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Author Topic: sstc changing frequency issue  (Read 20518 times)

Offline Jeg

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sstc changing frequency issue
« on: January 12, 2014, 10:51:58 AM »
Hi guys, i wish you a happy new year! :)

I recently built this Armstrong oscillator using an IRFP460 Mosfet. With 6 turns primary and 2 turns feedback, seems to work great and most important...cold! I tried to lower its frequency from 5MHz to 3MHz, by changing the primary coil turns, but things didn't go very well... I am thinking to leave the tank coil as it is, and change the caps. What is your opinion on this? Any drawbacks? Which is the most efficient and simple way to do it?

Tnks
Jeg

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sstc changing frequency issue
« on: January 12, 2014, 10:51:58 AM »

Offline MarkE

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Re: sstc changing frequency issue
« Reply #1 on: January 12, 2014, 11:13:16 AM »
You can slow it down by adding capacitance from the MOSFET gate to the MOSFET source.

Offline Jeg

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Re: sstc changing frequency issue
« Reply #2 on: January 12, 2014, 11:42:10 AM »
That's the definition of simplicity! Thaks MarkE, !!! :)


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Re: sstc changing frequency issue
« Reply #2 on: January 12, 2014, 11:42:10 AM »
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Offline gyulasun

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Re: sstc changing frequency issue
« Reply #3 on: January 12, 2014, 12:39:10 PM »
Hi Jeg,

I think it is your 1000 turn coil which basically defines the 5 MHz oscillation frequency...  You have an LC tank: L is the 1000 turn coil and C is the self capacitance of this coil (including any capacitive reactance transformed into the tank from the whole circuit), all the other components serve to maintain the oscillation energy in the LC tank via the coupling coils of 2 and 6 turns. 

Obviously you can vary some of the other C components, it will influence the 5 MHz a little but the main thing would be to rewind the 1000 turn coil: perhaps use a 4 or 5 cm coil bobbin to wind the 1000 turns using longer wire of course.  (I know it is not a pleasant thing to wind so many turns again...)

Alternatively, you could use a small piece of RF quality ferrite core and insert it into the grounded end of the 1000 turn coil (obviously Tesla did not use any core in his secondaries but then he used higher diameter coils like 1.5 cm)  but at least you can gain some experience what components change the 5 MHz the most.

Gyula

Offline Jeg

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Re: sstc changing frequency issue
« Reply #4 on: January 12, 2014, 01:04:23 PM »
My gosh Gyula! This is the worst scenario! I'll give it a try today starting with MarkE proposal. If it wont work as i 'd like then i will use the ferrite idea before i start winding again!
Thanks

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Re: sstc changing frequency issue
« Reply #4 on: January 12, 2014, 01:04:23 PM »
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Offline MarkE

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Re: sstc changing frequency issue
« Reply #5 on: January 12, 2014, 01:16:10 PM »
The MOSFET gate to source capacitance appears in parallel with the gate winding.  Adding more capacitance from the gate to source or using a MOSFET with more capacitance slows the circuit down.  If you do not have a lot of inductance in the wiring between the gate winding and the MOSFET and/or any additional capacitance you add, then the additional capacitance reflects through the transformer.

Offline gyulasun

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Re: sstc changing frequency issue
« Reply #6 on: January 12, 2014, 06:37:58 PM »
Hi Jeg,

Please do the tests as MarkE suggested of course, I did not mean to dissuade you from anything what others tell you.
My opinion is that if you increase the gate-source capacitance you may reach an oscillation limit whereby the oscillation frequency may start decreasing to a certain degree indeed due to the reactance transformation action (I mentioned this in my first post) what both coupling coils do towards the 1000 turn tank coil  but then the increasing shunting effect of the high capacitance value you connect between the gate-source would eventually stop the oscillator:  always test the start-up by switching off and then on the circuit when increasing the capacitor to check the normal start-up.  The shunting effect means that the AC voltage feedback to the gate from the 1000 turn tank goes lower than a certain treshold which would be just needed and just enough to start and maintain the oscillations for the tank.

Addition:  the biggest frequency change from your 5 MHz to the lower frequencies like 3 MHz could also be made by adding a 'capacitive load' to the upper 'hot' end of the 1000 turn coil which is now floating.  Even adding a HV capacitor of a few picoFarad like 8-10 pF in parallel with the 1000 turn would reduce the 5 MHz significantly.  But Tesla did not do it: he used capacitive load (a meatal ring hat )  to fine tune his secondary but this loading capacitance was not connected to the grounded end of his secondary. The question is what you wish to use the HV from the 1000 turn coil, maybe you can use some HV capacitors in  parallel.

