Re: Hot Primary

["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 Gregg,

I did a quick estimate for primary current in your system, assuming your
main gap is firing at 12 kV. Your primary inductance is around 3.2-3.3
uH, and your bang size is about 3.6 Joules. This means that your peak
primary current may be as high as 1400 - 1500 amps! Your tank circuit is
also operating at a relatively high frequency (~350-400 kHz), so the
skin depth of the copper tubing in your primary winding is only a bit
over 4 mils. The above combination certainly explains why your TC
primary gets hot. 

Some suggestions:
================
1. Replace your existing secondary a longer one using a finer wire
gauge. This will increase its inductance and voltage handling
capability, while also lowering its operating frequency. [Even better,
consider using a larger diameter coilform to further increase
inductance, power handling capability, and reduce operating frequency]. 

2. Combine the new secondary with a toroid that has a relatively large
outer diameter for additional topload C. For example, if you made a
4.25" diameter secondary winding 28" long using 24 AWG, and topped it
with a 22" x 5" toroid, the combination would drop your secondary
frequency to about half its present value. The longer secondary also
makes it easier to handle more power or longer sparks.  

3. The previous changes will now permit you double the number of turns
on the primary, halving your peak primary current (which, at 700 amps,
is STILL fairly high for a coil this size). BTW, making the same changes
but using a 6" diameter coilform will drop your secondary frequency to
around 130-140 kHz, permitting you to tune at 8 primary turns, and
reducing your peak primary current to a more reasonable 500 amps. 

4. While most of your losses are still coming from the primary spark
gap, you may also want to consider migrating to 3/8" copper tubing to
further reduce primary losses. 

Hope this helped and best regards,

-- Bert --
-- 
Bert Hickman
Stoneridge Engineering
"Electromagically" Shrunken Coins!
http://www.teslamania-dot-com

Tesla list wrote:
> 
> Original poster: "Gregg Adams by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<network-at-nexband-dot-com>
> 
> To all who responded to my question, thanks and here's the specs on my coil:
> 
> Secondary: 4.25" / 22 gauge / 22 inches in length
> Primary:  flat sprial / .25inch copper tubing / tapped at 3rd round out
> Power supply:  dual MOT's with voltage doubler(12kv) / not sure on the
> amprege
> Tank cap:  .05uf -at- 12kv / 2 strings of 6 geek caps(.15uf -at- 2kv) I have more
> just not hooked up.
> Spark gap:  ARSG / 10k rpm angle grinder w/6 inch disc and 4 contacts
> Performance: 40+ inches
> 
> My primary is getting hot enough that you can't leave your hand on it for
> more than a few seconds.  Also, I'm having to run alot of topload now.
> 
<SNIP>