Author Topic: looking for advice  (Read 5520 times)

Offline Garyf

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looking for advice
« on: November 14, 2017, 04:04:19 PM »
I assembled a Onetesla TS for someone else to gain some experience & insight.  It worked well so now I’m trying to build my own drsstc with a goal of 2-3 foot discharges.  Started with a 7” by 24” primary, 700 turns of #20 wire (purchased some years ago on ebay), 270khz with the topload.  Have a full H board and driver (screamer) with 20N60A4D, 170 volt bus.  Primary is a pancake of 7 turns of 3 conductor #12 sheathed cable (Romex) with ends soldered together. (Equivalent single wire size =#6, but more surface area.)  Using a OneTesla interrupter with 13-55 microsecond bursts. Tank capacitor is 4 series CDE 942s,  .1uf 2000v, = .025uv.
Appears to be well tuned, but only getting about 12 inch arcs.  Looking for the weak link.  I measure 150-200 amps from the OCD current transformer.  Tried doubling the bus voltage but blew up the 20N60s.  The pulsed collector current spec for the 20N60 is 380 amps, but that is probably for hard switching.  Soft switching would increase this rating by how much?  What should I set for my OCD?
In order, what would make the greatest improvements?
1.   New secondary with finer wire, more turns?
2.   New IGBTs…60N60? 110N65? I would prefer to stick with TO-247.
3.   Copper tube primary? (current primary resistance measured at .02 ohms)
4.   Primary more turns? Fewer turns?  I don’t see a lot of guidance on primary impedance.
The insulated romex primary makes the LC tank harder to adjust, but I like it for safety, and I don’t think I need a strike rail.  I know I’ve wandered off the well traveled path in a couple of respects, but I think these are reasonable mods.
Can anyone advise?

Offline Mads Barnkob

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Re: looking for advice
« Reply #1 on: November 14, 2017, 04:43:35 PM »
Hi Gary and welcome to HVF :)

First I would like to ask for some pictures of your current coil, bridge etc, some more details that makes it much easier to put out shortcoming and where to improve.

TO-247 devices are not the most powerful in the world, but they are often fast and more suitable for smaller coils. Your secondary sounds very big for a TO-247 bridge, so it could be a simple impedance mismatch between inverter and its load.

From the datasheet ( https://www.promelec.ru/pdf/HGTG20N60A4D.pdf ) you can see there is a pretty harsh current derating for over 200 kHz operation (FIGURE 3.  OPERATING FREQUENCY vs COLLECTOR TO EMITTER CURREN), did you see my DRSSTC guide about calculating IGBTs switching speed?

to your questions!

1: yes, make a smaller secondary
2: 60N60 would properly be a good switch, since you need better current handling at those frequencies.
3: in a DRSSTC it is of paramount importance to be able to fine tune the primary, the sweet spot is within 5cm on the coil
4: if you want to learn some more about primary impedance, see this thread and be welcome to ask more questions in that thread: https://highvoltageforum.net/index.php?topic=113.0
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Offline Garyf

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Re: looking for advice
« Reply #2 on: November 14, 2017, 06:13:37 PM »
When I get the performance I want I'll pretty it up, but here it is.  Bridge and driver boards are from Tom Blitch, his parts list called for HGTG20N60A4.  I note that device has a particularly high Icm. (380 amps)  I understand the derating at higher frequencies, but the effect of soft switching on the specs is not clear to me.
https://www.teslaceph.com/screamer
Yesterday I put my voltage doubler circuit on the Variac so I can adjust bus voltage up to 420 volts.  I haven't had the courage to crank it up full yet because replacing IGBTs is a nuisance. I have a small breakout point on top and the arc jumps to a wire connected to the light fixture.  At some point I guess I should set it on the garage floor and turn it up....

Offline Mads Barnkob

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Re: looking for advice
« Reply #3 on: November 14, 2017, 08:52:44 PM »
You made your secondary circuit about 40% larger than the original screamer, but you will not get longer sparks without also upgrading the inverter, worst case you have actually made something too hard for the inverter to drive.

In my guide ( http://kaizerpowerelectronics.dk/tesla-coils/drsstc-design-guide/igbts/ ) I explain how you can do softswitching derating, it is a lot of assumptions based on experiences but its about the best quick method there is.

There is actually only two things I think you should fix right now
- make a copper tubing primary, you need to be able to fine tune it, you got no way of tuning at the moment.
- do not use small alligator test leads for secondary ground connection!



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Offline Garyf

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Re: looking for advice
« Reply #4 on: November 14, 2017, 10:53:55 PM »
Thanks for your advice & experience! I will get some copper tube and try fine tuning.

This was my previous tuning method:  I used a scope and signal generator across the primary with no capacitor and identified the secondary resonance with a small scope probe antenna near the topload.  About 285 khz.  I've tried a number of primary configurations and turns, and with each one I drive the series LC with the same signal generator, looking for a null to identify the primary resonance.  I series/parallel various capacitors until I get something that closely matches the secondary.  I checked it by driving the driver board directly with the signal generator and noting the response while sweeping the frequency.

