Author Topic: A few question for my first DRSSTC  (Read 196 times)

Offline nabzim

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A few question for my first DRSSTC
« on: September 02, 2019, 03:36:56 AM »
**cross-post from the Facebook group 'TESLA COILS are BOSS' ...
Working on my first DRSSTC, voltage-doubler half bridge, CM200 coil, very, very similar to Gao Guangyan's DRSSTC-2 (shoutout Loneoceans Ggy). Planning on a bus voltage of about 396 VDC (from 140V variac). I might also try using my 208/240V transformer to step up the input from 140VAC to 161VAC, which would give me a bus voltage of about 457 VDC...

Currently test-fitting components of the bridge, and I have a few questions:

Q1.
I am wondering if my DC bus bars might be too close together.
The closest point measures 0.18" or 4.6mm between the positive and negative DC bus bars.
Should I shave them down a little to increase the gap?

Q2.
In Guangyan's design, a 10k-Ohm resistor is connected across the tank circuit (bridge output), to drain remaining MMC charge before the next pulse.
He used a nice ceramic wirewound resistor. I have a cheap wirewound resistor with an aluminum heatsink.
I was planning on mounting it straight to the IGBT heatsink, so it would be RF-grounded. However, I am concerned that this may be dangerous, as the resistor leads are only 0.18" from the edge of the resistor's heatsink (coincidentally the same distance between the bus bars).
Is this a real danger to have the primary/mmc leads so close to RF-ground?
I could easily insulate it with plastic cutting-board and nylon screws to leave the heatsink floating.

Q3.
Considering the "general wiring scheme" put forth by Steve Ward, which is now a standard. I had been aware that the negative DC Bus rail is coupled to RF ground through a 100nF cap, so when I ordered my MMC caps (942c20p15k), I included one extra to use for this purpose, but I now realize that .15uF might be a problem, since it's not the specified .1uF standard.
My other problem... I had not noticed that Steve's "general wiring scheme, as well as Mad's "DRSSTC GROUNDING" schematic (shoutout Mads Lundbæk Barnkob), both show a second 100nF cap placed across the DC Bus terminals. This part, I have yet to acquire.
What purpose does this cap serve? (Wouldn't the IGBT snubber already fulfill this duty?)
Does it need to match the value of the RF-ground-coupling capacitor?
Can someone please recommend good caps to use here, cheaper than Cornell Dubilier? (I'd like to save my extra MMC cap as a replacement part.)

Thank you all!
« Last Edit: September 02, 2019, 03:42:05 AM by nabzim »

Offline Netzpfuscher

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Re: A few question for my first DRSSTC
« Reply #1 on: September 02, 2019, 08:46:12 AM »
Q1: 4,6mm is enough. But you can add a foil for extended isolation.
Q2: The resistor is specified for 1250V. You don't need extra isolation. If you want, you can insulate the Terminals with shrinking tube.
Q3: I think the best way to do the RF path to ground are TVS diodes. With capacitors the not so small ripple from switching gets coupled to earth which can trip a RCD. In my coil I use 8x 100V TVS diodes in series from each inverter output to earth.

Offline Mads Barnkob

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Re: A few question for my first DRSSTC
« Reply #2 on: September 02, 2019, 07:12:40 PM »
Q1: For ~5mm to flash over, when it happens, the 5mm was never your problem and you have bigger problems on your hand. It is fine.

Q2: You do no need to heat sink this resistor, it is peak loaded with 16W on a 50W specification and at a low duty cycle, if you worry too much about RF ground on the heat sink, just find a small heat sink just for the resistor.

Q3: Too high capacitance also has a few bad side effects if your ground is broken or bad, I have zapped myself a few times due to that cap. I got no experience with TVS instead, are they fast enough to eat it/breakdown/avalanche?

I only have the lists of good MMC capacitors on my site, but generally those make good snubber capacitors as well. http://kaizerpowerelectronics.dk/theory/good-mmc-capacitors/
http://www.kaizerpowerelectronics.dk - Tesla coils, high voltage, pulse power, audio and general electronics

Offline dexter

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Re: A few question for my first DRSSTC
« Reply #3 on: September 03, 2019, 11:59:38 PM »
Can someone please recommend good caps to use here, cheaper than Cornell Dubilier? (I'd like to save my extra MMC cap as a replacement part.)

i just bought some of of this caps: https://www.ebay.com/itm/12PCS-0-68uF-680nF-2000V-MKP-TESLA-Pulse-Capacitors-Arcotronics-C4AS-IBGT/173956817574?hash=item2880a262a6:g:37YAAOSw18Jb7q5a
rated for 25A RMS
you can check the seller catalog for other specs

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Re: A few question for my first DRSSTC
« Reply #3 on: September 03, 2019, 11:59:38 PM »

 


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