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re: 20 joules at 100 bps vs 4 joules at 500 bps



Original poster: FutureT@xxxxxxx

repost of my bps comparisons posting from
Date: Mon, 22 Feb 1999 18:01:18 -0700
All,

I did some tests at a higher power level. First I installed a .0148uFcap with the 120 bps sync gap, then compared with the 400 bps rotary gap. At 1370 watts, the spark reached 54" at 120 bps, but at 400 bps, the spark could not reach 54" even at 2000 watts. But it came close. However, the spark was brighter and more flamelike at the high break-rate, and wanted to go upwards towards the ceiling. The ceiling is 36" above the top of the toroid.The sparks looked completely different at 120 bps.At this low break rate, the sparks were not as bright, but they tended to surge and grow, and emitted from different spots around the toroid. The sparks floated around slowly. The high break-rate sparks tended to be all about the same length and,seemed to want to remain together like a giant blow-torch or flame, and reminded me a little of tube coil sparks(high powered tube coil sparks of course). So all of this was the same as at the lower power level, but on a larger scale. I tried a higher break rate of about 600 bps, but the sparks got shorter, due to sagging voltage in the power supply. Of course,in any case, the voltage on the caps was always lower at the highbreak-rates, because the system was unable to benefit from resonant charging voltage build up. Next I replaced the 1500 watt 14.4kV potential transformer with (2)1500 watt 7.2kV potential transformers in series for a "stiffer" power supply. With this set-up I got 54" sparks at 1000 watts at 120 bps.At 400 bps, the sparks were very bright, but never reached 54" but they hit the ceiling a lot and seemed more powerful than with the one transformer.Then I replaced the 20" toroid with a 6" by 26" dryer duct toroid, This arrangement gave 58" sparks at 1050 watts, this I believe is a record for efficiency for my coils, based on my formula:spark length (inches) = 1.7*sqrt input power, wallplug (watts).The formula predicts 55", but this coil gave 58" sparks. I attribute the greater efficiency to the use of two transformers, which reduces the losses.Note; those with NST powered systems are cautioned to never use the NST power rating written on the NST in my formula above. Many NST systems use resonant charging and actually draw double or more the NST rated power input. The only way to know the input power of any Tesla coil is to measure it with a suitable wattmeter.If a wattmeter is not available, at least use an ammeter, and multiply by the input voltage to obtain input VA (volt-amps). This will not correct for the power factor error, but will give a ballpark measurement. In my coils, input current distortions and/or high frequency components did not seem to affect the meter accuracyby much. My formula above uses true input wallplug power. I measure the power before any variacs, ballasts, etc. I then tried using 400 bps, but the sparks jumped to the ceiling too often and would not reach 58", so this made it impossible to properly measure them accurately, although I did get a general idea. At 600 bps, the sparks did not get shorter, due to the use of the stiffer power supply. The sparks seemed bright and strong at these higher break-rates.I used the same wattmeter as my tests from last weekend, which should give reasonably accurate results.Table of results:

Cap (uF)     bps     watts      toroid (inches)     spark (inches)

.007         120      620                5 x 20       42

.007         400      1000               5 x 20       43

.0148        120      1370               5 x 20       54

.0148        400      2200               5 x 20       less than 54

.0148        600      2200               5 x 20       weaker

.0148        400      3000               5 x 20       59 one time

.0148        120      1000 (2) xfrmers   5 x 20       54

.0148        120      1050 (2) xfrmers   6 x 26       58

.0148 400 2000 (2) xfmers 6 x 26 went to ceilingbut approached 58

.0148        600      2000 (2) xfrmers   6 x 26       not weaker

.0148        400      1000 (2) xfrmers   6 x 26       36 (low cap Volts)

.0148        400      1050 (2) xfrmers   5 x 20       41

.007         400      1000 (1) xfrmer    3 x 10       30

In the second to last test above, I of course had to keep the variac turned down somewhat to hold the power level at 1050 watts, so the bang size was probably the same as in the last test, so it'sreally the larger toroid that is increasing the spark length in the second to last test. It's hard to know how much voltage was onthe caps in any of these tests.For a given input power, the low break-rates seem to outperform the high break-rates by a significant margin in all these tests, as far as input power vs. longest spark length is concerned.I found that at high break-rates, I had to use a much smaller ballast inductance than at 120 bps. At high break-rates, I used about 2mH to 10mH. At 120 bps, I used about 15mH to 25mH.The ballast was adjusted for best TC efficiency, and smoothes operation in all cases. High break-rate systems have greater losses due to:1. . I do not know how much of the highbreak-rate under-performance results from losses, and how much may be inherent to high break-rates (if any). I would not be surprised if the losses at high break-rates are much lower(relatively speaking) in large coils compared to small ones.It is possible that by using 120 bps, and scaling up a TC in size,that the spark lengths might outperform the predicted lengths given by my formula. Let's see....around .03uF should begood for a 100" spark, at 120 bps, at 2800 watts, using a 5kVArated 14.4kV pole pig for low loss. This is assuming the sparklength can outperform my formula. A 7.5" x 35" toroid will be needed. This might be too optimistic but would be interesting to try. John Freau