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Author Topic: sstc changing frequency issue  (Read 17610 times)

Offline Jeg

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Re: sstc changing frequency issue
« Reply #15 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

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Re: sstc changing frequency issue
« Reply #15 on: March 08, 2014, 02:37:22 PM »

Offline TinselKoala

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Re: sstc changing frequency issue
« Reply #16 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.


Offline Jeg

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Re: sstc changing frequency issue
« Reply #17 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.



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Re: sstc changing frequency issue
« Reply #17 on: March 08, 2014, 06:44:39 PM »
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Offline TinselKoala

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Re: sstc changing frequency issue
« Reply #18 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.

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Offline Jeg

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Re: sstc changing frequency issue
« Reply #19 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

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Re: sstc changing frequency issue
« Reply #19 on: March 08, 2014, 07:20:30 PM »
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Offline Jeg

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Re: sstc changing frequency issue
« Reply #20 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

Offline Jeg

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Re: sstc changing frequency issue
« Reply #21 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!!!


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Re: sstc changing frequency issue
« Reply #21 on: March 15, 2014, 05:55:40 PM »
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Offline Jeg

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Re: sstc changing frequency issue
« Reply #22 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?

Offline Jeg

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Re: sstc changing frequency issue
« Reply #23 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?

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Re: sstc changing frequency issue
« Reply #23 on: March 17, 2014, 02:03:16 PM »
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Offline Jeg

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Re: sstc changing frequency issue
« Reply #24 on: March 23, 2014, 12:48:24 PM »
sorry guys I need to make some editing on this.
Thanks
Jeg
« Last Edit: March 23, 2014, 10:00:19 PM by Jeg »

Offline Jeg

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Re: sstc changing frequency issue
« Reply #25 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

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Re: sstc changing frequency issue
« Reply #25 on: May 11, 2014, 02:19:43 PM »
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Offline Groundloop

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Re: sstc changing frequency issue
« Reply #26 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.

Offline Jeg

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Re: sstc changing frequency issue
« Reply #27 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

 

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