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JimG's Smaller DRSSTC

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Bennem

Mon Apr 24 2006, 07:56AM

Registered Member #154 Joined: Sun Feb 12 2006, 04:28PM
Location: Westmidlands, UK
Posts: 260

Wow!
Now that IS tiny!
I'm putting together a tiny DRSSTC, but in comparison, mines huge!.....lol
Here's my driver and combined H-bridge of HGT1N40N60A4D's
All the best with your project!

EEYORE

Mon Apr 24 2006, 11:44AM

Registered Member #99 Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637

Wow, nice work guys!

Better keep the on time really low for that full bridge!Ive never seen 40n60s made into such a small full bridge...
Now im getting the mini bug!
Matt

Steve Ward

Sat May 27 2006, 02:20AM

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

Ive got some results to report now as well. This was inspired by Jims work of course. The secondary is 1.3"x3", with a 1"x3" (roughly) toroid. Im using FGH50N6S2D IGBTs from fairchild in a half-bridge. My driver is the simplest i could do, just a pair of gate drivers (UCC27321/2). The feedback CT (primary feedback) is fed through a .1u cap, then clamped to the power rails with some 1n4148 diodes. The interrupter plugs right into the enable pins. The GDT is 1:2:2, using 6 primary turns and 12 secondary turns. The tank cap is 9.2nF at the moment. The primary coil is on 2.375" diameter pipe, 5.5 turns. The operating frequency is around 1.2MHz. Sparks reach out to 7" pretty regularly. Im running very long pulse widths, of up to 1mS. Anyway, i dont want to take over this thread, but since its a very similar project, i figured id make a post about it.

Pictures will be held in this folder, and may be updated as progress continues:

link

ragnar

Sat May 27 2006, 05:16AM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425

Aw Steve, that's just adorable. Love the protoboard and the primary, too.

JimG

Sat May 27 2006, 04:29PM

Registered Member #52 Joined: Thu Feb 09 2006, 04:22AM
Location: Austin TX
Posts: 57

Cool small coil, Steve. I've been a bit leery about going over 100us for fear of burning out my tiny IGBTs, but after seeing your long pulse sparks I've been curious how long of a pulse I can do with the small capacitors that I'm using.

I've been doing a bit of overtime at work so I haven't had a chance to work on my mains powered tiny coil. Hopefully I'll have enough time this weekend to finish it.

Steve Conner

Sat May 27 2006, 04:47PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

OMG, invasion of the mini DRSSTCs. :-o Steve, the streamers from yours look almost like VTTC sword streamers but not quite. Could you rig it to fire half-cycles of unsmoothed AC line with a staccato timer type circuit, and see if the streamers turn straight?

JimG

Sun Jun 04 2006, 05:32PM

Registered Member #52 Joined: Thu Feb 09 2006, 04:22AM
Location: Austin TX
Posts: 57

My camera isn't sensitive enough to get spark pictures off of this coil so I've attached a video of the sparks. It's not quite up to where Steve's coil is, though, but I'm still tuning it.

I'll try to post an update later today.

]1149442005_52_FT600_tnydrsstc2sparks.wmv[/file]

Composite image taken from video stills.

1149454789 52 FT1630 Tinydrsstc2composite

Even with the changes I've made I'm still getting flashovers at higher voltages. Hopefully I can fix this without changing any hardware by adding a feature to my interrupter re-write that will increase the on times for a single burst to promote streamer growth from the breakout point.

My tiny IGBTs aren't heatsinked so they get a little warm to the touch after a few minutes of running. Hopefully allowing for shorter on times and longer delays will help keep them a little cooler.

JimG

Mon Jun 12 2006, 05:47AM

Registered Member #52 Joined: Thu Feb 09 2006, 04:22AM
Location: Austin TX
Posts: 57

I've modified my old interrupter to do three 40us on/50us off pulses and then a 200us pulse. I can now crank my variac all the way up to its max rating of 130V with no flashovers to the primary. The streamers are also a lot brighter now.

Photo with variac set to 100v with 4.25 inch arcs to ground, 2 second exposure.

Photo with the variac set to 120V with a streamer reaching out to 6 inches, 1.5 second exposure.

1150086889 52 FT600 Tinydrsstc2streamers

Steve, I don't think I'm going to make it out to 7" like you did without a bit more work. Interrupter timing seems to be pretty critical to the streamer performance more than the voltage since I can get better streamers now at 80V than I could with the old settings at a little over 100V.

During a few test runs I measured the voltage in my 350 uF supply capacitor. It starts off at 176V and drops to 106V after a single shot, so I'm sure I can squeeze a little more out of it.

Steve Ward

Mon Jun 12 2006, 07:30AM

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

Hey Jim,

The only way i could get the 7" sparks was with the really long pulse lengths of at least 250uS. Anything that would seem resonable for a tiny DRSSTC (say 10-50uS) wasnt enough for maybe 2" sparks. Also, im running 350VDC on the half-bridge, so that may also give me an edge. Im not sure what i want to try next with my mini... maybe a larger tank C and less L. Im running 100A peak right now which is pretty safe, so maybe i should just leave it alone. Unfortunately, i cant run much more than 10bps before the IGBTs get really hot

. Perhaps a better cooling system would be a nice upgrade.

At Conner: i might try a half-wave voltage doubler to see if i can get straighter sparks. It could possibly be really cool.

Im still working on how to package the little guy... I want to keep it as small as possible.

JimG

Mon Jun 12 2006, 05:55PM

Registered Member #52 Joined: Thu Feb 09 2006, 04:22AM
Location: Austin TX
Posts: 57

I figure 170V DC on my full-bridge should be almost equivalent to 350V DC on your half-bridge. I went with a full bridge because the IBGTs take up less space than a second capacitor.

On my previous coil I certainly saw an increase in the streamer length when I went from 7.5nF to 20nF for my tank cap and measured about 100 Amps in the circuit. I haven't measured the current on the current one using a 22nF cap, but I'll probably do that tonight.

I have been a little concerned with overheating on this coil since I decided to put the IGBTs back to back on the board to cut down on space while the previous one had a small heat sink epoxied behind the IGBTs. I've been thinking about using some thermal epoxy to affix some sort of ceramic heatsinks to IGBT bodies, but haven't started looking into it yet.

I can't wait to see how you coil turns out, Steve. Your DRSSTC.5 was part of the inspiration for me to want to make a DRSSTC using a very small
capacitor.

Update:

I've had a chance to measure the current and it seems to average somewhere between 100 and 150 Amps. There are a few cycles where the current spikes to between 200 and 240 Amps.

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