# Re: LTR confusion...

```Hi Scott,

LTR caps are very carefully chosen so that the given transformers are just
barely able to get them to full firing voltage.  Too much cap, and the
voltage will be lower.  Too little, and something my go overvoltage and
energy is wasted.

Finding the perfect value is a pretty complex trial and error computer
simulation thing.  However, having done it all many times, there seems to
be a simple relation:

Set the synchronous rotary gap to fire 3.5 mS late.  I use a strobe light
and have my motor marked of in mS late dwell but trial and error turning of
the motor does fine too.  Simply set it for the longest arcs.  Some people
have gotten static gaps like your RQ gap to work too but they really should
use synchronous rotary gaps IMHO.  Perhaps those that use static gaps could

The cap value can be found with the follow simple formula:

C = 0.46nF x A

C = The optimal LTR cap value

A = The available current from the neons in mA.

Note that the voltage makes no difference.  Only the supply current
determines the needed cap size.

So, in your case with 120mA (four 30mA transformers) you get...

0.46 x 120 = 55.2nF

Perhaps a nice MMC would fill in the rest of the needed capacitance.

It will fire at about 21kV so with a 55.2nF cap at 120hZ you will be
pushing a nice 1460 watts into your primary cap and about 750 watts into
the arc (I think that is a very good indicator of a coils power).  I think
you will get about 65+ inch arcs from that.  Plan on this being a fairly
powerful coil.  Definitely not a tabletop machine ;-))  My single 15/60 LTR
coil stands four feet high but the arcs can still come around and hit the
floor...

I worry that your 20 inch high secondary and little toroid may be too small
for arcs that long.  You may just keep striking down the to primary.  Try
to keep the toroid as high as possible above the secondary to get more
distance from the terminal to the primary.  Be sure to use safety gaps that
are properly set.  LTR coils are very good at handling secondary to primary
arcs but the grounding must be good and safety gaps are essential.  You may
want to run the numbers in WinTesla and all just to get a good idea where
the primary needs to be taped and all that.  Sometimes a normal coil needs
some changes to handle the higher power and large primary capacitor.  You
will be delivering a surprising amount of power into your relatively small
coil.  Perhaps you should redesign it to be more like a small pig system
coil (bigger) since that is getting near the power level this system will
be running.  You are about double the expected power in an LTR system so
imagine it is a 3.6KVA (240mA at 15kV) machine.

400mF of PFC caps will drastically reduce the input current into this
system too so you can run it off a 20 amp breaker (~18 amps).

Cheers,

Terry

At 06:51 AM 01/08/2000 -0600, you wrote:
>Hi all...
>
>
>Im getting ready to make another coil but this time im going to try to
>squeeze everything I can out of it ...
>
>my main question is regarding LTR ... is this the value of the main cap
>that is to be larger than what the power supply delivers?
>
>Im using 4-at- 15KV/30mA units....  calcs for this assy call for a .0212 mF
>cap. The cap available to me is .026mF  -at-40KVDC ( ive used this cap with
>my pole pig unit so I know it can handle the load)
>
>Is this cap value good enuf for a LTR situation? or do I need more mF?
>
>the secondary will be 8"X20" with a 6X18 Toroid  and an RQ gap set fan
>cooled primary will be flat with 1/4" Cu tubing at 1/4" spacing   around
>14 turns.
>
>