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Solid States Devices => solid state devices => Topic started by: Jeg on January 12, 2014, 10:51:58 AM

Title: sstc changing frequency issue
Post by: Jeg 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
Title: Re: sstc changing frequency issue
Post by: MarkE on January 12, 2014, 11:13:16 AM
You can slow it down by adding capacitance from the MOSFET gate to the MOSFET source.
Title: Re: sstc changing frequency issue
Post by: Jeg on January 12, 2014, 11:42:10 AM
That's the definition of simplicity! Thaks MarkE, !!! :)

Title: Re: sstc changing frequency issue
Post by: gyulasun 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
Title: Re: sstc changing frequency issue
Post by: Jeg 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
Title: Re: sstc changing frequency issue
Post by: MarkE 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.
Title: Re: sstc changing frequency issue
Post by: gyulasun 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
Title: Re: sstc changing frequency issue
Post by: MarkE 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.
Title: Re: sstc changing frequency issue
Post by: TinselKoala 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.


Title: Re: sstc changing frequency issue
Post by: Jeg 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



 
Title: Re: sstc changing frequency issue
Post by: MarkE 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.
Title: Re: sstc changing frequency issue
Post by: Jeg 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
Title: Re: sstc changing frequency issue
Post by: MarkE 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.
Title: Re: sstc changing frequency issue
Post by: Jeg on January 13, 2014, 02:40:51 PM
Tnks MarkE... i hope i'll do it tonight so to discuss it tomorrow. :)
Title: Re: sstc changing frequency issue
Post by: Jeg 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?

STGW40N120KD
Title: Re: sstc changing frequency issue
Post by: Jeg on March 08, 2014, 02:37:22 PM
I recall this in case someone knows the way to improve the power of the above circuit. In the meanwhile I reduced the power supply to 48v and it works in a safety voltage level. One more question is why frequency changes according to the power supply level? Does it have to do with the current that loads the mosfet's gate capacitance? More voltage more current faster switching capability, higher resonant frequency?

Tnks
Jeg
Title: Re: sstc changing frequency issue
Post by: TinselKoala on March 08, 2014, 03:48:14 PM
I've found this kind of circuit to be very sensitive to a number of things. Coupling between primary and secondary is very important, on my coil the primary has to be at just the right height and spaced just right, too tight or too loose will really affect the power throughput. The positioning of the pickup-feedback coil is critical too, mine works best when this coil is actually underneath the primary-secondary arrangement, and I use just a single turn for this pickup. Also the mosfet is critical, probably because of the capacitance. I finally settled on the same IRFP460 and found that the IRFP460N variant would not work as well. The circuit winds up driving the mosfet gate with a swinging AC signal and the gate protection zeners don't always work, as Jeg has found out. I've put an analog ammeter in my circuit and I know never to allow the current in that part of the circuit to go over six amps or the mosfet will do what Jeg has described.
Looking at the secondary's HV field output by using a scope probe connected to a bit of aluminum foil some distance away from the coil, and also the mosfet drain signal, can be very diagnostic. When the circuit is running properly the mosfet drain will be a series of sharp fast spikes and the field pickup from the secondary will show a beautiful and clean sine wave.
There is something about the "4 Mhz" frequency that this circuit likes. Push it in either direction and you get different performance. Higher frequency and the output is more like a radio transmitter (duh) and you get less corona and sparks and stuff but you can get good effects lighting external CFLs and such. Lower frequency and you need larger coils, more control over coupling, more terminal capacitance, and again, the character of the corona display changes.  There are better designs for lower-frequency SSTCs than this one, I think.

However this design may work really well for a "plasma speaker". I got amazing fidelity at very low volume from an audio-modulated "Jacob's Ladder" ; excellent speaker effect even when the HV output was so low that no corona was visible.

Title: Re: sstc changing frequency issue
Post by: Jeg on March 08, 2014, 06:44:39 PM
...When the circuit is running properly the mosfet drain will be a series of sharp fast spikes...

If you set correctly the two coils at primary's side in means of resonance ala Don Smith (wavelength related), there are no Spikes anymore. Just a clean sinewave.


Title: Re: sstc changing frequency issue
Post by: TinselKoala on March 08, 2014, 07:04:56 PM
If you set correctly the two coils at primary's side in means of resonance ala Don Smith (wavelength related), there are no Spikes anymore. Just a clean sinewave.

Maybe that's why you are blowing mosfets and not getting much output, whereas I don't blow the mosfet (any more, knock on wood) , it switches cleanly at a low voltage. To get good output from a secondary, any secondary, the rate of change of the current in the primary has to be as fast as possible. Driving with a sine wave is very _inefficient_ in terms of making HV output and it also stresses the mosfet more than the right kind of spikes.

