Author Topic: DC Power Tesla Coil (Read 31378 times)

Jeg · « **on:** April 29, 2013, 12:48:15 PM »

Hi to all
I have a ready to use tv flyback to create 20KV HV at 1mA. With this Transformer i will charge a cap through a charging coil and a de-q-ing diode.

According to the equations i found on this page http://www.richieburnett.co.uk/dcreschg.html
i calculated a huge inductance coil (Lp) and i don't know if i made a mistake on somewhere.

At the end of the page the last equation calculates Lp. I used as Vdc=20KV, Pmax=(20KV X 1mA)=20W, and BPSmax=500.
With these numbers i find an inductance of 8114H !!!! Is that possible???

TNKS
Jeg

TinselKoala · « **Reply #1 on:** April 29, 2013, 01:53:06 PM »

What your numbers are telling you is what I think I told you some time ago: the flyback transformer does not supply enough current at the rated voltage to work effectively in a SGTC with a rotary break.

Instead of the flyback, put in a MOT at 2 kV, 600 mA, and see what you get.

Jeg · « **Reply #2 on:** April 30, 2013, 06:31:36 AM »

Hi Tinsel thanks for the reply

I re-calculated inductance with the numbers you gave me, and finally became a normal value of 0.8H!

Ok. I will search a little for finding a good mosfet or igbt driver for my MOT and i ll be back again later.

tnks

TinselKoala · « **Reply #3 on:** April 30, 2013, 06:58:31 AM »

Just plug the MOT into a Variac, you don't need a driver. Use another MOT primary in series, secondary shorted, to act as a current-limiting choke, or just be careful on your input current.

Jeg · « **Reply #4 on:** April 30, 2013, 07:11:30 AM »

Quote from: TinselKoala on April 30, 2013, 06:58:31 AM

Use another MOT primary in series, secondary shorted

You mean to connect the second MOT in series with the secondary of the first MOT as a ballast? Is it still deadly dangerous even with this current limiter?

TinselKoala · « **Reply #5 on:** April 30, 2013, 07:22:49 AM »

Sorry, no, you use the second MOT, with its secondary shorted, in series with the _primary_ of the main MOT. This is just like using a big choke on the input to limit the input current. MOTs are not current-limited the way flybacks are, so if your output impedance is low, it's easy to draw so much current in the primary that breakers trip or the smoke leaks out. SO you use the second MOT primary as an input choke, just to limit the inrush current when the discharged cap bank is looking like a direct short across the MOT output. Then downstream from the MOT on the HV side you put the FWB, and then the HV choke and de-Q diode if you are using them.
I don't have another MOT so on my coil I put an ammeter around the MOT input lead, after the Variac, and I watch the peak currents and don't let it go over 15 Amps. I've still managed to blow a couple of fuses in the Variac, though. Putting the second MOT primary in the circuit with its secondary shorted will prevent the input current from going much over 10 Amps and I should be able to run continuously without problems at that point, but first I have to find the second MOT !!

Jeg · « **Reply #6 on:** April 30, 2013, 01:15:11 PM »

Hi Tinsel
If i put the second MOT as an HV choke in series with the secondary, will it also react as a current limiter? I ask because i will need a charging coil in the secondary anyway, so if it works as a current limiter i will save some space and some extra work!

What FWB stands for?

Jeg

TinselKoala · « **Reply #7 on:** April 30, 2013, 05:35:00 PM »

Quote from: Jeg on April 30, 2013, 01:15:11 PM

Hi Tinsel
If i put the second MOT as an HV choke in series with the secondary, will it also react as a current limiter? I ask because i will need a charging coil in the secondary anyway, so if it works as a current limiter i will save some space and some extra work!

I don't know if it will work that way, but you can certainly try it, although I'd be a little worried about heat buildup. I've only heard about using the second MOT's primary in series with the mains supply to the working MOT, as I described above. But experiment, by all means... just be careful, the MOT is deadly.

