Re: Really big brick - Off-line SISG coil?
Steve Conner, Wed Oct 11 2006, 04:07PM

Hi Aaron

I used two of the slightly smaller CM600HA-24H in parallel in my OLTC2. After a lot of tinkering I got it to run at over 4kW of power and produce 80" sparks. Your brick is probably good for somewhat more. The peak current rating seems to be just a guideline, since the peaks in a Tesla coil circuit are much shorter than the 1ms pulses that Powerex specify them for. Mine were rated at 1200A peak, but I think I ended up running them at something like 4500A peak

When I got my CM600HA-24Hs, I built a test fixture with some MO caps and a single turn loop inductor to pulse current through them:

The OLTC is really not too different to the SISG. As far as I'm concerned, the main difference is that I developed the OLTC to use DC resonant charging. This makes the most efficient use of the components, but Terry's SISG trigger circuit can't cope with that (it would fire just once and then stick on forever) I'm working on that, but I am stuck for a way to do it without ruining the simplicity of Terry's circuit :(

Re: Really big brick - Off-line SISG coil?
J. Aaron Holmes, Wed Oct 11 2006, 10:18PM

Thanks, Steve!

4500A is a lot of amps! How do those Geek caps fare at those hideous currents?

A simple ~100W design might be:
- Voltage double my 240V, 60Hz mains
- 600V firing voltage
- Terry's "Piranha" charging topology (second edition)
- This all suggests a 60bps firing rate, yes?
- 10uF + 600Vpk + 60bps => 108W coil power
- ~1uH single-loop primary => ~1900A peak primary current!!!
- ~50kHz resonant frequency (seems "ok")

All this for what will surely be some pretty wimpy sparks!! But hey, if it works at all, it'd certainly be one of the simpler OLTC-ish..."things"...out there The primary peak current makes me worried for the tank cap, though. Even the 600VDC-rated CDE942C's top out at around 1500A. I'd prefer not to have them going off like firecrackers! Would the 600VDC ones survive? They come in 2.5uF, so I'd only need four of them! That would sure be convenient.

EDIT: Actually, I'm thinking about the cap current all wrong, aren't I? Haven't done much MMC'ing. The per-string current is the peak current divided by the number of strings, yes? So with four "strings" (of one cap each ), I only see ~500A per cap. Ok, maybe that's not so scary after all.

Regards,
Aaron, N7OE

Re: Really big brick - Off-line SISG coil?
Steve Conner, Wed Oct 11 2006, 11:42PM

Hi Aaron,

I used between 14 and 32 parallel "strings" of one 1uF cap, so they didn't see too much current each

Re: Really big brick - Off-line SISG coil?
Terry Fritz, Thu Oct 12 2006, 03:30AM

Hi,

First to test it.

Measure the resistance from the gate to the emitter and collector in both directions (positive to negative and negative to postive). In all cases, the gate resistance to the other terminals should be "infinite" or look like a say 200nF capacitance (!!!). If there is resistance, it has been damaged but "might" still work.

Take a 9V battery and charge the emitter to gate to -9v. The collector to emitter should look open. Then charge the gate to +9v and the collector to emitter should look shorted. If it does that without any trouble, it is good Typicaly if these bricks fail, there is case damage as in cracking. But the electrical test is really good for detecting damage. Surplus parts like these are typically removed when another brick blows and they replace all of them. In many cases the other bricks are fine.

It could be used as an SISG easily and you can probably just add another SIDAC to make it fire at 1200V. Then just use it as a OLTC type thing. The 200nF needs some thought though since that is 40X normal for an SISG IGBT. I will have to think on that You probably can just eliminate the 100nF cap and hope the turn-on is "slow" enough to allow it to reach say 24V on the gate before the IGBT takes over. Or, go to a 1uF cap...

Cheers,

Terry

Re: Really big brick - Off-line SISG coil?
Finn Hammer, Thu Oct 12 2006, 03:19PM


Could you explain why it will stay on forever?

I thought the SISG circuit could work this way, if it was fed trough a charging choke with a voltage lower than half of the Sidac`s breakdown voltage:
A Thyristor, or an IGBT, is connected to short out 3/4 of the Sidacs. The first ringdown is initiated when this SCR or IGBT is triggered.
When the double voltage, due to the charging choke arrives at the primary cap, it is stil not enough to force breakdown of the Siac`s, so the trigger elements still have to be operated for another ringdown.
This would enable full remote triggering of the coil.
I have an idea for a trigger circuit.

