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Messages - Hydron

Pages: [1] 2 3 ... 11
1
Beginners / Re: Keep or throw away ?
« on: March 19, 2019, 08:14:33 PM »
Most likely not even usable for an amplifier - they will not have very good SOA for linear operation. I vote for scrap - that's what I did with my pile of large power BJTs.

2
Dual Resonant Solid State Tesla coils / Re: Strange gdt waveforms
« on: March 18, 2019, 02:27:35 PM »
I personally don't have an issue with it, especially in this context - I certainly understand the emotion of getting a long and tricky project going!. If anyone feels differently though then please feel free to get in touch with Mads about it.

As for the coil - looks like it's working quite well if you're getting that sort of performance 8)

3
You are right - I should have specified "and add a bit more film capacitor decoupling for the high frequencies".

I was thinking along the lines of induction cookers etc - they get good power factor by deliberately not including a large smoothing capacitor - high frequency currents are supported by smaller film caps but the overall input voltage follows the mains supply, leading to reasonable power factor.

4
Wow, that's some serious arcs coming from a relatively modest transformer! Definitely beats what I managed with 4 microwave transformers in series.

Have you considered removing the electrolytic smoothing cap? Might make it sound scarier to have the hum from the mains ripple.

5
Transformer (ferrite core) / Re: Piernas's Fast ZVS Mazzilli Driver
« on: March 12, 2019, 10:18:03 AM »
I'm not an expert on this stuff (I'm a physicist not an electronics engineer). With "dead-time" and "cross-conduction", you mean to avoid enabling both Mosfets at the same time?
That's exactly what it means - deadtime is the enforced period between the top switch turning off and the bottom turning on (or vice versa), while cross-conduction is what happens when both end up on at the same time, shorting out the supply (generally a bad thing :P)

6
Dual Resonant Solid State Tesla coils / Re: UD 2.7C Build
« on: March 06, 2019, 07:19:40 PM »
If you look on eBay you should be able to find some cheap skinny spdif optical cables for audio use. The 2.2mm (I think) diameter is suitable for these POF optical links, especially short ones

7
Dual Resonant Solid State Tesla coils / Re: Primary coil connection
« on: March 02, 2019, 06:08:54 PM »
I don't think the connection makes much of a difference - I would pick whichever is easier the manage mechanically.

My QCW coil will actually have both ends of the primary at HV potential - I have split the MMC into two halves and the primary is between them. It will also use phase shift modulation so I am interested in your results. I currently have a prototype running using the UD3 driver but there is still a lot of work to do!

8
Dual Resonant Solid State Tesla coils / Re: Blow up your top load !
« on: February 28, 2019, 10:04:27 PM »
This has been done, though not by me. I think people have used papier mache over a tube before the aluminium layer for structural integrity, but it all sounds like a lot of work.

For my smaller coil I bought a spun toroid off eBay, for my larger one this wasn't practical so I used ducting. For my first go I used alu duct and while it worked it was _very_ fragile. I've now replaced it with some single-wall stainless steel flue liner which is massively stronger, though more difficult to work with. This stuff is easy to get it in the UK, though difficult to source in NZ where the coil was. Ended up finding a UK eBay seller offering international shipping via the eBay global shipping program - normally a ripoff but in this case a rather large box was only £12 to the other side of the world (must be done by weight rather than volume!).

9
I was gonna say that my SKM200GB125Ds were fine at ~750A or so (might have hit 800, was a while ago so forget exactly).

10
Dual Resonant Solid State Tesla coils / Re: Bleed R's for MMC caps ?
« on: February 24, 2019, 12:25:05 AM »
yeah that did occur to me - hence the planned addition of a neon indicator (which should run on DC OK) as a separate safety feature.

So far I have only had it running up to ~100V in testing, though I did manage to short it out at ~50V or so with a test probe (which promptly disappeared into vapour!).

I'd rather not run so much capacitance, but for a QCW I need to have enough to avoid the bus voltage drooping too much over a single pulse and a thumping great capacitor bank was the easiest (and cheapest, given I got them free) way to do it.

11
Dual Resonant Solid State Tesla coils / Re: Bleed R's for MMC caps ?
« on: February 23, 2019, 03:04:49 PM »
Yes I know about that well - I have 4x6800uF for my qcw coil and an even larger bleed resistor - takes a long time to drop to a safe level. I do however have a high efficiency led in series with the bleeder R to tell me when there is voltage present - stays obviously lit until well after its safe. May also add a neon indicator to be doubly sure.

