Author Topic: My first DRSSTC on bricks  (Read 7237 times)

Offline Netzpfuscher

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Re: My first DRSSTC on bricks
« Reply #40 on: April 19, 2019, 09:51:51 PM »
There is no software function which alters the pulse width. If you can't set parameters while the coil is running it is much likely due to corrupted bytes on the serial line. That's why I added the MIN-Protocol to the firmware, it's a protocol like TCP with checksums and re-transmission of packets. But with a BLE adapter you can't use this, it's included in the new ethernet and wifi interface.

I cannot say very much about the stability of a UD3 on perfboard, we only have two UD3 coils with a proper PCB. On the big coil with 1200A primary current the driver is around 40cm away under the primary coil in a metal box. It works without problems, only the UART connection sometimes catches false chars (50cm CAT5 cable to the fiber modules) we solved this problems with the MIN protocol. But in essence it works also without.

The new layout I'm working on is a 4 layer PCB and should be even more stable, because of the inner power planes.

I like to put the inverter in a separate box (without the MMC). It gives more flexibility, less EMI in the driver, less weight to carry and I can change the resonators to the needs. I use thick welding cables (twisted) and welding connectors for the connection to the resonator.

Offline Teravolt

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Re: My first DRSSTC on bricks
« Reply #41 on: April 22, 2019, 07:24:49 PM »
Hi Laci, are your mmc caps soldered to the aluninuim because leaded solder will not flow on aluminuim and you may have a bad connection. get some copper wire or tubing or brass and solder to that.

Online Laci

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Re: My first DRSSTC on bricks
« Reply #42 on: April 22, 2019, 07:48:58 PM »
Thanks Teravolt,I paid attention to your previous suggestion on this and rebuilt the MMC since then.This time I soldered the parallel caps to a copper wire,enhanced the connections with a lot of solder and soldered the thick wires from inverter and to primary too.You might be able to see this on this picture:


Online Laci

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Re: My first DRSSTC on bricks
« Reply #43 on: April 26, 2019, 09:39:32 AM »
I put the driver into a metal enclosure.After trying it out,signal noise appeared again.It looks like there's too much noise if the enclosure is floating and even worse if I ground it to mains earth,because the PSU for the driver is on the same ground.Any ideas about this?




Offline Netzpfuscher

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Re: My first DRSSTC on bricks
« Reply #44 on: April 26, 2019, 10:03:03 AM »
The red and black wires on the right side are from the power supply? Is the red wire connected at the top of pcb? Is the supply for the UD3 connected at the bottom of the right pcb?
With such a configuration you couple all of the switching noise of the GDT drivers into the UD3. Connect the red wire at the bottom.

Have you scoped the AC part of the 5V rail? I bet you see the switching there. This could explain the problems with the analog signals. The ADCs are referenced to the 5V rail.

Online Laci

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Re: My first DRSSTC on bricks
« Reply #45 on: April 26, 2019, 10:19:06 AM »
The + of the PSU goes to the buttom + bus of the output stage(4xNMOS,4xPMOS + capacitors)and the black goes to the upper - bus.The multiple wires from the 4xdriver ICs goes to the gates.The wires which have the ferrite bead are feedback and the other white wires are the PSU wires too.

There's plenty of noise on the +5V!
« Last Edit: April 26, 2019, 10:54:42 AM by Laci »

Offline Netzpfuscher

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Re: My first DRSSTC on bricks
« Reply #46 on: April 26, 2019, 11:07:59 AM »


please change the layout to a star configuration. Like this:


Online Laci

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Re: My first DRSSTC on bricks
« Reply #47 on: April 26, 2019, 04:05:09 PM »
I think the 5V noise got lowered to an acceptable level after soldering the 5V regulator close to the input,adding some ceramic capacitors and some thicker wires.

