Author Topic: How much power?  (Read 2272 times)

Offline Terry

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How much power?
« on: January 30, 2024, 08:07:39 PM »
I'm looking to shoot 16 to 24 inch arcs in random directions from the top load continuously, and am fairly convinced that a spark gap build is the way to go. My first step is to build the power supply, but I don't know how big it needs to be.

The OBIT says to deliver 6KV and 35mA (RMS) continuous. Can I expect to need 1,2, or 10 of these in parallel to meet the objective?

Is the popular dual MOT rig enough? Can I short circuit protect it with a dimmer switch?

Thanks in advance.

Offline Mads Barnkob

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Re: How much power?
« Reply #1 on: January 30, 2024, 10:17:45 PM »
I think it was John Freu that suggested a spark length formula, for well designed spark gap tesla coils.

Spark length in inches and power in Watt.

P = (L / 1.7)^2

L = 1.7 * SQRT(P)
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Offline MRMILSTAR

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Re: How much power?
« Reply #2 on: January 31, 2024, 05:08:10 AM »
The John Freau equation that Mads cited is a commonly used one. Another way to get an estimate is to use JAVATC to design a hypothetical SGTC. One of its outputs is an estimate of maximum streamer length.
Steve White
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Retired electrical engineer

Offline Hysteresis

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Re: How much power?
« Reply #3 on: January 31, 2024, 03:34:48 PM »
One of my Tesla Coils produces streamers 16-24 inches. That coil uses a 15KV at 60ma NST.

Offline klugesmith

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Re: How much power?
« Reply #4 on: January 31, 2024, 11:43:32 PM »
Two posts above demonstrate a familiar misreading of OBIT and NST nameplate values.
For those internally-ballasted transformers, nameplate gives Open Circuit kV and Short Circuit mA.
In normal steady state operation, the current is close to nameplate value and the voltage is less than 1/10 of nameplate value.

For resistive loads, the maximum output power is reached with about 2/3 of nameplate voltage "at" about 2/3 of nameplate current.
On NST reported by Hysteresis, that reduces the maximum V*I product from 900 W to 400 W.   
Freau formula gives 51 inches and 34 inches for those powers.

[edit] Here's a report from 4hv forum, characterizing a 15-60 NST with resistive loads.  I got up to 425 watts out with nominal primary voltage.
https://4hv.org/e107_plugins/forum/forum_viewtopic.php?p=1&id=182849
That was near end of calendar year 2018, as 4hv orbital decay approached the critical point. Wow, it's been 5 years since then!
« Last Edit: February 01, 2024, 12:15:51 AM by klugesmith »

Offline Mads Barnkob

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Re: How much power?
« Reply #5 on: February 01, 2024, 07:53:53 AM »
Two posts above demonstrate a familiar misreading of OBIT and NST nameplate values.
For those internally-ballasted transformers, nameplate gives Open Circuit kV and Short Circuit mA.
In normal steady state operation, the current is close to nameplate value and the voltage is less than 1/10 of nameplate value.

For resistive loads, the maximum output power is reached with about 2/3 of nameplate voltage "at" about 2/3 of nameplate current.
On NST reported by Hysteresis, that reduces the maximum V*I product from 900 W to 400 W.   
Freau formula gives 51 inches and 34 inches for those powers.

[edit] Here's a report from 4hv forum, characterizing a 15-60 NST with resistive loads.  I got up to 425 watts out with nominal primary voltage.
https://4hv.org/e107_plugins/forum/forum_viewtopic.php?p=1&id=182849
That was near end of calendar year 2018, as 4hv orbital decay approached the critical point. Wow, it's been 5 years since then!

Time flies fast! HVF is from March 2017 :)

Do you still have the original files and could possible repost it here? All there is left on 4hv is the thumbnails, cant see the large pictures.
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Offline klugesmith

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Re: How much power?
« Reply #6 on: February 01, 2024, 07:03:41 PM »
There's not time this morning to re-post the whole story of NST I-V behavior. 
Here's the chart for nominal 15 kV 60 mA transformer with 277 volt primary.
THere are data points for primary voltages of 70, 140, 210, 277, and 290 volts.
The finite load cases are four different resistors, and series string of 120-V Christmas lights, and some luminous tube element.
Yellow curve represents constant power of 425 watts.

« Last Edit: February 01, 2024, 07:09:22 PM by klugesmith »

Offline Terry

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Re: How much power?
« Reply #7 on: February 06, 2024, 10:51:35 PM »
You guys are awesome, Thanks! I've got a good feel for it now.

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Re: How much power?
« Reply #7 on: February 06, 2024, 10:51:35 PM »

 


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