[Prev][Next][Index][Thread]

CAPS & SPARK GAPS

 * Original msg to: Esondrmn-at-aol-dot-com


 Es> Richard,
 
 ES> Why does the spark gap not want to fire with more capacitance 
 ES> in the circuit? 
 
 RQ> The larger capacitance does not fully charge up with too small of a
   > power supply feeding it. Since the cap is never allowed to fully
   > charge, the voltage peak in the capacitor is not large enough to
   > jump the gap. 

 ES> Are you saying that the 12kv 120ma power supply cannot supply current
 ES> fast enough?  Normally with one tank cap (approx. .018mfd) and one
 ES> cylindrical spark gap and 12kv 120ma, the gap starts to fire at about
 ES> 50% power.  With two caps in parallel (wish I had two:-) ) will I need
 ES> 240ma to still get it to fire at 50%?

A 12kv 120ma 60 cycle power supply will effectively charge a .026 uF 
capacitor, so you should have no problem charging a .018 uF cap. Other
factors may be brought into play: neons frequently fail slowly before
giving up the ghost entirely. It is not uncommon for a secondary winding
output on a neon to start dropping off, then go dead entirely. Irregular 
gap performance is a symptom of neon degradation, whether it is one 
transmformer alone, or one winding in a bank of transformers.

 ES> This brings up another point - these cylindrical gaps seem to be
 ES> somewhat fickle. Sometimes they will fire at 30% power and sometimes
 ES> 75% power with no circuit changes. Last night I was running at 50 to
 ES> 75% power (trying to get some run time on my last cap) and noticed
 ES> this.  

The gap electrodes can't move, so something else must be fickle. 
Pull your neons out of the bank one at a time, and test the outputs 
by drawing an arc from the HV bushing to the case on every secondary 
winding. Look for a weak or sputtering output from one or more windings. 

 ES> You said "to save your capacitors until they are truly broken in I
 ES> would drop down to 9kv neons. You will find it very difficult to 
 ES> blow a homemade rolled cap of this design with a 9 or 10 kv input".  
 ES> Are you saying that I could apply 9 or 10kv continuous (2 or 3 
 ES> minutes at a time) now without blowing my last good cap? At what 
 ES> power level? Could I apply 200ma now (2kva)? 

Yes, these caps will hold up at 9kv without much problem if you do not
go out and deliberately over stress them. Don't run with the main system
spark gap opened up all of the way, and don't turn the coil on and leave
for coffee. At 9kv the capacitor will fully charge with less current.
What this means is that you are limited in the power that can be processed
by the capacitor. At 9000 volts, 120ma will fully charge a .036 uF cap
so you are pretty much limited to 1 kVA. 9000 volts -at- 60 ma will charge
a .018 uF capacitor.

 ES> If I run at 10kv for a month, will the cap really be ok to run at 
 ES> 12kv after that?

You know that these capacitors will hold up to 12 kv right now.
You have run these caps at 12 kv for a month, and did not encounter a
dielectric failure until you pushed your peak power and duty cycle.
You have to use limited duty cycles and limit the amount of spark
gap you run with for about six months if you feed your coil 12kv. 
Even after fully broken in: I still limit the peak powers produced 
from these units (by using less gap), or I limit the duty cycle (only 
for short run times with long rest periods). If I want big bang peak 
powers (in excess of half a megawatt) I run four units or more (I own 
14 of these capacitors) in series/parallel.

You know these caps work. You just have to baby them over 10kv, or
build more and run them in series/parallel where you can say "damn
the voltage!".

Richard Quick
... If all else fails... Throw another megavolt across it!
___ Blue Wave/QWK v2.12