But of course, if it doesn`t quench after the first ringdown, this won`t work.


Cheers, Finn Hammer

Re: Really big brick - Off-line SISG coil?
J. Aaron Holmes, Thu Oct 12 2006, 05:03PM


Thanks Terry, Steve. Wish I'd read this last night! Now I'm at work and itching to get home and try it out!!

Terry, one SISG question: Where does the 785A/us figure in the SISG PDF come from? (this in your 250kHz 500A Ipk example). I'm trying to understand this section of the doc a bit better now so that I can work out the implications of the capacitor (e.g., if 1uF were used).

Thanks!
Aaron, N7OE

Re: Really big brick - Off-line SISG coil?
Steve Conner, Thu Oct 12 2006, 09:36PM

Hi Finn, all,

These are the problems I expected to happen with DC resonant charging:

1: Terry's SISG won't be able to run with discontinuous current unless it's modified to trigger on command. This is because it's triggered by voltage, and once the inductor current falls to zero, the voltage will quit increasing. Therefore, the closest you can get is to tune the voltage such that it fires exactly as the charge current stops. This won't be possible to do exactly due to circuit tolerances, therefore in practice the SISG would always end up firing before the charging current stopped, and hence the charging current would never stop.

2: The average voltage across an inductor is always zero. (not counting IR drop, but at the high voltages and low currents in Tesla coil circuits, it may be possible to neglect this.) Put another way: Inductors don't ballast DC!

3: Terry's circuit stays on for rather a long time, during which time the inductor is shorted across the power supply and the current will ramp up quickly. This could cause boost converter action which would be self-reinforcing.

Due to the interaction of these three factors, I'd expect a SISG running off DC to run away, with the breakrate increasing and inductor current ramping up until the SISG stuck on permanently and blew the power supply out. It might be possible to find a power supply voltage setting that gave stable running, but I'd expect it to be very touchy. I thought of some ways to tame it:

1: Use command resonant charging to force discontinuous current. Unfortunately this needs a high voltage switch that can be triggered, and if we had a reliable triggerable HV switch, we could just have...

2: Achieved discontinuous current operation by making the SISG itself triggerable. This would make it more complicated.

3: Add resistive ballast in series with the charging choke. This wastes power, though, and would probably just tame the instability a little rather than getting rid of it altogether.

4: Use hybrid DC/AC resonant charging with a full-wave rectified unfiltered HV transformer. Run this off a lamp dimmer type of circuit to cause zero periods in every half cycle where the SISG can have a chance to recover. This is really just command resonant charging with the HV switch moved to the primary side of the power transformer and the breakrate fixed to 100/120Hz as a consequence. (You should find a dimmer setting where the breakrate is exactly 100/120.) The charging choke could be replaced with an ordinary ballast on the primary too.

I got a couple of SISG modules from Terry, and I hoped to try this stuff out, but the more I look at his original design, the more it seems to "just work fine" as it is.

Re: Really big brick - Off-line SISG coil?
Terry Fritz, Fri Oct 13 2006, 03:00AM

Hi,

Terry, one SISG question: Where does the 785A/us figure in the SISG PDF come from? (this in your 250kHz 500A Ipk example). I'm trying to understand this section of the doc a bit better now so that I can work out the implications of the capacitor (e.g., if 1uF were used).

See this:



It is the change of current vs. time equation. A calculus derivative.

Stever McStoner Mc Connerr... said :o))

Something like the SISG might not run well off a pure DC source. But a little resistance will probebaly drop the source "enough"... Think a simple RC filter there for isolation. In the PIRANHA, the MOT's inductance really is the only isolation along with minimal curent limiting from the MOT.



It woorks good!! That giant 165nF primary cap is not "trivial" to charge!!! The "DC" supply would have to charge it back to say 6800V in 500us for it to "hang on".

Cheers,

Terry

Re: Really big brick - Off-line SISG coil?
Steve Conner, Fri Oct 13 2006, 10:18AM

Terry, I never quite understood your voltage doubler charging arrangement. As far as I can tell, it has to either fire at 60bps, or else only charge half of the tank cap for each bang. In the latter case it would waste 3/4 of the energy capability of the cap?

