Re: High Voltage (fwd)

[William Beaty <billb@eskimo.com>]

On Wed, 24 Sep 1997, Lowell Herr wrote:

> Would anyone be able to send me some answers on the question below. 

I can try...

> 	I have a question I haven't been able to find a satisfactory answer to, 
> but thought you might be able to help.
> I build a Tesla Coil high voltage generator over the summer and want to 
> measure the output voltage.  I have read that a) voltage affects spark length 
> only and b) power output affects spark length.

Voltage is mostly relevant for measurements using a spark gap.  For sparks
like 18" long, you'd need spark gap electrodes with radius of curvature
much greater than 18" (a bit huge and expensive, and they'd load down the
coil output and reduce the potential wrt ground.) 

Once a spark has ignited, the voltage necessary to sustain it is much
lower than 30,000v/inch, so this number doesn't have much meaning.  It
takes a high voltage to ignite a spark, but then the spark can grow to
great to length without a proportional rise in driving voltage.

It does take energy flow to maintain an arc, and the longer the arc, the
more energy per second.  I've seen estimates for long Tesla Coil
discharges ranging between 300w/foot and 1Kw/foot.  I don't know if this
applies to 18"  sparks, I think it's more valid for sparks many feet in
length. 

Here's a possible way to measure voltage.  I've never had a chance to try
it myself, and don't know how accurate it is.  It should give a rough idea
of output voltage.  A probe plate or sphere is mounted on insulators a few
feet from the coil, and a voltage reading is made between the probe and
ground while the coil operates.  To calibrate the probe, disconnect the
top electrode from the tesla coil secondary wire but leave it atop the
coil.  Connect a signal generator to the TC upper electrode and to ground,
and apply a sine wave at the approximately the same frequency as the tesla
coil.  Take a reading at this low voltage.  The oscilloscope will give a
reading that's proportional to the signal generator voltage, and once this
ratio is known, the tesla coil voltage may be calculated from the scope
reading taken during coil operation.  For example, if a 10Vpp signal is
applied to the TC terminal and gives a 10mVpp reading on the scope, then
the scope actually reads 1000 times less than actual voltage.  A 100KVpp
tesla coil output would then be measured as a 100Vpp reading on the scope. 

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