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DRSSTC peak powers and spark length

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Dr. Dark Current

Sat Aug 11 2012, 03:50PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Hi,
what peak power should I expect the tank circuit of a standard interrupted DRSSTC to draw to reach eg. 50cm or 1m sparks? Just need a ballpark figure... I'm mostly experienced with CW/QCW coils but I think a pulsed DR will need higher peak powers to reach the same spark length. Thanks for sharing your measurements.

Goodchild

Sat Aug 11 2012, 04:17PM

Registered Member #2292 Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795

Well that widely depends on your tank impedance and drive time. You can drive for say 200u or 2500u and have low peak or drive for only like 2 or 3 cycles and have higher.

For an average DR I would say about 250A to 300A

Mads Barnkob

Sat Aug 11 2012, 04:23PM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

I have gathered some data on different DRSSTCs and on a graph that delves to about 300-400A for 1 meter spark.

Dr. Dark Current

Sat Aug 11 2012, 04:24PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

So you can get longer sparks if you drive the same amount of power for a longer time? I thought they only get brighter...
The current figures you said are for 320 volts output from the bridge (H-bridge on rectified 230V)? Or was it a different voltage?

Mads Barnkob

Sat Aug 11 2012, 04:39PM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

Dr. Dark Current wrote ...

So you can get longer sparks if you drive the same amount of power for a longer time? I thought they only get brighter...
The current figures you said are for 320 volts output from the bridge (H-bridge on rectified 230V)? Or was it a different voltage?

Thereabout 320VDC.

Sparks also get brighter at longer ontimes, but they certainly also grow in length. Primary capacitance size has a larger role in arc thickness as this is were the energy released for each strike comes from.

Thomas W

Sat Aug 11 2012, 05:50PM

Registered Member #3324 Joined: Sun Oct 17 2010, 06:57PM
Location:
Posts: 1276

Mads, have you got this Graph, it would be interesting to see how it predictes untried power levels, going beyond what DRSSTCs have been able to peak currently,

~~Thomas~~

Mads Barnkob

Sat Aug 11 2012, 06:31PM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

__=|(:3)-|--{__ wrote ...

Mads, have you got this Graph, it would be interesting to see how it predictes untried power levels, going beyond what DRSSTCs have been able to peak currently,

~~Thomas~~

I still think of it as incomplete and it does not predict anything ahead as the upper limit is the largest coil plotted :) I will share it some day, but not now.

I suggest you look at Gigantor for a absolute limit of what amateurs can make :)

Ben Solon

Sat Aug 11 2012, 07:36PM

Registered Member #3900 Joined: Thu May 19 2011, 08:28PM
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Posts: 600

Limit? Naw

just whats reasonably feasible.

Goodchild

Sun Aug 12 2012, 08:18AM

Registered Member #2292 Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795

Mads Barnkob wrote ...

Primary capacitance size has a larger role in arc thickness as this is were the energy released for each strike comes from.

You know this is interesting that you bring this up because I'm not convinced that is true. I have built both very low impedance DRs and very high impedance DRs and I don't find that to be the case. With regular DRs I notice no difference in streamer thickness vs the size of the tank cap.

Furthermore if this was true this means a very very small tank cap (say maybe 10nF) should make very weak thin sparks, this can't be true as shown by QCW operation. With only 10nF you get sparks thicker than a regular DR that has a tank cap an order of magnitude larger.

I'm curios to what lead you to your conclusion?

Steve Conner

Sun Aug 12 2012, 09:42AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

My approach has always been to assume a burst length between 150 and 300us, as Steve Ward and I managed to convince ourselves that this was some sort of sweet spot for efficient spark growth.

When designing a DRSSTC for a particular spark length, I use John Freau's equation to go from desired spark length to average power, and since I assumed a burst length I can calculate what peak power I need to achieve that average power. The peak power then dictates the design of the inverter and tank circuit. (you can also estimate the RMS tank current at this stage)

I define peak power as bang energy divided by burst length. Mjollnir has a peak power of about 50kW in that sense. A marketing guy would say DC bus voltage * peak output current, which would be 70kW.

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