Steve

Re: Really big brick - Off-line SISG coil?
J. Aaron Holmes, Fri Oct 13 2006, 03:28PM

Test results (brief): It appears to work! No unwanted DC resistances, and the thing appears to turn on Didn't get to play as much as I wanted, but it seems I do actually have the fixins for an OL-SISG Still need to tinker a bit with the MMC figures, which is one reason why...


...this question interests me. In the more "classic" voltage doubler config with an SISG + primary across it, it seems intuitive that 60bps is what you'd end up with. Does this change in some subtle way with PIRANHA? You're talking about many 100's of bps. Of course 60bps isn't all bad; the caps will certainly be a lot happier!

Regards,
Aaron

Re: Really big brick - Off-line SISG coil?
Terry Fritz, Sat Oct 14 2006, 12:59AM

J. Aaron Holmes wrote:

Test results (brief): It appears to work! No unwanted DC resistances, and the thing appears to turn on

Cool! It should be good!!

Steve said:

Terry, I never quite understood your voltage doubler charging arrangement.

"I" never quite undersood how PIRANHA's charging works either The "computer" figured it out while I was randomly trying different things. But the actual coil does work just like the MicroSim model, so it "does" work.

Here is what I know:

The MOT voltage can be fairly low (like 105VAC) to help keep the MOT out of saturation.

The primary cap is fairly "resonant" with the MOT which helps charge the two 330nF caps up to 1.5X the voltage. The primary cap value is not super critical and 200 to 400nF seems to work prety good too.

The system charges as a voltage doubler referenced to ground but discharges "floating". So the caps really don't discharge with respect to ground. Having the other cap discharge to a lower voltage helps the next cycle fire. It is almost "preserving" the firing voltage.

The BPS is pretty sensitive to the actual line voltage (variac). One can get from 60, 120, 240, "other" BPSs easily but it is touchy if you are trying to hit one exactly. Often the firing is fairly chaotic. ~240BPS seems like a nice area. The streamers "like" the higher BPS and the chaotoic nature of the thing.

Here is a Microsim model:




The firing rate at 106VAC input is ~220 BPS as shown here:



But look at those cap voltages!



The waveforms on the two caps are "messy" but they add up to the bottom waveform which is able to fire the 165nF combined capacitance at 220 BPS at 6900V!

Nothing really gets hot and the 100 ohm resistors just get a little warm. The system power is

220 x 1/2 165e-9 x 6900^2 = 864 watts.

The MicroSim model gives an RMS voltage of 106.066V and 9.668Arms:



So the VA is 1025.5. If we look at the real input power:



We get 911.6 watts. So the power factor is 88.9 Which is pretty good

The "loss" is 911.6 - 864 = 47.6 W The RMS current through the 100 ohm resistors is:



430mA so the power dissipation is 2 x 0.43^2 * 100 = 37 watts. That only leaves 10Watts on the IGBTs and primary ressitance! 0.25 ohms there seems "high"! It is proably a little lower! The IGBTs stay "cold" in the fan breeze.

So there, now you know just as much about it as I do It is a bazaar thing and it works super good!! But the charging stuff is not "easy" to comprehend. I hope to figure it out "someday"

The PIRANHA is the only coil I use now and I have no desire to get out any of the others. Simple, light, never breaks, runs forever, and is a super good performer off a single happily running MOT. It is a scary coil in that it goes from dead silent to full power instantly, and it is loaded for bear!!

Obviosuly, it is the greatest Tesla coil design on Earth I should probably advertize it more, but it will come in its time It is a little tougher to get across since between the MOT and the coil, "nothing" is "conventional" at all... And I can still say that an SISG coil has "still" never blown up

Cheers,

Terry

Re: Really big brick - Off-line SISG coil?
J. Aaron Holmes, Sat Oct 14 2006, 02:18AM


Heheh....well, that means the title of "First To Blow Up An SISG" will probably be mine The more I look at the primary currents necessary to get any kind of coil power out of 600Vpk, the more scared I get!! Maybe I shouldn't just plunge in like this, but I'm so impatient!

I'm seriously considering going back and getting the other one of these bricks. There were two. $100 total (for two of these) seems pretty good if they work. Then I could scale up a bit with a MOT or something if things start to go well.

If I get a good bonus this year (and hide some from the wife), maybe I'll have to pick up one of those 6.5kV bricks to play with. Haven't seen any used ones yet, though, and I've been watching for a while...

Regards,
Aaron, N7OE