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Re: A SSTC Ground ?



Original poster: Finn Hammer <f-h-at-c.dk> 

Comments inbetween:

Dan wrote:
 >
 > Forget about taking any type of sensitive measurements within the vicinity
 > of an SSTC.  Protect all electronic devices and equipment.
 > Use an analog meter if you need to take measurements.  Its not worth the
 > risk of damaging an expensive scope, etc...
 >
 > Dan

It is more difficult to measure under these circumstances, for example
does a Pearson 101 monitor report much more noise at 25MHz than a 2100
does. Which is strange since the 2100 has a -3dB point at 20MHz whereas
the 101 drops off at 4 MHz.
Running the coil in half wave rectified input, the scope simply looses
controll of itself. The reported amplitude tracks the trigger level, and
if not triggered, the signal amplitude rises and falls in a 5 sec.
cycle.

But I don`t have an education as a radar engineer, so I need
measurements more than usual, to understand what is going on.

For example: does timing the bridge from an antenna automatically
produce zero current swiching?

Apparently not necessarily so:

The scope grounded at the bridge negative bottom,
trace one:voltage across the 4th FET,
trace two: current in coil

http://home5.inet.tele.dk/f-hammer/voltacross4-vs-current-antenna.jpg

Heavy oscillations at switching, 60 degrees current lead, 339,9KHz

Current lead means the coil is driven below Fres.


Turning up the power, increasing the discharge -> lowering the
frequency:

http://home5.inet.tele.dk/f-hammer/voltacross4-vs-current-antenna-morepower.jpg

same faults, but frequency lowered to 338.1 KHz. So it tracks FRes. but
appears to aim too low still.


However, driving the coil from the oscillator rather than the antenna:

http://home5.inet.tele.dk/f-hammer/voltacross4-vs-current-oscillator-morepower.jpg

Fairly clean swiching, 10 deg. current lag, more current into coil,
345.3 KHz

I could have adjusted the coil to switch at 0 degrees, but it worked
best at ~10 deg. lag.

I don`t yet make any conclusions of this, other than it is possible to
make measurements, that it is useful.
I know that Stephen (hi!) has made great results with the same circuit,
so it can be tweaked into working, probably by adjusting distance from
coil and antenna length.
But without measurements?
I would like to stick a cap in the ground lead and derive the sync.
signal from there, as suggested by mr. Herrick, and it is the problems
associated with the ground lead that prompted this thread. I would like
to see that signal on the scope first, before piping it into the
comparator.

However, I`l probably try to see it on a cheap analog scope at low
power.

There is a construction picture of the h-bridge with it`s piggyback
driver here:

http://home5.inet.tele.dk/f-hammer/Resize-of-sstc1.jpg

With 2 AMD K6 coolers it runs wonderfully. The gate resistors are not in
place on the pic.

This is fun

Cheers, Finn Hammer






 >  > SSTC gang.
 >  >
 >  > I`ve got my IFRP460A H-Bridge up and running.
 >  >
 >  > I am running DC in, and everything gets "electrical" when the coil is
 >  > running.
 >  >
 >  > I find it hard to establish a ground reference, because even short
 >  > lengths of wire pick up enough potential to arc trough insulation.
 >  >
 >  > My present grounding is only 2 meters away from the coil, and yet it
 >  > arcs to the table, myself, coax from scope to Pearson Current Monitor
 >  > etc. etc.
 >  > I fear for my scope.
 >  >
 >  > What are you guys using as a ground?
 >  >
 >  > Cheers, Finn Hammer
 >  >
 >  >
 >  > The coil is based on the schematics posted on Aron and Justins site.
 >  >
 >  > This means beefy driver ( tc4428/ IRFP9540/ IFRP530 )
 >  > Source of oscillations is a TL494 triggering an AD8561 comparator.
 >  > (Sometimes an antenna is doing the triggering)
 >  >
 >  >
 >  >
 >  >
 >  >