12
Without watching the video I'm guessing popcorn :P

13
Dual Resonant Solid State Tesla coils / Re: Bleed R's for MMC caps ?
« on: February 22, 2019, 04:34:14 PM »
Actually it's very quick to discharge - the caps are only 6.2nF so the time constant is only 63ms - enough to render the array safe in well under a second, and even enough to perform the job of the inverter bleed resistor too given the low pulse rep rate of the QCW coil this MMC has been built for. From a safety perspective a 10M resistor should be good enough for even capacitors into the low-uF range if you give them a minute or two to discharge.

For 18 caps I would 100% put them in, even if you have to put two in series 36 resistors will not take long to solder up.

14
Dual Resonant Solid State Tesla coils / Re: Bleed R's for MMC caps ?
« on: February 22, 2019, 10:07:23 AM »
Thinking about it more, I personally would add the bleed resistors if only for safety when working on the MMC/coil. It is unfortunately quite annoying to solder them all (I once put together a MMC with 512 bleed resistors - two in series for each cap!).

You will need to try and find resistors with a suitable "overload" voltage or put two in series - their normal rated voltage for steady state is often significantly lower than the MMC capacitor rating but we are not running them in steady state operation.

If you have PCB design experience then you could maybe get something made by a cheap chinese PCB supplier to hold the caps and bleed resistors - they are extremely cheap, especially if you only have 2 layers and 1oz copper, and you can always use both layers (and even stack a couple of identical boards) to get the amount of copper up. See https://github.com/Hydron-4hv/QCW_MMC_PCB/raw/master/Reference%20Outputs/Schematic%20%26%20BoM/QCW_MMC_PCBA-(NO_WE_PRESSFIT).PDF for one I did (I built up 8 of these boards!).

Finally I would not worry so much about the current sharing bit in Mad's guide - this is important for sharing between low impedance parts such as bus capacitors or semiconductors, but the impedance of the MMC capacitors is high enough that they should share pretty much perfectly regardless of connection point unless you do something wrong. Do however space the caps out a bit, and try not to have the resistors touching the caps too.

15
Dual Resonant Solid State Tesla coils / Re: Bleed R's for MMC caps ?
« on: February 21, 2019, 06:06:46 PM »
Individual bleed resistors would eliminate the danger of getting a zap off a MMC cap due to uneven stored charge, whereas across the bridge gives less guarantees. Not sure how much of an issue this is in reality though.

I would definitely recommend a resistor across the bridge as Mads said, especially if you are using a UD2.x style driver. Only needs to withstand the bus voltage at your max duty cycle so it's easy and cheap to add.

16
Dual Resonant Solid State Tesla coils / Re: My first DRSSTC on bricks
« on: February 18, 2019, 06:38:29 PM »
CT between inverter and primary is fine - the place to avoid is between primary and MMC due to high voltages present there.

As for the MMC it is indeed likely the weak part - 4kV sounds like a low rating for a coil that needs brick igbts, and will limit your max current.

17
Coupling the non-MMC end of the primary would put a significant load on the bridge - take a look at the impedance of the typical coupling caps (e.g. 0.1uF) at the switching frequency. I suspect it would also cause EMI and safety issues, as any current through this capacitor would have to flow via the mains wiring to wherever the earth-neutral bond point is.

Given that the issue doesn't seem to be the strike current (on the order of a few amps only) flowing through the igbts (which have anti parallel diodes included to provide a path when off) but was more about voltage stress between the bridge and heatsink (ground) when hit, the standard method seems to do the trick.

18
Transformer (ferrite core) / Re: Homemade flyback on 60N60DLF full bridge
« on: February 11, 2019, 08:13:07 AM »
The M10 bolts are excused! Those are some nice arcs, though there always seems to be something missing without the 100Hz hum from mains :P

19
Voltage Multipliers / Re: Russian Marx generator test center torn down
« on: February 08, 2019, 09:19:57 AM »
Sad to see the destruction - I assume it was long decommissioned though?

Very #yolo method of knocking it down, I wonder if the Russians learnt something from Fred Dibnah?

20
Have had a quick look through it, is some nice work but I'm not sure I subscribe to the idea that the MMC layout has a big effect on current sharing.
I would have thought that any sharing issues would only arise if the reactance of each capacitor string is similar in magnitude to the parasitic resistance/inductance of the connections between the strings.
Given that the reactance of a 100nF capacitor is ~16 ohms at 100kHz this should completely swamp any effects of the connections (unless they are _really_ bad) and thus force nearly perfect current sharing.

I'd personally focus more on keeping enough distance between capacitors, at least when being run at high RMS currents, so that there is less chance of hot spots.

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