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4hv.org :: Forums :: Tesla Coils
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REAL votage of a tesla coil? One for Mr Ward or Mr Connor I think!

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plazmatron
Fri Feb 19 2010, 06:23PM Print
plazmatron Registered Member #1134 Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351
I'm curious, when guestimating DC EHT, the rule of thumb is around 1kV per mm of park length.

What rule applies to guestimating TC voltage?

I have a couple of small DRSSTC`s kicking about, and I find it difficult to believe that 30cm of spark is really 300+ kV !

Thanks,
Les
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dex
Fri Feb 19 2010, 06:49PM
dex Registered Member #2566 Joined: Wed Dec 23 2009, 05:52PM
Location:
Posts: 147
The "Rule of tumb 10 kV/cm" applies, very roughly,only to a negative DC spark-over distance in very nonuniform gaps (like needle-to plane).
For positive DC sparks I think it is twice as less (~5kv/cm).But coil sparks have alternate high frequency nature.The biggest problem is the spark growth phenomenon over consecutive shots. It is very tricky to estimate voltage by spark lenght even in a single shot of a Tesla coil.
Better thing you can do is to measure bottom current of the resonator and aproximately calculate top volts from Ls and operating frequency .
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Steve Conner
Fri Feb 19 2010, 09:00PM
Steve Conner Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Plazmatron & dex, you're right, it's hard to estimate the voltage of a Tesla coil. Measuring base current as suggested would work.

Also, if you have a toroid with a smooth surface, and the coil manages to break out from it without a breakout point, you know it must be producing a field of about 30kV/cm at the surface, so you can calculate the voltage from that. You just multiply by the toroid's minor radius. This isn't particularly accurate either, though. Little imperfections in the surface can lower the breakout voltage quite a bit.

But you know it's never going to get much above 30kV/cm either. If you tried, more and more streamers would appear until the toroid was a solid mass of fire, so it's an upper bound too.

And some DRSSTCs will just blow up or burn if you try to run them without a breakout point.

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HV Enthusiast
Fri Feb 19 2010, 09:11PM
HV Enthusiast Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
The real voltage of a Tesla coil is MUCH MUCH less than you think it is for all the reasons stated above.
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dex
Sat Feb 20 2010, 08:58PM
dex Registered Member #2566 Joined: Wed Dec 23 2009, 05:52PM
Location:
Posts: 147
Steve McConner wrote ...

Measuring base current as suggested would work.
Actually this I found the best way of determining top volts without direct voltage measurement (which is very problematic in the case of Tesla coil due to loading effects of the equipment).From steady state AC circuits analysis we know that:

Vm = 2*Ï€*I*f*L = 6,28*I*f*L

L is a low frequency secondary inductance,I secondary base current,f resonant frequency.

SGTC or DRSSTC currents, are not in a steady state regime.They are transients.Good news is that for systems with coupling k<0,2,simulations with RLC lumped elements show that the error of the simple formula above is less than 2 %!
However,here comes the problem.Due to distributed nature of secondary coil,current is not uniform from the base to the top.For bare coils (no top load) with ratios 3<H/D<5 the error of this method be as large as 20-30 %.Salvation comes in the form of fact that secondary coil is terminated by relatively large capacitive top load.If capacity of top load exceeds Ces of bare coil than the error of the formula is less than 6%.
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