Quote

What FWB stands for?

Jeg

Full-wave bridge. Made of HV high current diodes. In my MOT DC coil they are the black bell-shaped things to the left of the capacitor bank.

Farmhand · « **Reply #8 on:** May 02, 2013, 07:46:45 AM »

Quote from: Jeg on April 30, 2013, 01:15:11 PM

Hi Tinsel
If i put the second MOT as an HV choke in series with the secondary, will it also react as a current limiter? I ask because i will need a charging coil in the secondary anyway, so if it works as a current limiter i will save some space and some extra work!

What FWB stands for?

Jeg

Yeah you can do "secondary side current limiting". See the link. http://www.kronjaeger.com/hv/hv/src/mot/index.html.

The resonant charging circuit does that. http://www.richieburnett.co.uk/dcresist.html

I use a MOT secondary for the charging inductor in my resonant charging circuit. I also have a 400 mH choke made from 0.5 mm wire with removable core in series with the supply, the two "Supply" MOT primaries are in "anti" parallel then another MOT's secondary is the charging inductor (this MOT can have it's primary shorted to reduce it's inductance). The power is limited to about 650 Watts, current about 2.7 amps in my setup at full power, the HV caps are actually 14 nF, frequency 750 kHz or so.

FWBR means a Full Wave Bridge Rectifier. My circuit uses one but it's drawn in two parts. (http://i227.photobucket.com/albums/dd168/Toey1/DualMOTPrimaryCircuit12.jpg)

Cheers.

P.S. I can calculate my power input by using the electronics assistant program to tell me the energy in the HV caps and multiply by the breaks per second.

Say I dump 8000 volts from a 14 nF cap 500 times a second, 0.448 Joules times 500 = 224 Watts. And it correlates with the energy meter with a bit of losses.
The MOT secondary charging inductor will only allow the supply to charge the caps at a certain rate so the input is limited that way.

Even 6000 volts into a one turn primary with 180 secondary and extra coil turns (combined) would give a transformation to 1 million volts.

Without any resonant rise.
My setup has one primary turn and about 200 turns in the secondary and extra coils combined.

Speaking of "Q" my secondary and extra coils combined have about 3 Ohms resistance and the top load is about 16 or 17 pF I think. And the setup has about 2.5 mH in the secondary and extra coils combined. Almost a Quality factor of 4000. http://www.circuitsage.com/tools/tool_view&tool_id=17

..

TinselKoala · « **Reply #9 on:** May 02, 2013, 02:33:32 PM »

Thanks, Farmhand, what you've described is indeed the "right" way to do a DC rotary SGTC using MOTs. Is this the coil that you showed in operation earlier? What happens when you let it pull 1500 Watts input, instead of limiting to 650?

I'm working on a zero budget so I have to design from what's on hand, rather than designing to theory and then building it with the right stuff.
I'm on a 115-120VAC main, so my 15A current input (manually limited) is equivalent to 7.5 A input in your 220-240 system. But I'm only using the one MOT so my output from the FWB is nowhere near the TC primary voltage that you are getting, I think.

Farmhand · « **Reply #10 on:** May 02, 2013, 10:07:44 PM »

Yes same coil, It's a fairly big one, not the smaller ones I sometimes use for experiments. It's a screamer, I had to go to 1500 BPS to get 650 Watts into it when the power levelled out. I'm not so confident to try more power just yet, I'm not sure all the components could take it. Because of the fairly high frequency the primary capacitance is small so for a fair sized coil that's not really enough I don't think. Can't use any less than one primary turn though.

The MOT's here have more primary inductance, 236 mH each, they vary a bit.
The supply MOT's are power factor corrected (almost) which helps for more output voltage, it took 39 UF to do it.
If each MOT makes 2000 volts and the two are adding voltage then the charging circuit gets 4000 volts to double. I have to set the safety gap pretty wide, about 7 mm.

