to start smaller, so i built a mini version. However it bothers me that the secondary coil resonates at @835kHz, that high number scares me a little so i stopped to ask a few questions. Is that frequency a bit high ? can i reduce it ? and what driver do you recommend for the primary to deal with such high frequency ?

[Zipdox]

No that resonance frequency isn't that high at all for a small coil.

[bogdan]

So what driver should i use ? i tried it with a 555, but it's not acurate at all at that frequency, i even tried it with an arduino, but the square wave...is not a square wave starting from 500kHz.

[Zipdox]

So what driver should i use ? i tried it with a 555, but it's not acurate at all at that frequency, i even tried it with an arduino, but the square wave...is not a square wave starting from 500kHz.

Using a fixed frequency oscillator is not a good idea. You should be using a driver that takes some kind of feedback.

[bogdan]

Well, i tried the  half-bridge steve ward, but the mosfet drivers just blew up in my face, and after a lot of troubleshooting i could not find anything so i abandoned the project...

[Zipdox]

Well, i tried the  half-bridge steve ward, but the mosfet drivers just blew up in my face, and after a lot of troubleshooting i could not find anything so i abandoned the project...

Schematic of what you built?

[bogdan]

here it is http://kaizerpowerelectronics.dk/tesla-coils/kaizer-sstc-i/

[davekni]

At 835kHz, gate driver chip failure seems likely.  The gate charge of a pair of IRFP460 FETs repeated at 835kHz likely translates to more power dissipation than the gate driver chips can dissipate.

SiC FETs typically have lower gate charge and switch quickly.  That's one option.  Another (or in combination) is to add discrete FET buffers to the gate driver outputs as in many DRSSTC designs, with heat sinks on the buffer FETs.

[bogdan]

Uhm...and in English please ? what does that mean for me ?

[davekni]

Sorry, I hadn't paid attention to this topic being in the beginners section.

As you have measured, small designs generally resonate at high frequencies, 835kHz in your case.  Drive electronics are more difficult at high frequencies, especially at high power.  Perhaps a Slayer oscillator running from 24Vdc would be a good starting point for a tiny coil.  I don't have personal experience with Slayer oscillators.  There are some examples on this forum.  Slayer oscillators use a single power transistor and use the secondary ground-return to drive the gate.

Another option for a small coil is to wind it with MANY turns of fine wire.  That's not easy either, but does reduce frequency.

Good luck!

[bogdan]

but a slayer will not give me any sparks, or very small ones, i want to have at least o couple of centimeters with this coil. If i want to make more turns, that means thinner wire, is it a bad thing ?

[davekni]

People do get sparks from Slayer driven Tesla coils.  I'd guess that 2cm is achievable, but don't have personal experience.  High frequency can make sparks at lower voltages.

Yes, fine wire does have a down-side.  The series resistance is higher, so secondary Q lower.  However, if you can get the frequency low enough to drive with a normal half-bridge, that may give better performance.

For half-bridge drive, adding a 555 to enable it for short times with longer gaps (low duty cycle) will allow gate-driver chips to survive high frequencies.  That might work even up to 835kHz of your existing coil, but will help even if you rewind to get down to 400kHz or 500kHz.  KaizerPowerElectronics web access isn't working for me at the moment, so I can't double check if enable modulation was already part of the schematic you tried or if there is a similar schematic including such.

[bogdan]

ok, i will look into the thing called slayer on steroids and see what results will i get, i will post them later on, hopefully i get enough time in this weekend. Thank you.

[davekni]

The "slayer on steroids" I've seen uses a gate-driver IC, so may have the same overheating issue.

Is a larger secondary diameter possible within your definition of a small coil?  Doubling the diameter will almost halve the frequency using the same wire gauge.

[bogdan]

i can test the circuit on my bigger coil, the frequency of that one is @141kHz, and if everything is ok, i can test the small one.

[bogdan]

So, i built this slayer on steroids circuit, and made a few tests, but, i don't seem to be able to tune it properly, i am posting some waveforms with oscilloscope directly on the coil, and with the power source set at 5V. The current draw is about 2A for all capacitors, except for the test run with no capacitor. Each photo has the capacitor near it. any idea is welcomed.

[davekni]

To be helpful, we need the specific schematic you implemented (schematic image or web link to the schematic) and an image or two of your setup showing where the antenna is placed and wired to the slayer oscillator.

[bogdan]

I hope you can tell from these

[davekni]

Have you tried reversing the primary coil leads?  Usually one polarity works better, and the opposite polarity may not work at all.

What value is R1?  I can't quite read the color bands in the image.

Oscillation without a capacitor was 213kHz, above your expected 141kHz secondary resonance frequency.  With the smallest capacitor you tried, 0.22uF, frequency was too low, 96kHz.  That implies a smaller capacitor would get closer to 141kHz, around 0.1uF or perhaps a bit lower.

Are the four IRFP140 FETs wired in parallel?  That adds up to more load than TC4426 is designed to drive.  May still work at 141kHz, but definitely won't at 835kHz.  A scope image of the gate waveform would be helpful, together with a drain waveform with the same configuration.

[bogdan]

"Have you tried reversing the primary coil leads?  Usually one polarity works better, and the opposite polarity may not work at all."
yes, i seem to get the best results this way

"What value is R1?"
8.9k, it was the one nearest to my hand

"Oscillation without a capacitor was 213kHz, above your expected 141kHz secondary resonance frequency.  With the smallest capacitor you tried, 0.22uF, frequency was too low, 96kHz.  That implies a smaller capacitor would get closer to 141kHz, around 0.1uF or perhaps a bit lower."
now this is strange to me as well, the 141kHz i got with a small capacitor, which sadly is gone. Could that be the reason? and the actual frequency of my coil is actually @210kHz ?? the test i made with the following capacitor shows different results, the capacitor is new, and also the tests are being made with a 12v power supply, also i get around 1cm of starting spark, and i can stretch it at @3cm. If i go lower than 100nF, either my caps are busted or something is wrong because they get really really hot, i will try to find some new ones.

"Are the four IRFP140 FETs wired in parallel?"
yes they are.