Measuring electric voltage in a Tesla coil

[omrid13]

Hello everyone
After building a relatively small Tesla coil I would like to know:
1. Is there a way to measure the output voltage?
2. What affects the output voltage more. The length of the pulse or the frequency of the pulses?
Thanks

[flyingperson23]

As far as I understand, secondary voltage is relatively unimportant. You can approximate it with a simulation or arc length in CW mode which would kill most coils. In normal drsstc pulsed operation, arc length is longer than a continuous supply of the same voltage would create. I don't think voltage has much relation to frequency, but primary voltage definitely increases with increased pulse length.

[omrid13]

So more pulse width gives more voltage in the primary coil which gives more voltage in the secondary coil and causes more voltage at the output?

[flyingperson23]

Sorry it looks like I was mistaken, what I was thinking of was Zpri*Iocd = Vmax across the MMC. Secondary voltage is I believe a little complicated due to it being its own resonator only weakly coupled to the primary. If you don't have a breakout point, for example, the secondary voltage will ramp up higher before breaking out as the breakout point gives much higher field for the same voltage. If you want long arcs, a high current (low on time, low primary impedance, big mmc) design is probably more effecient; the opposite of that would be better at playing music.

[davekni]

1. Is there a way to measure the output voltage?

There are hard ways to get somewhat accurate at least for the beginning as breakout starts.  Hydron placed a scope on top of the top load to measure charge:
    https://highvoltageforum.net/index.php?topic=117.0#msg780
I've done somewhat similar experiments using home-made fiber-isolated scope probes:
    https://highvoltageforum.net/index.php?topic=1263.msg9259#msg9259
    https://highvoltageforum.net/index.php?topic=1950.msg14588#msg14588
However, you can get reasonably close by measuring current in ground lead to bottom of secondary.  If top load is reasonably large, electric field will be somewhat uniform down secondary, so secondary winding capacitance is a small contribution compared to top load capacitance.  Current in secondary (amplitude and frequency) and secondary inductance are enough to calculate voltage.

Measurement will be even more accurate if a scaled version of primary current is added to secondary current measurement.  Scale factor is mutual_inductance / secondary_inductance.  This adjusts for voltage created by transformer action from primary to secondary.  Inductive drop across secondary due to secondary current is generally a lot larger, but adding in scaled primary current may change result by 10% or even 20%.  Resulting current (from which voltage is calculated) may be higher or lower depending on relative phase of primary and secondary currents.

One way to add in scaled primary current:  Add a third primary CT output stage (or entire third CT) with its own adjustable burden resistor (or fixed burden resistor followed by POT to reduce voltage).  Wire this CT output in series with secondary CT output.  (Secondary CT of course has its own burden resistor.)  To adjust scale factor (POT on primary current):  Run coil with secondary shorted (top load wired to ground).  Adjust POT until sum of the two CT outputs is zero.  Swap polarity of one CT if zero can't be reached.  Will never be exactly zero due to phase shift in CTs, but should be closer than secondary current alone.

2. What affects the output voltage more. The length of the pulse or the frequency of the pulses?

For typical DRSSTCs, top voltage drops as arc grows.  Arc lowers secondary Q and makes a better breakout point.  Highest voltage is when breakout starts.  Voltage will be higher with shorter or no breakout point.  Fast voltage ramp also increases peak voltage.
Frequency also matters.  Higher frequency reduces breakout voltage.  However, high frequency and low voltage can make long arcs.  My normal DRSSTC at 80kHz and ~600kV initial peak voltage makes ~2m arcs when tuned for music, 3m when optimized for arc length.  My 450kHz QCW coil is about 40kV peak and makes arcs in the 2 to 2.5m range, perhaps slightly longer when I push the pulse width.  For QCW coils, voltage may increase slightly as arc grows and more voltage is needed to overcome arc resistance.

[Mads Barnkob]

Someone, many years back, made a dual antenna with amplifier, to measure the voltage field of a topload. But I can for the love of god not find it, I guess the homepage is long gone and the knowledge is lost

It was made on a PCB and looked like this, black is traces and red were coax cables.

[Uspring]

One way to add in scaled primary current:  Add a third primary CT output stage (or entire third CT) with its own adjustable burden resistor (or fixed burden resistor followed by POT to reduce voltage)...

That's a very neat way to measure the top voltage. You can even measure the arc current when you scope primary and secondary current:

I_arc = I_sec + L_sec*C*d²I_sec/dt² + M*C*d²I_pri/dt²

L_sec, M and C are the secondary inductance, the mutual inductance and the top load capacitance respectively.
The main problem is, that the first 2 terms are large and almost cancel each other, so that the result is susceptible to measurement errors when the arc current is small. But the measurement is much simpler than having electronics on the top load.

[omrid13]

Thanks for all the answers.
Is there a recommended pulse width and frequency in order to get the maximum voltage from my Tesla? Is there a recommended controller? Most of them are that I play too much with these parameters, I burn the MOFST

[Mads Barnkob]

Someone, many years back, made a dual antenna with amplifier, to measure the voltage field of a topload. But I can for the love of god not find it, I guess the homepage is long gone and the knowledge is lost

It was made on a PCB and looked like this, black is traces and red were coax cables.

I actually succeeded in finding the page, it was Terry Fritz that did a lot of experiments. I was thinking about this http://hotstreamer.deanostoybox.com/TeslaCoils/MyPapers/planant/waveant3.html and http://hotstreamer.deanostoybox.com/TeslaCoils/MyPapers/smallar/smallar4.html

But really, check out all the great stuff from Terry at: http://hotstreamer.deanostoybox.com/TeslaCoils/MyPapers/MyPapers.htm and just for the fun of it, all the old tesla coil websites mirrors: http://hotstreamer.deanostoybox.com/