In the image below, the top trace is the Drain trace at 10V/div  and the bottom trace is the pickup from a piece of foil a meter away from the secondary, displayed at 50 V/div. Input was 22 volts, current not recorded.
Title: Re: sstc changing frequency issue
Post by: Jeg on March 08, 2014, 07:20:30 PM
This is mine. Check the time between peaks...
The time diff between the dotted lines is 25.5ns, but if you notice they are not centered. The actual diff between peaks is 18.5ns
Title: Re: sstc changing frequency issue
Post by: Jeg on March 08, 2014, 10:38:06 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.


Gyula I am sorry for not doing this earlier but I just notice that. You have helped me many times in the past and I really admire you for your knowledge in electronics. What I meant was that the worst scenario was to rebuilt my secondary as I do that by hand and it really kills me! I hope that there is no misunderstanding here.

Best regards
Jeg
Title: Re: sstc changing frequency issue
Post by: Jeg on March 15, 2014, 05:55:40 PM
Hi to all :)
The following images are from the same Radu Motisan Oscillator circuit. Shots were taken with oscilloscope's ground clip attached to probe's tip forming a one loop receiver coil.

Six turns main coil, two turns tickler coil.
Every shot is with different cable length attaching coils to the main circuit.
Power supply is 36v by 3X12V batteries.
Mosfet Irfp460

Every Mosfet changing even same brand, needs re adjusting the cable lengths to achieve the same as below results. After some training on this I can achieve the right resonance in a couple of hours by trimming and trimming and trimming the cable length. To make this I burned over 15 Mosfets because when resonance is being achieved the build up of voltage is enormous that even fast fuses can not catch it. Finally I stabilized that with 36 volts supply, and pot in place of the 1K resistor and of course a current meter to watch what happens on drain.

Just watch the coil's oscillation transformation in relation with the different lengths. And finally watch at the end the perfect resonance at the natural frequency of the wire. Around 3Mhz switching mosfet frequency, and about 50MHZ oscillation at the coils at 60v!!!

Title: Re: sstc changing frequency issue
Post by: Jeg on March 16, 2014, 08:39:32 AM
I am thinking of modulating it by supplying it 36V rectified 50Hz signal from main without smoothing cap. This teqhnic will help to take 50Hz at a secondary output...i think! What is your opinion guys?
Title: Re: sstc changing frequency issue
Post by: Jeg on March 17, 2014, 02:03:16 PM
Has anyone use this Mazilli driver? Does anyone know the frequencies that can catch with air coils?
Title: Re: sstc changing frequency issue
Post by: Jeg on March 23, 2014, 12:48:24 PM
sorry guys I need to make some editing on this.
Thanks
Jeg
Title: Re: sstc changing frequency issue
Post by: Jeg on May 11, 2014, 02:19:43 PM
Hey guys
I need your opinion for analyzing more this Armstrong - mosfet oscillator. More specifically, i'd like to know about this pi filter consisting of 0.01uF, 100uH, and 0.68uF. Is there any way from this to find out the cut off freq. that Radu used? What is the effect of this filter on the overall efficiency of the oscillator? Is 0.68uF cap affecting the osc. freq of the drain coil? What if i discard both capacitors and keep only the chock as in mazzilli driver?
The same questions applies on the use of the 0.22uF Cap. Whats its use there?

What i don't like on this oscillator is its instability, and the same time its capability for exploding mosfets without efforts :D Is it possible without many modifications to use an IGBT instead of the mosfet?

I could ask this questions on a general purpose electronic forum, but inside this forum there are members with so much knowledge that is very rare to find them somewhere else. ;)


Tnks
Jeg
Title: Re: sstc changing frequency issue
Post by: Groundloop on May 11, 2014, 03:00:11 PM
Hey guys
I need your opinion for analyzing more this Armstrong - mosfet oscillator. More specifically, i'd like to know about this pi filter consisting of 0.01uF, 100uH, and 0.68uF. Is there any way from this to find out the cut off freq. that Radu used? What is the effect of this filter on the overall efficiency of the oscillator? Is 0.68uF cap affecting the osc. freq of the drain coil? What if i discard both capacitors and keep only the chock as in mazzilli driver?
The same questions applies on the use of the 0.22uF Cap. Whats its use there?

What i don't like on this oscillator is its instability, and the same time its capability for exploding mosfets without efforts :D Is it possible without many modifications to use an IGBT instead of the mosfet?

I could ask this questions on a general purpose electronic forum, but inside this forum there are members with so much knowledge that is very rare to find them somewhere else. ;)


Tnks
Jeg

http://www.electroschematics.com/835/rfsim99-download/

GL.
Title: Re: sstc changing frequency issue
Post by: Jeg on May 12, 2014, 08:03:07 AM
http://www.electroschematics.com/835/rfsim99-download/

GL.

Cool soft GL! Already downloaded and installed! This self oscillator was very educative to me as I learned how a constructive resonance build up can happen. It is just time for improvements on this, as its instability already cost me more than 60euros in mosfets!!! :D