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Ground strikes at DRSSTC

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MRacerxdl

Wed Sept 24 2008, 09:27PM

Registered Member #989 Joined: Sat Sept 08 2007, 02:15AM
Location: SÃ£o Paulo, Brazil
Posts: 476

Hmm, ground problems can be, I wasnt know that a poor ground makes problem with drsstc's.
I am using a 1x1 m aluminium foil outside of my house in the ceramic ground. And a wire to the DRSSTC. The target is in the same ground but near the DRSSTC.

Reaching

Thu Sept 25 2008, 08:31AM

Registered Member #76 Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458

ive had the same issues with my older drsstc 6 grounded to a ladder which i used as my target too. its difficult to explain what happened there but i remember that i heard a very uncommon noise just before my bridge blew up after a fat yellowish strike to the ladder. i think it is because of the closed loop and the significant voltage drop in this case.
i used secondary bottom feedback in my drsstc 6 with no synced driver or modulator and with no overcurrent protection at all, so the current in the primary can peak to whatever it would peak depending on the R of the LC circuit. maybe thats your problem.

after i changed the igbts in my bridge and grounded the secondary of the drsstc on a copper waterpipe everything was just fine.

to prevent such failures i recommend that you "can" ground your "smaller" drsstcs just at the mains ground if you can be sure that the ground connection of the house is fine.
ive had no such failures for a year or two after i used the mains ground and changed completely to primary current feedback with overcurrent protection and so on,.

MRacerxdl

Thu Sept 25 2008, 12:59PM

Registered Member #989 Joined: Sat Sept 08 2007, 02:15AM
Location: SÃ£o Paulo, Brazil
Posts: 476

Hmm, but the biggest problem is here in SÃ£o Paulo all mains ground is attached to neutral on the mains =/ (norm from the local power distributor), that is a stupid thing to do, but is that the power distributor does ...

Any other way that I can ground? I have a good soil in front of my house, but it will need 10m or more of wire to get here to test.

Firefox

Thu Sept 25 2008, 02:23PM

Registered Member #1389 Joined: Thu Mar 13 2008, 12:50AM
Location: Pittsburgh, PA
Posts: 346

My dad is a contractor here in the US of A, and electrical code here says that you need a single 8 foot (2.5m) ground rod for any residential building, if I recall correctly. Fortunently for you, the ground doesn't need to be a single piece, because you don't have to worry about any code. So just drive 3 meters worth of ground rod into the ground, tie it all together with a solidering torch and some wire, and youve got a ground isolated from the neutral of your power.

Linas

Thu Sept 25 2008, 03:42PM

Registered Member #1143 Joined: Sun Nov 25 2007, 04:55PM
Location: Vilnius, Lithuania
Posts: 721

something like that

MRacerxdl

Thu Sept 25 2008, 04:01PM

Registered Member #989 Joined: Sat Sept 08 2007, 02:15AM
Location: SÃ£o Paulo, Brazil
Posts: 476

Here are my DRSSTC Setup (the highside)

I was noticed about one thing, I didnt put the reverse diodes in parallel of the gate resistors (Steve Ward and other people put it) that matter something in that problem?

Dylan

Fri Sept 26 2008, 12:46AM

Registered Member #1024 Joined: Sun Sept 23 2007, 10:56AM
Location: Northern NSW, Australia
Posts: 95

Mestre Racerxdl wrote ...

--==SNIP==--
My Primary cap is 50nF and the Inductor is 8.5uH, its a small coil.

--==SNIP==--
(Its at perfect tune, I have measured, the primary is running at 244Khz so the secondary.)

What peak currents have you calculated based on this primary tank?
And assuming no OCD, are these within or close to spec for your IGBTs?

Dalus

Fri Sept 26 2008, 11:28AM

Registered Member #639 Joined: Wed Apr 11 2007, 09:09PM
Location: The Netherlands, Herkenbosch
Posts: 512

Reaching already stated wat was going on, he just needs a good solid ground.

MRacerxdl

Fri Sept 26 2008, 02:10PM

Registered Member #989 Joined: Sat Sept 08 2007, 02:15AM
Location: SÃ£o Paulo, Brazil
Posts: 476

Yeah, the current its VERY low, if I calc with Reactance, I get only 24A of peak on 320V BUS, the IGBT is rated for 200A Pulsed

Yesterday I was turned the coil on again, but with the breakpoint pointed to the top, I didnt get any problems.
But Reaching Coils have another style of arc, I dont know why, that can be ground too?

Steve Ward

Sat Sept 27 2008, 05:10AM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

Yeah, the current its VERY low, if I calc with Reactance, I get only 24A of peak on 320V BUS, the IGBT is rated for 200A Pulsed

I bet your calc is wrong.

The peak current is limited by the "DC" resistance and time. Given infinite time, its limited by only the resistance as the reactive components cancel.

What is your L and C combination? You can figure out the surge impedance as Z = sqrt(L/C). This tells you the peak current on the FIRST half-cycle of operation using I = V/Z. Then, each half-cycle basically adds that same current again, so its 2X, then 3X, and so on. The secondary streamer formation limits the peak value usually, but its very likely to be higher than 24A!

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