My guess is if I just remove my ballast coil or replace it with one that has less resistance or inductance then it will be able to use more power. I should try it because it sounds like fun. When I short the primary of the MOT I use for the charging inductor the secondary reads 4.7 Henries. With the primary open it's over 20 Henries. I use it open.

Three MOTs makes the setup awkward to try to lift or move. The downside to MOT's. Speaking of using what is laying around, I made the rotary spark gap from wood and the rotor is cut from a "cutting board" plastic one. It's got a small universal motor from a floor polisher in it, with 50 volts DC it can do 6000 rpm and it has 8 shorting bars on it, It also has two breaks at 180 degrees so I can double the rate that way.

I'll crank it right up soon. I'll be needing a fix sooner or later.

Cheers

Jeg · « **Reply #11 on:** August 26, 2013, 12:03:12 AM »

Hallo again..

After the summer vacations i started again the HV Experiments!

Well, i have a mind trouble, so if anyone can guess a solution to my difficulties i will appreciate it!

So! I built a dc power supply using a MOT with 2 more mots as a ballast at the input. (I don't have a variac)
I used two microwave oven caps in series as a smoothing cap. So far so good. It gives me a 2.2KV DC Output as expected. Next stage is a de-Q-ing diode followed by a 4th mot as a charging coil, and 3 mot's caps in series as a tank cap. The problem is that it also gives me a 2.2KV output and not 4.4KV as expected! I tested the diode the coil and the Caps and everything seems to work right. Where is the mistake? The circuit is following...

TinselKoala · « **Reply #12 on:** August 26, 2013, 12:13:48 AM »

You probably have way too much capacitance. Disconnect the three series MO caps and then see what the voltage is. Then use a capacitor that is properly sized for your primary coil.
My DC coil uses a single MOT and a single FWB, no de-Qing diode (I tried one but found no improvement) and about 160 nF total capacitance in the primary circuit, but I am also using a fixed 2-element spark gap with blown quenching.

Farmhand · « **Reply #13 on:** August 26, 2013, 03:56:51 AM »

Tinsel is correct, the final caps you have, have possibly around 330 nF which is a lot.

Also if you have too much ballast on the input then when the setup is loaded not enough current will flow
to properly charge the caps. Which is why I tune the MOT Primaries to resonance, or power factor correct them,
that makes the transformation solid with less voltage drop. With all the ballast in your primary circuit the inductance
might be too much, and the primary circuit may well be tuned to too low of a frequency, or if you are using the secondaries for ballast
to the primary of the MOT then the resistance will be way too much for the voltage applied. Three MOT primaries with no capacitors
should be ok and not tuned too low, I put the PFC caps directly across the MOT primary. There is some trial and error due to system particulars.

But if you use the primary coils of the other MOTs as input ballast it should be OK.

Try shorting the unused primary of your secondary side MOT which is the charging inductor. That will reduce the charging inductors inductance,
alternatively the small LV coil can be shorted. for less decrease in inductance.

If your primary has only 4 uH and the transformer frequency is, lets just say 400 kHz then you would only need
40 nF of final HV caps. If the primary coil had 12 uH and freq. is 400 Khz then you would only need 13.5 nF.
It's best to tune the primary a bit lower in frequency than the secondary oscillating circuit, because loading with streamers
lowers the secondary resonant freq.

Jeg · « **Reply #14 on:** August 26, 2013, 12:35:27 PM »

Thanks guys! Your experience is really valuable and i appreciate it!

I was just hopping to avoid building an MMC bank, by using what i readily have. And what i have are 5 mots and 5 MO caps! Anyway, i hope the next days i will finish the supply and loading part of the system.

In the following pic you will see at the primary side a MO pcb which includes a line filter and a current limiter (via resistance and relay). But there is one more watt resistance of 3.3K, feeding a 24V zener for the relay pwr supply. And this resistance gets so hot that you can not touch it after some few seconds. Is that normal?
Tnks again

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Author Topic: DC Power Tesla Coil (Read 31378 times)

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