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RE: capacitor questions!!!
Hi Bob,
You wrote:
>The plans for my old tesla coil provided by Morris and Lee include a chart
>used to select the right sized capacitor, independent of the tank frequency.
>It is a chart that shows the voltage attained across the HV transformer
>and capacitor (with a 15kv neon sign supply) due to _60Hz_ resonance. For
>small variations in the capacitor size, we see the voltage range from
>15 up to 30KV.
>
> 24kv | - -
> | - -
> 21kv | - -
> | - - this is from memory, units
> 18kv | - - are off, I'm sure.
> | - -
> 15kv |- - -
> |
> |
> |______________________________
> .001 .002 .003 .004 uF
>
>One problem I have with this curve is the fact that the slope is still
>increasing with higher capacitance.
>Thanks for any insight. According to this chart, the caps we're buying
>would not work in my circuit, since the neon-sign secondary would surely
>breakdown (even if the caps can take it).
Well, Bob, this is an area I need to study more. I'm glad you brought it up.
I'm not sure I fully understand the figures you are talking about above. First,
when the old instructions were talking about the voltage resonance rising, I
assume they are talking about how it rises around the point of resonance. That
is certainly true. But our tank circuits are resonating at a high frequency,
not at 60 Hz. That is why we have to tune the primary, to resonate it at the
correct frequency to excite the secondary. ( I know you know this already, I'm
just repeating it as I write to get it straight in my mind.) Resonate rise at
60 Hz? Bot, I don't know. I wouldn't think so, but I certainly don't
understand every aspect of these circuits, but I'm trying. :-)
So what does happen as we add capacitance ( or inductance)? The resonate
frequency gets lower. How does that effect resonate rise? I'm not sure myself.
At first glance, it would seem that the voltage rise would simply be at a lower
frequency. Consider this, a larger capacitor should take a longer time to
charge at the same voltage, and hence hold more energy than a smaller one. So
when it pulses, it pulse with more energy, ie. - joules. So, the rushing
electrons are coming out with more force. Does a larger capacitor drive the
resonate voltage higher? Common sense would indicate it would. But, again,
these are microsecond peaks charging a resonate circuit.
One thing is for sure, these peaks can't be allowed to get back to the
transformer, so we build low-pass filters, as you know. That will stop the high
frequency, high voltage from getting back to the transformer.
The only limitation I know of comparing capacitor size to transformer size is
the possibility of not having enough current at a given voltage to charge the
caps. I have heard mention that the main caps should be charged for at least 3
time constants. The higher the voltage, the shorter the time constant and the
more current needed to properly charge the cap. I ran 15000 volts through a
freind's program that does this calculation, and it said a little under 200 mA
is needed to properly charge the cap, assuming a non-rotary gap. ( I don't know
on what he bases this calculation. I'll contact him and find out. ) One thing
all agree on is that is better to have too much current available from your
transformer(s) than not enough. You want it to be able to keep up so you don't
get a voltage drop.
This is an interesting subject. I would like to discuss it more. Your an EE,
right? Well, I'm studying to be one. ( Although I am 34 years old, I figured it
is not too late. I can still breathe. ) Time to break out those old
enginnering text books and start crunching numbers. And what fun it is! ;-)
Please, information and feedback from others on these points would be
appreciated.
>Did anyone ever supply those requested specs (dc rating and shot
>life)?
No, I didn't. But here it is as written by Condenser Products. These are VERY,
VERY conservative ratings.
1. The DC rating by safe allowable voltage stress across margins of capacitor
sections is 60 KV.
2. By standard DC stress level per mil of dielectric is 136 KV. The
approximate hous of life at:
90% reversal 1,200 PPS 500 hours
90% reversal 400 PPS 1500 hours
80% reversal 1,200 PPS 2500 hours
80% reversal 400 PPS 7,500 hours
These figures are based on 15 KV RMS.
Now, Some posted much higher figures on the shot life ratings of these caps. I
don't remember who it was. It could have been Ed Sonderman. Was it you Ed?
The shot life figures are open for interpretation. I am awaiting a call from
Condenser Products now to discuss this matter further. We don't really use it
the way that this test indicates. I'll let you know of my findings.
>I still don't understand the '100% reversal' comment.
The 100% reversal refers to the capacitor being charged 100% full on one side,
being pulsed, and then being charged 100% in the opposite polarity, and then
pulsed. This is typically not the case. With a rotary gap at high speed, the
cap is pulsed many times on one side during the first 180 degrees of the input
waveform. Then it is reversed during the second 180 degrees of the input
waveform. So you get maybe 5-15 pulses in one polarity and then it reverses.
But, again this is true only of rotary gaps.
>I don't even
>understand the significance of the 'pulses per second'. These are running at
>100khz. Does this mean you can't use a static gap? Obviously, I'm still
>missing some theory, so be kind:)
Pulses per second refers to how many times it is charged in the tank circuit by
the spark gap. It is resonating at a higher frequency, such as 100 kHz. With a
rotary gap, it can be pulsed many times per second. With a static gap, it will
depend upon the gap spacing, hence the breakdown voltage ( I think ). The
rotary is the way to go because a more even charging of the cap can be acheived.
It will take some stress off the cap. Instead a few very high current, very
high voltage pulses, a rotary can even out the voltage and current peaks and
valleys, giving better power throughput. Those that use variable speed rotaries
will tell you that as they speed up the rotary, the sparks from the toroid get
longer. This seems to be indicative of the better power throughput of a rotary.
Instead of banging the resonating circuit with just a few ( probably as low as
120, due to input frequency ) charging pulses per second with a static gap, it
can be hit with many more even and smaller pulses ( maybe 500 - 1000 PPS )
allowing the circuit to be energized more efficiently and therefore resonate at
a higher energy level. With the rotary, more of the energy of the input
waveform can be utilized. This doesn't mean you can't use a static gap. I'm
just pointing out the advantage of a rotary. ( Well, I digress here on the
spark gaps. Back to the capacitor. ) Therefore, from many coiler's
observations, the more pulses per second, the longer the discharges.
Comments?
Scott Myers