Gyula

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Re: sstc changing frequency issue
« Reply #6 on: January 12, 2014, 06:37:58 PM »
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Offline MarkE

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Re: sstc changing frequency issue
« Reply #7 on: January 12, 2014, 07:15:01 PM »
The increase in capacitance needed is roughly 2.7:1.  A high voltage capacitor can be made by stringing lower voltage capacitors in series.  This is probably not a very good idea with a Tesla coil where the voltages can get very high.  Fashioning a parasitic capacitor as just a disk or a ring connected to the high voltage is a good solution if you have some metal stock around and are handy with tin snips and a file to smooth the edges.  Increasing the diameter of your ball will also increase the capacitance.

As long as the primary and secondary are tightly coupled you should be able to increase capacitance on the low voltage primary without much trouble.  You will have problems if there is a lot of leakage inductance in the transformer and or the wiring from the transformer to the capacitor as added capacitance will then increase the phase shift of that low pass network and enough of that will kill the oscillator.  So keep whatever you use for a capacitor close to the transformer primary.

Offline TinselKoala

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Re: sstc changing frequency issue
« Reply #8 on: January 12, 2014, 07:17:40 PM »
All of the above, plus you could also use more wire on the secondary, like going to 1200 turns. This is very similar to my "Sassy ClassE" sstc.
You will get different quality in the corona as you lower frequency. If you lower frequency by adding terminal capacity, you may not be able to get corona breakout, but you may get lots of electric field power transmission to CFLs and neons, etc, near the coil.



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Re: sstc changing frequency issue
« Reply #8 on: January 12, 2014, 07:17:40 PM »
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Offline Jeg

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Re: sstc changing frequency issue
« Reply #9 on: January 13, 2014, 01:51:28 PM »
Hey guys
Every time i learn something new from you. Thanks a lot!

I fix it by providing more capacitance between gate and source as MarkE said, but also i will make a new secondary with more turns as Guyla and Tins proposed. It is really fan working on these Frequencies @ MHz range!!! What surprised me is the big burn on my hand just by touching Drain at 360V peaks of 3MHz! It was really a nasty burn!!!

My next try is to inject audio frequencies in to this little but powerful oscillator, for AM modulation. I will start by connecting the audio gen after the 15K resistor at where it meets the 1K resistor. At this point there is a DC voltage of about 2-5V (pot regulation in parallel with the 1K resistor), and an AC signal of the feedback coil at 3MHz. I guess i have to put some filter arrangement before injecting it, so to prevent the High freq. oscillation to come in to my audio gen. Have anyone try it? Are there any other critical points that i have to consider?

Tnks
Jeg



 

Offline MarkE

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Re: sstc changing frequency issue
« Reply #10 on: January 13, 2014, 01:55:32 PM »
Jeg, you can make your life slightly easier by lifting the 0.22uF capacitor from ground and inserting a choke there.  You can then inject your audio at the junction of the capacitor and the choke.

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Re: sstc changing frequency issue
« Reply #10 on: January 13, 2014, 01:55:32 PM »
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Offline Jeg

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Re: sstc changing frequency issue
« Reply #11 on: January 13, 2014, 02:06:28 PM »
I really like when my life is getting easier! ;D

Is there any way to calculate this choke before to start winding?
Do i have to make a resonance between choke and cap at these low audio freqs?

Tnks
Jeg

Offline MarkE

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Re: sstc changing frequency issue
« Reply #12 on: January 13, 2014, 02:15:47 PM »
Pick the choke inductance based on the impedance of your audio source and the lower frequency limit that you want to go down to:

L ~= 0.025/(Fcutoff^2*0.22E-6)

Once you know the inductance, you can buy an inductor, or use the tables for whatever core you choose to determine the number of turns.

Another way to do it would be to replace the 0.22uF cap with something smaller like 0.022uF, and then just use a 2.2uF or larger nonpolar capacitor to couple your signal source at the junction of the 1K and 15K resistors.  A 0.022uF capacitor will roll off high frequencies at about 8kHz.  A 2.2uF coupling capacitor from your source will roll off on the low end at about 80Hz.  If you want to go lower than that, just use a bigger coupling capacitor.

Offline Jeg

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Re: sstc changing frequency issue
« Reply #13 on: January 13, 2014, 02:40:51 PM »
Tnks MarkE... i hope i'll do it tonight so to discuss it tomorrow. :)

Offline Jeg

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Re: sstc changing frequency issue
« Reply #14 on: March 06, 2014, 06:56:35 PM »
Hi again :)
At the above self oscillating mosfet circuit, when I matched the two coils in terms of wavelength, the oscillation became a clear sinus which immediately started to grow. After 2 seconds the oscillation became so big that mosfet explode. I think because of overdriving the gate above this 20V limit even I had a 18v zener between gate and sourse. I am thinking to substitute mosfet with a very tough IGbt. Does anyone know the way? I test it like that but it didn't work. Is there any other self oscillating circuit appropriate for IGBT?

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