Quickly put everything together and now I get noise from the secondary.Its base is connected to mains ground for testing.If I connect the enclosure to mains ground the waveforms gets worse,but if I only connect the heatsink to mains ground I see some improvements like longer arcs and more current flowing.Should the ground rod connected to the enclosure and heatsink solve this issue?I'll get the rest of the components next week,then hopefully make this ready for a mains test,unless you say that the ground rod would solve the noise,then I test it before. :D

Offline Netzpfuscher

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Re: My first DRSSTC on bricks
« Reply #48 on: April 26, 2019, 04:26:00 PM »
The secondary needs a separate ground. No connection to the driver/bridge an what so ever.
Use the water pipes or the pipes from the heater or a rod in the garden.

Online Laci

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Re: My first DRSSTC on bricks
« Reply #49 on: April 27, 2019, 08:21:51 PM »
First light...on video!
I'd say about maximum 40-50cm sparks under 100us ontime at 250A limit!This 1:1 spark to secondary size is already promising I think. :)
Still many things to do,like thermal protection,RMS current measurement and a WIFI or ethernet interface for better control.For now the app works great,but the BLE distance decreased significantly after the metal enclosure to around 5m,which is still enough in a room.

The largest improvement that can be done is a better MMC.My dream was to find some screw terminal high pulse duty capacitors at affordable price.These are quite rare,for cheap even rarer.If the current WIMA FKP-1 caps can keep up the stress,then I'll probably buy plenty of these in the future to get some 8kV and current up to like 800A.Is this too much for a 12.5cm dia. and 50cm tall coil?! ::)

The grounding is still questionable.Now the secondary base and the heatsink is connected to the ground rod,but the driver enclosure is floating.The general DRSSTC grounding diagram suggests to connect the driver enclosure to the same ground,but I get a lot of noise in the gate drive with that...Also I don't have strike rail and the bus negative to ground capacitor placed.


A pretty spark from the video:


Video: https://mega.nz/#!AKgCGSaD!bLG60IWPkzon7V5Krxq0VxgPT3-mpxzL3VsgE7QD4Fc


(Sorry for the bad quality,the photographer job is the next.  :P)


Offline Mads Barnkob

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Re: My first DRSSTC on bricks
« Reply #50 on: April 27, 2019, 09:23:56 PM »
Congratulations on first light!

800 Apeak will properly work just fine, you just have to monitor your secondary coil temperature when pushing it around its limits, you might need a larger/wider topload to get the longer sparks away from the primary coil and to shape a larger magnetic field to keep the sparks from flying inwards.

Grounding everything together only works if you have a low impedance ground rod that it all connects to, else it will find a way for the noise to couple in through the other parts, I have also used floating driver enclosures before but I have not seen a difference when throwing out large sparks from grounded or floating shielded. I did however not have any measurements on the drivers imposed noise from when doing so either, so its just a qualified guess based on it not exploding and taking a beating :)

Maybe your app could use one of the external buttons on a phone as trigger? So you dont have to control it with a turning up/down of the pulse width.

http://www.kaizerpowerelectronics.dk - Tesla coils, high voltage, pulse power, audio and general electronics

Online Laci

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Re: My first DRSSTC on bricks
« Reply #51 on: April 27, 2019, 09:41:30 PM »
Thanks,Mads!

A larger topload with less sharp edges is in the plans.

The ground wire is relatively thick;thicker than the primary and around 6-7m long.The ground rod is made out of aluminium,about 1m long with many holes in it.All in all I think this grounding is pretty low impedance and something else causes the problem,but if it's good floating then I don't care. :)

The external buttons are great idea,I never thought of this!Since processing MIDI files in app inventor looks not really possible,I plan to switch to something more professional like android studio,that might take some time.

Online Laci

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Re: My first DRSSTC on bricks
« Reply #52 on: April 29, 2019, 09:42:07 PM »
After spending hours on choosing the greatest and cheapest capacitors to upgrade the MMC,I realised they all have small dVdT rating.With heavy spark loading the frequency should drop to around 110kHz which is 9us period.In this period are two cycles so the rise and fall times of the primary current are 2.25us.At a high voltage,like 8kV,they should be rated over 4kV/us if we are lazy and assume the primary current rises linearly.

