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Cap choice sims for static gaps




After posting my 200BPS rotary gap simulation results a couple of days
ago I am now presenting the results of my static gap capacitor size
simulations.  I have been following the 2 Questions on Resonance thread
and am very interested.

Here are some simulation results which show the effect of using different
sized tank capacitors with a STATIC SPARK GAP.  First the gap is set
to a low firing voltage which would minimise resonant voltage rise.

"Neon sign transformer -
 10kV Static gap, capacitor selection"

Actual measured Neon Xfmr parameters:	6Kv / 125mA
50Hz resonant cap:			66.3nF

Simulation results:
	C = Capacitor value in nF
	V = Capacitor firing voltage
	P = Power throughput (= 200 * 0.5 * C * V * V)
	VA= Mains supply VA
	pf= Power factor (P/VA,  kind of like a measure of efficiency)


C [nF]	BPS	V [v]	P [W]	VA	pf

16	583	10000	466	809	0.576
20	468	10000	468	832	0.563
22	452	10000	497	872	0.570
24	424	10000	509	886	0.574
28	366	10000	512	957	0.535
30	342	10000	513	976	0.526
32	318	10000	509	966	0.527
34	302	10000	513	1011	0.507
36	298	10000	536	1009	0.531
38	268	10000	509	1025	0.496
40	256	10000	512	1044	0.490
42	240	10000	504	1038	0.486
44	230	10000	506	1074	0.471
48	210	10000	504	1113	0.453
52	190	10000	494	1063	0.465
56	198	10000	554	1262	0.439	<----- Only fired on neg
cycles !
60	160	10000	480	1120	0.429
64	142	10000	454	1193	0.381
68	132	10000	449	1180	0.381
72	130	10000	468	1293	0.362
76	106	10000	403	1241	0.325
80	104	10000	416	1081	0.385
90	96	10000	432	1235	0.350	<----- Only fired on pos
cycles !
100	68	10000	340	1298	0.262	<----- Missed several half
cycles
120	49	10000	294	1224	0.240	<----- Missed several full
cycles
130	-	-	0	1506	0	<----- Gap would not fire

Conclusions can be drawn from these results more easily if each parameter
is plotted on a seperate graph against capacitor value on the X-axis.

It is easy to see why people beleive that a large capacitor allows more
power to be got from a neon transformer.  Compare the results for both
a 20nF capacitor and a 72nF capacitor.  Both capacitors charge to the
same voltage (determined by the static gap,)  but the firing rates are
obviously different.  The point of interest is that the simulation
indicates identical power throughput (Bang energy*BPS)  however the
supply VA is 55% higher for the 72nF capacitor !

Some people will ask "Where did the extra 55% of power go ?"
Well, it is not real power but reactive power.  Although there would be a
55% increase in current from the "wall socket" the power delivered to
the sparks remains the same.  It is merely the power factor which is
made worse.  (The system is "less efficient".)

Also for those who beleive it is not possible to exceed the face plate
rating with a static gap,  I would say YES as far as real power
throughput is concerned provided the gap is set small.  Nowhere in this
wide range of capacitor values does a point occur where the full 750W
faceplate rating of power is processed. HOWEVER, if you only measure
the current in the power cord then the VA product will almost always
exceed the faceplate rating !

During my simulations of the static gap,  I noticed several things:-

1. The gap always seems to fire quite chaotically.  Sometimes a half
   cycle will be skipped,  and it will fire more times in the next half
   cycle.  This must be down to the behaviour of the circuit and not
   randomness in the spark gap breakdown voltage as I have always
   believed.  (This spark gap is ideal and fires at exactly 10,000 Volts.)

2. With certain capacitor values I was able to get the simulated gap to
   fire totally on positive cycles, (see comments to right of results
   above.)  I believe this is down to the "perfect world" nature of a
   simulation,  and a bit of real world randomness would probably stop
   this from occuring.  (But maybe not ?)

3. The randomness of the gap firing over many cycles makes the supply
   cord current fluctuate by a small amount over time.

4. The power factor is always much lower than I found during my 200BPS
   sync rotary simulations.  This goes along with peoples comments about
   supply current dropping when a rotary is installed.


I then repeated these tests with a higher firing voltage for the static
gap:

Vg= 25.3 kV

C [nF]	BPS	V [v]	P [W]	VA	pf

32	105	25300	1075	1497	0.718

I have omitted the rest of the results to keep this already too long
post down to size.  However it can be seen that the firing rate for
this 32nF capacitor is lower than before but it charges to a much higher
voltage.  The power processed now does exceed the faceplate rating by
over 40% but the transformer is destined for a quick demise due to the
300% overvoltage and 200% over current of its windings !!!
(Notice the power factor has actually gone up too,  but it never comes
 close to that of the sync rotary.)

So what does this prove.  Well,  that matched capacitors are not needed
for best performance on static gap systems,  and that wider gap settings
produce better performance at the expense of irregular firing and risk
of overvolting components.   Big capacitors draw more current but the
simulation suggests that no more real power is processed.  Maybe choose
a small cap and get power from the higher break rate.

It seems to me that static gaps and rotary gaps are indeed two totally
different ball games.

PS.  If this post is too offbeat,  not appropriate or just plain WRONG
     then don't hesitate to shut me up.


				- More simulation results from

					Richie in sunny Newcastle.