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Re: [TCML] Spark gap loss

Hi Steve,

Thanks for the link - very interesting paper.

I really didn't address your question of energy loss or efficiency in my earlier post. IN the late 1990's I did a series of experiments on my static spark gap coil. These were designed to measure energy losses during primary-secondary energy transfers.

As you are aware, a full energy transfer from Pri -> Sec (or vice versa) for a SG coil typically takes 2.5 to 4 RF cycles to complete. By setting the coupling coefficient (k) to one of the "magic" values (such as 0.133, 0.153, 0.18, or 0.22), complete Pri -> Sec or Sec -> Pri energy transfers can be done within an integral number of RF half-cycles. Using a magic value avoids stranding some energy in either the primary or secondary, simplifying primary V or I measurements and resulting energy calculations.

k:     RF cycles:   RF Half-cycles (N):
0.133    4               8
0.153    3.5             7
0.18     3               6
0.22     2.5             5

My test coil was set up with a k of 0.18. During testing, I purposely reduced the bang size so that the secondary did NOT break out to prevent streamer losses from introducing errors.

Spark gap coils never "ideally" quench at the first primary notch (single Pri -> Sec) energy transfer), especially when the secondary is prevented from breaking out. So, after all the system energy has been transferred to the secondary, the gap continues to conduct, and energy in the ringing secondary now transfer "backwards", ringing up the primary tank circuit. After another "N" RF half-cycles, all remaining system energy now resides back in the primary circuit. By comparing the initial peak voltage in the primary circuit versus the peak voltage (or current) returned to the primary circuit after a full Pri -> Sec -> Pri energy transfer, I was able to calculate the total energy lost during a round-trip cycle. A wideband current transformer could used to measure primary current to make similar energy calculations.

For my coil, the portion of system energy lost when making a primary to secondary energy transfer (from ALL causes) was about 15%. Most of the losses likely came from the spark gaps, since my system used a series vacuum gap with 8-12 static gaps. Because of the large number of gaps, my losses may be higher than for a typical rotary gap. If you have a HV divider or wideband CT to measure primary capacitor voltage or tank circuit current and an oscilloscope, you can duplicate these measurements for your system.

Although my coil transferred 85% of primary bang energy to the secondary, a well designed SG coil with fewer gaps might hit 90%, perhaps more...


Steve White wrote:
I was reading a thesis that studied spark gap losses recently. Although the test apparatus (electrode material, diameter, and gap spacing) does not match exactly what you would find in a typical tesla coil, I found the results very interesting. The closest to the tesla coil scenario was the following from the paper.

1. Electrode material: copper-tungsten
2. Electrode diameter: 2.5 cm
3. Gap spacing: 1.4 cm
4. Trigger voltage: ~30K volts
5. Trigger energy: ~1K joule
6. Air at 1 atmosphere

The test results at these conditions measured an energy loss of about 7%. If I extrapolate these results to my 4800 watt coil, I am losing about 336 watts in the rotary spark gap. This is less than I imagined since I have always been lead to believe that the spark gap was very lossy.

Does anyone else have any other data which shows the loss caused by a spark gap?
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