The WIMA FKP1 series have the best dVdT rating I could find.I planned to buy 20pcs 0.047uF 2kVDC of these,the same as the current ones,just with 5% tolerance(the current ones have 2.5%).If I combine all of them,I get a 4 in series,7 in parallel MMC,rated 0.0822uF and 8kVDC.

The interesting thing starts here.With the heavy spark loading I mentioned at 110kHz, the MMC's reactance is 17.6 ohm.The highest MMC voltage I would allow is 8125V with the 325V bus.From these,according to ohms law the maximum current is 8125/17.6 = 461A. This was a bit disappointing after making the calculations and the only way I can think of to increase the maximum current is increasing the MMC capacitance.
Am I right in the calculations and should I really increase the MMC's capacitance for higher peak currents?Perhaps knows somebody about affordable capacitors for MMC I can buy?
« Last Edit: April 29, 2019, 09:43:48 PM by Laci »

Offline Phoenix

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Re: My first DRSSTC on bricks
« Reply #53 on: April 29, 2019, 10:04:02 PM »
Hello Laci

Your calculations about the MMC's reactance at 110kHz are correct, but there is a very important thing that happens in an LC circuit: The reactance of the capacitor and the reactance of the inductor cancel each other out. If the LC circuit gets exited with the exact resonance frequency, the impedance will be 0 Ohms, but you will still have the ESR of the Primary and the MMC limit your current. The impedance will be 0 because with each cycle the voltage increases and so does the current. Theoretically, the LC circuit would ring up infinitely.

If you use more capacitance, you will have a faster current rise and you could get more peak current with the same ontime and number of primary cycles. The more time you give your DRSSTC to ring up, the more peak current you will get. If you use less capacitance it will just take more cycles to reach the same current than with more capacitance. Of course you need to make sure that the voltage rating of your MMC gets not exceeded during the ringup. The calculation about the maximum peak current you can allow with the MMC is correct.

I think the Wima FKP1 are a really good choice for your coil. I have also used these capacitors for my DRSSTC I. But I do not know how much RMS current your chosen capacitors can handle, Wima doesn't provide datasheets with a lot of information. Have you already tried the MMC calculator from Mads? It is really good for doing the calculations.

EDIT:

I just calculated your primary inductance and I think lowering your primary inductance using less turns and increasing your MMC capacitance to get to the 110 kHz would be a good idea. If you use 4 Wima FKP1 1600V 100nF caps in series and 8 of these strings in parallel, you get 200nF at 6400V. You could go up to 880A with this MMC.

Here you can buy Wima FKP1 capacitors new and for a very good price: https://www.reichelt.at/pulse-capacitor-fkp1-1600-100n-p7733.html?GROUPID=7175&START=0&OFFSET=16&SID=94XE7uy6wQAT4AAEAgW0ke260e2724a5569dec4ba647f3dc371a5&LANGUAGE=EN&&r=1


Greetings
Phoenix
« Last Edit: April 29, 2019, 10:41:41 PM by Phoenix »

Online Laci

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Re: My first DRSSTC on bricks
« Reply #54 on: April 30, 2019, 12:09:19 AM »
Thanks for the useful informations,Phoenix!

I did my calculations until the limitations of the MMC,but you are right about resonance.

The current rise time is not critical for me as I want to run with long pulses,up to 800us with freewheeling.

I used Mads MMC calculator hundread times for now. :)

Many thanks for your calculations and for the webshop.The capacitors are really cheap indeed,although the 10 euro shipping is a bit painful compared to the them.I have to do my calculations about these.
At first I thought 0.2uF is too much and will require only a couple of turns.After running a JavaTC simulation it showed that one less turn,so 5 will be at 113kHz which is good!

