Re: Terry's DRSSTC - Frequency/Pulse Width Controller

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Original poster: Terry Fritz <teslalist@xxxxxxxxxxxxxxxxxxxxxxx>

Hi All,

I didn't get very far today on the Frequency/Pulse Width Controller.  I got 
too wrapped up in the protection stuff.

Here is the IGBT over temperature circuit:

http://drsstc.com/Design/OverTemp.gif

It simply uses a LM35CAZ temperature IC that puts out 10mV/degree C.  I 
compare that with a DC 0 to 1.5 volt signal from the pot for a temperature 
range of about 0-150C that can be easily pre-adjusted.  The open collector 
output of the comparator is buffered to give complimentary Over Temp output 
signals.  R3, C1, and R5 form a hysteresis effect.  If the setpoint is 
100C, the circuit will trip at about 102C and not reset until the 
temperature drops back down to about 92C.  Sort of cool and prevents fast 
oscillations.  A simple LED indicates when the circuit is active.  I am 
using little IGBTs with little heat sinks.  VERY cheap, but have to shut 
down if the temperature goes out of control.  Best case is they can pump 
500 amps just like the big boys and remain happy ;-))

For the Over Current part:

http://drsstc.com/Design/OverCurrent.gif

An off-the-shelf high speed CT (DigiKey) just feeds two comparators with a 
positive and negative trip point.  A LMC555 timer provides a "timeout" if 
an over current is detected.  I have it set to 500mS but it could be just 
about anything like 500uS if one were to want to restore the signal 
immediately for the next bang.  These two circuits would go together on a 
card very happily since they could share ICs.  The CT signal could also be 
used to drive the inverter logic.

Just simple goofy logic stuff today, but I thought real hard and long on it 
%;-))

I know of a digital way to do the Frequency/Pulse Width Controller but it 
is a little messy.  I could use a Basic Stamp and add an LCD display and 
all too ;O))  A microcontroller could do a lot of stuff here really.  But 
it adds a whole new set of complexities most people are not used to.  I'll 
think more on this.  I could not quite get any current source ideas to work 
yet.  Steve Conner has a nice circuit too...  I am curious as to how well 
he like the linear control???  But a basic stamp could just set the BPS 
rate and has pulse width output too... Could read out IGBT temp which is a 
real issue...  I am very tempted just to use a microcontroller...  I have 
Rabbit RCM3000 modules too for some of the 55MHz IEEE floating point math 
stuff, but that would be way too much power unless I wanted the "black box" 
thing to record all the data right before it blew up...  Lots of unknowns 
about what is the coolest way to control things.  A fully programmable 
controller might be the way to go to find out...  The "box" could "protect 
itself" so the user could pick any BPS and pulse width solution they 
wanted.  From Dan's simple LM555 thing to a laptop ;-)  I think I need to 
add a many signal military connector to get to the remote box...  I 
certainly have the armored cables and shielding from my robot stuff...  The 
days of "computer controlled" Tesla coils, may have arrived!!!

I am "really" putting stuff on the drsstc.com site too ;-))  I "need" to 
move everything there since my DRSSTC notebook is now so messy I can't find 
stuff in it anymore...

Here is the input stuff and the parts list so far:

http://drsstc.com/Construction/InputBoard/InputSection.gif

Note the blocking inductor between the RF and AC line ground...  It "might" 
be a "good" idea...  We always want the RF and line grounds to be the same, 
but separate...

http://drsstc.com/Construction/MasterPartsList.xls

Yeak!!  $725 so far ;-))  But It is all top of the line stuff...  Probably 
hit $1000 easy.  The rest of the parts are cheap but the PC boards will add 
up now still.  But the box should be able to run a wide variety of coils 
and be really cool!!!  I am using all the best ideas and state of the art 
stuff.  It should run most lower frequency coils in the 1.5kW range and I 
can easily lift it with an out stretched arm ;-)

I am sure that DRSSTCs can be extremely reliable.  They are a very simple 
form of switching power supply.  There are no great tricks to it...  A 
little harder to come up with at the "amateur level", but we will get there 
given a little more time and 1/1000th the R&D budget ;-))

Cheers,

	Terry