Re: Terry's DRSSTC - New H-Bridge :-)

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Steve Ward <steve.ward@xxxxxxxxx>

Hey Terry,

Just a few comments.



> So I cranked it up into a bare primary circuit with about 3 ohms of 
resistance:
>
> http://drsstc.com/~terrell/pictures/NewBridge-01.gif
>
> Note that the voltage driving square wave is super clean!!  Almost "too"
> clean...  It started out clean with the last bridge too, but then got
> noisy...  Hmmmmmmm  ?;-|  I wonder if there was some damage to the old
> bridge I did not detect...  Gee whiz, I checked and double checked
> everything...  Think more on that...  But clean is good for now ;-)

I cant help but wonder if maybe you accidentally switched on/off the
HF reject on the scope?  Those fancy digital scopes have so many
options, it would be easy to not notice some minor setting like that
;-).  Anyway, that waveform looks more like something you get out of
pspice, not out of power electronics!

>

> Notice the 1.4MHz unloaded ring down:
>
> http://drsstc.com/~terrell/pictures/NewBridge-02.gif
>
> Just a resonance "somewhere".  Not sure at all where.  Does no harm, but I
> should probably "find it"...

Seems like it would be due to your buss caps and inductance there?
Maybe too much ESR in those big lytics is allowing the buss L to
resonate with the snubbers?  Thats the sort of thing i just ignore
really... everything is off at that point.

>
> The current limit circuit still trips too soon.  Probably noise on the
> protection inputs due to these super fast switching times!!!!

I still think you should keep a low impedance from your current
detection CT to the inputs of your comparators, and put about 100 ohms
right at the input of the comparator to ground.  That keeps the
impedance low there and the 100 ohms doesnt really mess with the
burden resistor considerably, though you should factor it in.  I just
watch the current on my scope and set the ODC to trip wherever i want
it to.  Right now its set to about 520A :-).  Only trips rarely,
unless im way out of tune!  Thank god for it when i accidentally tap
it a turn lower than it should have been!  oops!


> Few question as to noise and such, but these are pure "science" problems
> that can be solved just by thinking *;-)  The performance is almost "way
> TOO good"...

I dont think you should worry that much about noise... im sorry but
its gonna get worse when you are actually driving a coil.  Sometimes i
think its magic that these things work at all in this environment, but
they do!  Im sure your setup is probably 10X more noise resistant than
mine with all unshielded electronics mounted just a few inches from
the primary!


>
> Cheers,
>
>         Terry
>
> *There is a potential "big problem" if a DRSSTC driver were to blow
> open.  The say 20KV voltage on the Tesla coil's primary caps could find
> it's way inside the DRSSTC "box" and do "massive" damage!!!  Simple MOVs in
> the right places will "catch" these voltages in the case of say an H-bridge
> nuke and just short them out.

Hey, how about some 1000V spark gaps or something like they use inside
of some appliances?  Interestingly, on my smaller DRSSTCs the bridge
usually fails short, blowing the fuse etc...  On the BIG coil,
apparently the 1400J of filtering caps just blasts the die connections
right off and the darn thing will still run on a half-bridge
afterward!  I was really confused when after getting 10' arcs i hear a
loud pop and now im only getting 4-5' of spark...  the fragments of
IGBT case on the ground were a decent indicator that something went
wrong ;-).  Im thinking that primary feedback should pretty much
eliminate most failures, it just *seems* more solid.

Steve W.

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