What about the number of series and parallel capacitors?The datasheet doesn't mention the current rating,but in theory the more capacitors in parallel,the less the ESR is or in these applications it makes not much sense?For example at 110kHz,800A which one should I choose, 0.2uF 6kV(more in parallel) or 0.15 8kV(more in series)? (in theory more in parallel)
« Last Edit: April 30, 2019, 12:11:16 AM by Laci »

Offline dexter

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Re: My first DRSSTC on bricks
« Reply #55 on: April 30, 2019, 12:32:24 AM »
What about the number of series and parallel capacitors?The datasheet doesn't mention the current rating,but in theory the more capacitors in parallel,the less the ESR is or in these applications it makes not much sense?For example at 110kHz,800A which one should I choose, 0.2uF 6kV(more in parallel) or 0.15 8kV(more in series)? (in theory more in parallel)

You can calculate Ipk=C*dV/dT

If 6kV is enough for your coil i'd go for 0.2uF (more capacitance and more caps in parallel)

Offline Phoenix

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Re: My first DRSSTC on bricks
« Reply #56 on: April 30, 2019, 08:01:39 AM »
Hello Laci  :)

I would also use more caps in parallel to get the 0.2uF 6400V MMC. Because you are planning to use freewheeling, you really need a high RMS current MMC, if you want to run moderate BPS values. As you said correctly, more caps in parallel decrease your ESR value. Low ESR is very important in such an application, because with more ESR there are more losses generated in the capacitors (P=ESR*IRMS^2).

Because you are building the same MMC like I did, just with less capacitors, I can really recommend you a very good design for equal current sharing between the capacitors. I am also using Wima FKP1 1600V 100nF capacitors (4 in series 15 strings parallel for 375nF at 6400V). Here you can see the MMC of my DRSSTC I:



As you can see, the connections from the copper rails to the inverter/primary are not on the same side. This ensures good current sharing.

Greetings
Phoenix

« Last Edit: April 30, 2019, 08:05:15 AM by Phoenix »

Offline Netzpfuscher

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Re: My first DRSSTC on bricks
« Reply #57 on: April 30, 2019, 08:36:53 AM »
If you find some old high voltage oil capacitors for really cheap they are worth a try.

I use a old capacitor in my coil and a bunch of them are used in a big SGTC and work reliable with low heating.

Online Laci

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Re: My first DRSSTC on bricks
« Reply #58 on: May 02, 2019, 10:25:11 PM »
I found this ebay listing with 0.1uF 2.5kV paper in oil capacitors:
https://www.ebay.com/itm/K41-1a-Paper-PIO-pulse-capacitor-0-1uF-10-2-5kV-Lot-of-2/251259264733?_trkparms=aid%3D888007%26algo%3DDISC.MBE%26ao%3D1%26asc%3D20131227121020%26meid%3Dc3967893df3249af835219ad5cd172cc%26pid%3D100009%26rk%3D1%26rkt%3D1%26mehot%3Dag%26sd%3D162844731444%26itm%3D251259264733&_trksid=p2047675.c100009.m1982

Does anybody have experience with this kind of capacitors?Are they any good for a MMC?

Another thing: in series connection the overall pulse resistance(dVdT) of the MMC gets multiplied by the amount of series capacitors? In this case the voltage gets shared by the series capacitors and they see a smaller voltage rise across them,so 1.8kV/us capacitors would already be good with 4 in series at 6kV across the MMC?
I just saw Futurist's DRSSTC1 and he used C4BS capacitors,which have around 1kV/us rating if I remember right and they worked fine for him...
« Last Edit: May 02, 2019, 10:49:41 PM by Laci »

Offline Max

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Re: My first DRSSTC on bricks
« Reply #59 on: May 03, 2019, 06:16:10 PM »
I always thought that paper oil capacitors had terrible losses at high frequencies...? Or in other words that PP film caps are the only type of caps that are suited for this frequency/power range. Apparently that's not true?

Kind regards,
Max

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Re: My first DRSSTC on bricks
« Reply #59 on: May 03, 2019, 06:16:10 PM »

 


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