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Registered Member #48007
Joined: Wed Jun 18 2014, 01:00PM
Location: Prilep, Republic of Macedonia
Posts: 33
For a while I was looking at SSTC's, and after one failed attempt, I saw that a simple, yet powerful SSTC can be made using the Mazzilli ZVS driver to drive the primary. my secondary resonant frequency is around 560kHz, and I'm planning on using the IRFP260 from International Rectifier. The zener diode I plan to use is 12V at 5W and the Vdd to gate resistor value I plan to use is 470Ohm. The toroid inductor I made by winding 24 turns of litz wire made by twisting together 4 wires of 0,5mm enamel wire, giving a total of 2mm litz wire, this wire is wound on a pale green/blue toroid core from a old smps. Would this work at such a high frequency, what wire thickness for the primary should I use? Any comments are much appreciated.
Registered Member #4074
Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335
The typical ZVS oscillator circuit should be capable of switching at 500kHz, but I'm not sure how stable it would be. The IRFP260Ns are certainly fast enough though.
The frequency that the ZVS oscillates at is determined by the values of the tank capacitor and the center-tapped primary coil, so it will need to be tuned in a similar fashion to a spark-gap coil. Its easiest to tune a SGTC by winding a primary with many taps and using a fixed value HV capacitor. For the ZVS oscillator it would be much easier to wind a fixed value center-tapped primary and vary the number of capacitors in the MMC/tank. A bunch of cheap box-type polypropylene/MKP capacitors would be ideal.
Registered Member #48007
Joined: Wed Jun 18 2014, 01:00PM
Location: Prilep, Republic of Macedonia
Posts: 33
I was mainly concerned that the mosfets won't switch fast enough with 400Ohms per gate, I plan to use two of the 200Ohm resistors in series per gate, so a 400Ohm of gate resistance... Another question, does the toroid look good enough, I have read that the pale green/blue toroids are iron powder ones and have a relative magnetic permeabilllity of around 75, is that correct? If it is, we're looking at ~110uH of inductance... As for the caps, I plan to use FKP caps, mainly because of their high current rating and that they are readily available (in my junkbox). As for the primary, what is the minimum number of turns I need to wind, also, I could lower the resonant frequuency of the secondary witha topload, no problem with that, but some folks have run Royer oscillators in the MHz, specifficaly the Mazzilli ZVS driver using RF mosfets. Around 6.5MHz to be exact, check it out!
Registered Member #4074
Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335
Thats a pretty neat little lamp. I'm currently building a desklamp using a big old 1970's xenon strobe designed for airport runways, but I've taken the easy way out by using a small 50Hz 12kV transformer.
As for the gate resistors, the gate capacitance and its series resistor can be seen as an RC circuit with a time constant. You can calculate how long the gate will take to charge and as long as the FETs don't spend more than approx 10-15% of each cycle in the linear region they shouldn't get too hot. Sorry, I'm currently a bit too drunk and tired to help you out with the math, most of it would probably be wrong haha. Online calculators should be good enough though.
The toroidal choke in series with the positive DC rail is fairly non-critical. A powdered iron core that can handle the amp-turns without saturating is all you need. Your core should be ok, depending on how much input current you're aiming for.
At 560kHz your primary wont need many turns. 1+1 to 4+4 turn center-tapped coil will be the most likely range, depending on the diameter, how many caps you want to used, desired input voltage and curent draw.
Hope that helps, sorry I couldn't get into specifics, starting to go to sleep in my computer chair haha.
Registered Member #48007
Joined: Wed Jun 18 2014, 01:00PM
Location: Prilep, Republic of Macedonia
Posts: 33
Thanks for the info, sure it helps, but wouln't 4+4 turns draw a lot of current? I aim to minimize current draw, wouldn't increasing the primary turns decrease current draw? Also, could I use a computer psu with the 12V rail to supply this circuit?
Registered Member #4074
Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335
Yes, more turns will result in a lower current draw. Impedance of an inductor increases with frequency. At ~30kHz a 4+4 turn primary (8 turns total) wrapped on the ferrite core of a flyback may result in a couple of hundred watts (ballpark figures). At 560kHz an air-core 4+4 turn primary shouldn't draw an extreme current. More turns means less capacitance required, and vice versa.
The PC supply might be good enough, it depends on how many amps the 12V rail can handle. Theres usually a sticker stating the rated wattage and output currents. You really want a supply that won't sag below 10V at full load, or your FETs will latch and explode. Test your supply before use by loading it to its maximum rating and measure the voltage drop across the dummy load.
EDIT: You mentioned minimizing current draw, do you have a specific power level you want to achieve? You will probably want at least 75-100W to produce some healthy plasma.
Registered Member #48007
Joined: Wed Jun 18 2014, 01:00PM
Location: Prilep, Republic of Macedonia
Posts: 33
I am aiming at around 200W power consumption, but you can see, in the "You Tesla Coil" thread I produced some "healthy" plasma with just 25W, (3cm to air, 7cm to ground) but that was a totally different design, about 200W would be nice... Also, I have two 22nF 1kV WIMA FKP-1 caps and two 1nF 1,6kV WIMA FKP-1 caps, I can combine them to form various capacitances...
Registered Member #4074
Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335
Your relay-switched coil is very nice looking, I love the design of the primary and secondary assembly. Interrupted coils like your relay design, spark gaps and DRSSTCs can produce very long thin streamers. My old spark gap coil was making almost 20cm open-air sparks with an input of about 200VA, as seen here:
The ZVS/Royer oscillator will be similar to other CW SSTCs. At 200W you will get a very hot silent plasma discharge several cm long. Make sure to post some photos and videos when its finished, should be an interesting build.
Registered Member #48007
Joined: Wed Jun 18 2014, 01:00PM
Location: Prilep, Republic of Macedonia
Posts: 33
Because of the small heatsink, maybe I should try a 555 timer circuit to turn on transistors that would ground the gates, that would look like the interrupted SSTC's, but I need to make it work first... I will surely post photos, without a doubt, first, a couple of questions... -Will a higher primary to secondary coupling coefficient work better, I want to figure out the diameter of the primary I will be making. -I have the secondary with a long steel wire as the discharge terminal, what would be the capacitance of that piece of wire, it has 1mm diameter and is 180mm long. A couple of pF maybe? I need an approximation of the capacitance so I could calculate the resonant frequency. An update, this coil has a self-resonant frequency a bit lower than the previous mentioned 560kHz, due to that the real world differs from an ideal one, in which the calculations were made and due to extra space on the form that I wanted to finish off... Here is a calculation of the coil itself and a picture of it:
Registered Member #48007
Joined: Wed Jun 18 2014, 01:00PM
Location: Prilep, Republic of Macedonia
Posts: 33
UPDATE: I have searched for a power supply that can give that output current at such voltage without sagging, and was unable to find any within my budget, but I have picked up one 12V 45Ah Lead-Acid battery from a car, it's not new but it hasn't been used a lot, only a couple of months. Could I use it as the power source?
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Royer SSTC