I received a Chinese product the other day that is remarkable for the price: a $20 LC meter with nanohenry resolution and a claimed 1% accuracy

https://www.ebay.com/itm/Digital-LC100-A-LCD-High-Precision-Inductance-Capacitance-L-C-Meter-Tester/252892255805?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2060353.m2749.l2648

I checked my primary inductance with it and found it to be 12.8 microhenry.  With my tank capacitance of .026uf, the primary frequency works out to 276khz.  I haven't addressed the recommended frequency separation yet, but I think I'm in the ballpark.  I will get some copper tube and see if I can improve the performance. 

I know my coil is oversized vs the screamer, I'm hoping I can make up for the greater size by increasing the bus voltage.  I do have some 60n60s I can try, but it's confusing because the Icm rating isn't as good.

I will also study your guide about switching speeds.  Thanks!!

Offline Hydron

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Re: looking for advice
« Reply #5 on: November 15, 2017, 01:31:53 PM »
A single bridge of TO-247 devices are a bit small for a coil of this size, though the Icm specification is surprisingly large on the devices you're using. They are also pretty quick IGBTs, and I'd expect them to handle 270kHz switching OK at lower currents.

The problem is that at peak currents in the 100s of amps (like you'd want for a coil this size) and high frequencies (100kHz+) any deviation from soft switching will put a lot of stress on the IGBTs though, especially in device with a limited ability to dissipate heat (TO-247s are good for through-hole parts, but not compared to stuff you mount with bolts!)

I'd have to have a think about what more you could do, but I think the high frequency of the coil and small IGBT devices are limiting your options for getting larger streamers without changing a lot of stuff.

Offline Garyf

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Re: looking for advice
« Reply #6 on: November 15, 2017, 06:31:17 PM »
More data: bus voltage 270 volts, 300 amp max (see waveforms...I use my old indestructible Tek scope for high voltage work). Ran continuous for two minutes, then checked temperature of the IGBT= 105F. (no fan used)  Discharge about 14 inches.  Seems to indicate good soft switching and additional current reserve. 

Offline Hydron

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Re: looking for advice
« Reply #7 on: November 15, 2017, 11:10:53 PM »
Ideally to judge whether switching is "soft" enough or not you'd want to capture bridge output voltage (differentially is best, but scoping one output is probably good enough) simultaneously with current. This might be tricky with an analogue scope unless it is a dual beam model. I wouldn't worry too much about using a digital scope on the coil - obviously keep it away from streamers but I've never heard of one getting killed when being used correctly.

I also saw that the "screamer" driver does not use phase lead to allow the switching interval right on the zero current point, which almost guarantees you're not actually getting perfectly soft switching. The fast IGBTs might mean you get away with it though, even at 270kHz. If you try and hard switch at currents greater than Icm you're asking for trouble (dead devices), but the high rating helps make this unlikely.

Offline Garyf

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Re: looking for advice
« Reply #8 on: November 16, 2017, 04:27:23 PM »
I don't have differential input on scopes, so I isolated the ground, attenuated output voltage on chan 1, raw OCD signal on 2.  Zero switching is not perfect....closer on the negative going than positive....

I've been thinking about how to predict the performance from the primary current.  At resonance, primary and secondary inductance and capacitance cycle the same energy back and forth. If you assume lossless transfer of energy from primary to secondary, you can say that
1/2 Lp(Ip)^2=1/2Cs(Vs)^2
and solve for Vs=Ip(Lp/Cs)^.5

So if you know primary current, primary inductance, and secondary stray capacitance, you can estimate secondary output voltage.  I can measure these values with my LC meter.  Plugging in, I get Vs=960Ip.  If my current is 300A, Vs=288kv.  At 25kv/inch, this is about a 12 inch arc, which is just about what I'm getting.  This suggests higher primary inductance and lower secondary capacitance lead to better performance.  Of course higher primary inductance requires higher bus voltage to drive the same current, so there's no free lunch.

I know this is simplified and assumes no losses, but it seems useful.  Thoughts?

Offline Mads Barnkob

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Re: looking for advice
« Reply #9 on: November 16, 2017, 07:27:38 PM »
Actually I have not spend much time doing theoretical modelling of Tesla coils, as you can only do steady state calculations.

The dynamic load conditions of a Tesla coil, the complexity of the spark formation, growth and continuation is a mix of plasma physics, high voltage, high frequency , humidity, pressure and random :)

Regarding estimation of output voltage we discussed that here: https://highvoltageforum.net/index.php?topic=123.msg775#msg775 which also takes examples from some real life topload discharge current measurements that is really interesting reading: https://highvoltageforum.net/index.php?topic=117.msg735;topicseen#new
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Offline Garyf

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Re: looking for advice
« Reply #10 on: November 17, 2017, 05:13:22 AM »
Thanks!  I'm thinking ahead to possibly ordering another secondary (smaller) from the ebay seller in michigan.  Its useful to see how the parameters would affect performance.
Any opinions about the quality of the michigan coils?
Regarding the voltage relationship to arc length, the attachment to your first thread was the best reference ive seen.  Im not sure how high frequencies change it, but at 60hz, the kv/mm rule works pretty well for my neon sign transformer at 15kv.

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Re: looking for advice
« Reply #10 on: November 17, 2017, 05:13:22 AM »

 


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post Re: 2x Panasonic Inverter Microwaves - what to salvage, dangers?
[General Chat]
Twospoons
March 17, 2024, 04:45:17 AM

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