Re: Bipolar tesla coil (Cap Size?)

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Bert Hickman by way of Terry Fritz <twftesla-at-qwest-dot-net>" <bert.hickman-at-aquila-dot-net>

Hi Nolan,

If you used exactly the same primary inductance per primary and the same
secondary and toroid as before, you'd need to cut your primary
capacitance in half to maintain tune. 

However, I'd recommend that you use the same tank cap as before to
maintain the same bang size and reduce expense, cutting the inductance
of each primary in half, and readjusting the coupling of each
primary/secondary pair for best coupling. I'd also add an off-axis
tuning inductor to help tweak tuning (especially if you are going to
move the location of the system often). 

Tuning a twin can be a bit of a challenge, so some instrumentation is
desirable. Some earlier posts covered the techniques. I'm attaching a
portion of an off-list posting to a fellow coiler a while back that you
may also find useful.

You want to tune your system such that each resonator/toroid pair and
the primary circuit all resonate at the same frequency. However, the key
to doing this without pulling out all of your hair is that you must tune
each tuned circuit completely _independently_ of the others, since
coupling between any primary and secondary circuit will confuse your
measurements, and energy coupling between secondaries via the common
secondary base connection ALSO confuses things. The solution is to
ground only one secondary at a time (the one you're tuning), and
physically remove the other secondary so that it's at least 20 feet away
and/or at a right angle to the first one (i.e., lay it in a horizontal
position). The following tuning process assumes that both secondaries
and toroids are physically identical such that they can be easily
"tweaked" to resonate at the same frequency (Fres).

1. First, try to locate your system so that the secondaries are equally
distant from walls or other conductive objects. Place ONE secondary
(with it's attached toroid) in position relative to its primary. Short
circuit the primary tank cap and also connect the primary to ground. Now
measure and record the secondary's resonant frequency by directly
driving the base of the resonator with a signal generator through a pair
of LED's. Tune for the lowest Fres. that gives you the brightest light
from the LED's. (There are other ways to tune, but this is very easy and
it works quite well) and write down the resonant frequency (Fres). 

2. Remove the first secondary and put it far away (>20 feet). Put the
OTHER secondary and toroid in its normal operating position with its
primary. Now measure and tune this secondary so that Fres is very close
to the same frequency as the first one. If you've made the coils
identically, they should already be quite close. Slight tweaks can be
made by raising or lowering the toroid slightly to change the effective
topload C. Make sure that you are at least 10 feet away while making
this measurement so that YOUR self-C doesn't influence the measurement.
If you can't bring the second resonator into tune this way (within a
couple kHz), you may need to remove some turns from this secondary (if
Fres for the 2nd resonator is too low) or from the first resonator (if
Fres of the second one is too high). By "tweaking" one or the other of
the resonators, you should be able to adjust them to the same frequency
while in their normal operating positions relative to their primaries.

3. Once you've tuned the loaded secondaries to a common Fres, remove
BOTH and move them far away from their primaries. You now want to remove
the ground connection from the primaries, disconnect the HV supply and
any RC filter connections to the primary circuit, and short circuit the
main spark gap so that you have a parallel LC circuit with the two
primaries and any tuning inductor connected in series along with all the
heavy cabling that interconnects them. The primaries should be in their
desired positions. Now tune the primary circuit so that it also
resonates at Fres. Ideally, each primary should have the same number of
turns and the same position relative to its secondary so that coupling
and inductances are balanced and the off axis tuning inductor (if used)
should be tapped at about the middle to provide tuning range either way. 

Once you've completed the 3rd step, your system should be in "instrument
tune". You can now reconnect the primary HV source, reinstall the
resonators, connect the secondary bases together (remembering to also
tie them to your common RF ground). Fire up the system at low power
using breakout points if necessary. You may need to do a bit of final
primary "tweaking" under higher power operation to compensate for
streamer loading. Once you've got them in tune, you can then begin
playing with coupling to bring the system up to best performance with no
racing sparks.  

Hope this helps - good luck and best regards,

-- Bert --
-- 
Bert Hickman
Stoneridge Engineering
Coins Shrunk Electromagnetically!
http://www.teslamania-dot-com

Tesla list wrote:
> 
> Original poster: "tmoore by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<tmoore-at-erie-dot-net>
> 
> Hello All,
> 
>     I have been working on changing my mini tesla coil in to a bipolar tesla
> coil the single coil can be seen at http://hot-streamer-dot-com/nolan/ . I was
> wondering if I were to create same exact components like the secondary,
> primary, toroid and the both shared the same spark gap, power supply and
> capacitor. Would it require double the capacitance the same or 1/2 of that
> of the original single coil design, if I were to keep the primary tapped at
> the same spot on both the primary coils, like how I have it while ruining a
> single coil what would it need 0.03uf like in the original or double that.
> Ok thanks a bunch oh and if you got any good tips for a bipolar tesla coil
> it will be appreciated.
> 
> Sincerely,
> Nolan Moore