Author Topic: mini tesla (hopefully not a fail)  (Read 2477 times)

Offline bogdan

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Re: mini tesla (hopefully not a fail)
« Reply #40 on: November 29, 2020, 07:30:41 AM »
Yes, the top-load was in it's place when i ran the test. 48.39kHz was the biggest until i reached 177.85kHz, in 100Hz steps the output goes away in about 5 steps so at @ 40 and 60 i have almost no output

Offline davekni

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Re: mini tesla (hopefully not a fail)
« Reply #41 on: November 29, 2020, 08:04:33 PM »
Great!  48kHz is the fundamental secondary frequency.  The goal now is to get the Slayer oscillator running around that frequency.  I'd start with 0.33uF, but also try 0.22uF and 0.44uF (two 0.22uF caps in parallel).  JavaTC indicates 0.38uF for primary resonance matching.  I think Slayer oscillators tend to work best with capacitors a bit under the resonant value.

I'm guessing that the higher-mode 178kHz showed higher amplitude because a node part-way up the secondary coil was closer to the scope probe.  Not sure.  Either way, the fundamental frequency of 48kHz is where the coil will run best.  (The frequency will not be exactly 48kHz, as the primary coil will shift the secondary frequency slightly.)
David Knierim

Offline bogdan

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Re: mini tesla (hopefully not a fail)
« Reply #42 on: November 30, 2020, 12:03:06 PM »
I have made some tests with a few capacitors, and i have managed to kill my IC...i don't know how... All i have now is the non inverting pair of this one, the TC4427, and i inverted the primary, all the tests are made with this configuration, the IC and MOSFETs are stone cold.

Offline davekni

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Re: mini tesla (hopefully not a fail)
« Reply #43 on: November 30, 2020, 06:56:13 PM »
Without labels, I can't tell which signal each scope image is measuring.  If the square-wave traces are of gate-to-source (Vgs), the voltage is too low to turn on the FET completely.  Vgs and Vds are the most useful plots to see.  If the other traces are Vds (drain-to-source), then the hump in the middle of low times would fit with low Vgs.  Otherwise, 0.33uF appears to be best.
David Knierim

Offline bogdan

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Re: mini tesla (hopefully not a fail)
« Reply #44 on: December 01, 2020, 09:29:24 AM »
Yes, the traces are what you said. I made the test with 6.5V because it was already drawing over 3A. Here is the test with the 33nF cap with 12V.

Offline davekni

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Re: mini tesla (hopefully not a fail)
« Reply #45 on: December 01, 2020, 07:44:14 PM »
Could you please scope Vds directly at the FET leads?  That will be a bit easier to decipher than measuring at the coil.

Also, a picture showing wiring from FETs to 0.33uF cap and primary coil would help.  (BTW, I presume you mean 330nF, not 33nF.)

After looking again at your scope images and running a quick simulation, I think I was guessing wrong as to which way around the scope is connected on the coil.  (Another reason to scope at the FETs, ground to source and probe to drain.)  After reinterpreting the images, I think 0.22uF is probably better.
« Last Edit: December 01, 2020, 09:20:39 PM by davekni »
David Knierim

Offline bogdan

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Re: mini tesla (hopefully not a fail)
« Reply #46 on: December 02, 2020, 08:55:06 AM »
Yes, i meant 330nF, my bad, sorry. When i connect the scope to the coil, i just put the ground and probe directly on the coil/cap connection, since my scope is on a battery, i don't have to deal with it's ground. Here are the tests and traces for gate and drain, the unlabeled picture is the gate trace of 330nF.

Offline davekni

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Re: mini tesla (hopefully not a fail)
« Reply #47 on: December 02, 2020, 08:12:55 PM »
Thank you for the scope images.  Those measurements on the FETs are easier to interpret.

Both 220nF and 330nF look reasonable.  330nF has a bit lower Vds, so a bit more headroom for increasing bus voltage, to around 30V for your 100V FETs.

Are you interested in downloading and learning the free analog simulator LTSpice?  I made a simulation of your Slayer circuit:


Plots look reasonably similar to your scope traces.  Here's Vds (green) and Vgs (blue) with 12V Vbus supply:


These are image captures.  I'll zip the simulation schematic if you decide to use LTSpice.  (There are other free analog simulators too.)

The current draw from VBus looks about right too.  Most of the power is going into secondary winding resistance.  Per simulation, increasing VBus to 30V gets just over 55kV peak on the top-load, likely enough to get some arcs.  Current goes up to 14A.  Do you have a supply that can provide 30V at 14A?  Or, if you have a large capacitor, charge it to 30V and use it for momentary power.  Unless you add a heatsink to the FETs, momentary operation is all that they will handle at high power.

Most SSTC designs include a 555 timer or other interrupter source to enable the driver for short periods of time.  That would make the average current draw much less than 14A at 30V, and is one of the key reasons interrupters are used.

The only other option that comes to mind for reducing power is to rewind the secondary with thicker wire.  That will reduce resistance and increase resonant frequency.  Doubling wire diameter should roughly cut secondary losses in half (for a given secondary voltage).  So, power will still be fairly high.  That points back to adding an interrupter.
David Knierim

Offline bogdan

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Re: mini tesla (hopefully not a fail)
« Reply #48 on: December 03, 2020, 12:05:36 PM »
I have used LTSpice before when i was playing with the induction heater circuit, i will try to find some higher voltage mosfets. Meanwhile i tried it with a 24V transformer, with the ground connected, and i touch only momentarily the positive lead of the transformer to the connection on the pcb...and my bridge rectifier is all over the room in tiny pieces. Fortunately the circuit is ok, but i did not hear any corona noises before it blew up.

Offline davekni

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Re: mini tesla (hopefully not a fail)
« Reply #49 on: December 03, 2020, 06:58:09 PM »
Good to hear that you have used LTSpice before.  Here's the LTSpice simulation schematic for you to try, zipped to make it compatible with attachments for this forum:
* slayer3.zip

Try running this circuit.  I believe the topology matches what you have implemented.  Once you are comfortable with exploring this, we can discuss adding interruption.  That will work better than momentarily connecting the power supply.
David Knierim

Offline bogdan

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Re: mini tesla (hopefully not a fail)
« Reply #50 on: December 05, 2020, 11:38:28 AM »
Thank you, i will try and learn from it as much as i can :)

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Re: mini tesla (hopefully not a fail)
« Reply #50 on: December 05, 2020, 11:38:28 AM »

 


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