Hi Bud,
it is difficult to create a large current in a low C and high L primary tank.
Power input from the bridge is proportional to primary current (and primary voltage)
so a small current won't transfer much energy into the primary.
Say, you are running the coil at the minibrutes typical frequency of about 160kHz
and want a current of say, 500A, also typical for this coil. The voltage on the
primary 5nF cap is then I/(2*pi*f*C) or about 100kV(!).
Aside from the cap spec, the 100kV value is important, since it tells you how
long the burst must last to ramp up to this value. Theoretically you can gain
a bridge voltage swing (i.e. 400V from -200V to 200V) for each switch
of the bridge. The time to ramp up until 100kV is thus 250 half cycles or
about 750us. This is the absolute minimal time since it disregards any losses
in the primary by resistances in the primary windings and the primary cap and also
the energy transferred to the secondary. Typically bursts should be at least twice as
long as this estimate.
Another issue is the primary copper loss resistance. For a low primary C you need
a large primary L, i.e. many turns. In my minibrute using a 100nF primary cap and about
7 primary turns I measured a primary loss resistance of about 0.2 ohms. This implies, that
at 500A currents I loose about a 100V of my 200V bus voltage alone by this. I'd need
about 30 turns for a 5nF primary cap making a primary current of 500A impossible.
When using long bursts a third problem arises: The bus electrolytics will be depleted
considerably, so you primary voltage will drop.
Generally I think it is not a bad idea to try out small caps for a DRSSTC. A 5nF cap at
100kV will hold an energy of 25J. A 0.2uF cap at 2.5kV (calculated from a 500A primary
current) will hold just 0.6J. Using a 5nF cap is pushing that idea to the extreme
and requires very long bursts together with a considerable effort keeping
primary loss resistances low.
Udo
----- Original Message -----
From: "Buds mohrstamping mail" <bud@xxxxxxxxxxxxxxxx>
To: <tesla@xxxxxxxxxx>
Sent: Friday, May 27, 2016 6:50 PM
Subject: [TCML] solid state tesla coil question
I recently finished the minibrute tesla coil and have it running well. The question is why does this coil operate better with a
large primary capacitor size of .21uf vs. a smaller cap size of say .005uf. I have tried this setup two different ways by using
two different secondaries and at .005uf the arcs are very weak and small. At .21uf the arcs are bold and long. I know for sure
that the primary and secondary were in tune on both setups.
Bud mohrman
Forrest Mohrman
Engineer
bud@xxxxxxxxxxxxxxxx || 440.647.4316
www.mohrstamping.com
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