I have been asked this question so many time I thought I aught to write down the answer.

Musical tesla coils are fairly common now and there are many videos of them working, but what causes the noise.

So lets start with the basics. Most electronic tesla coils don’t run continuously (Ill come back to CW coils later). To maintain a large spark output current in the primary circuit is key, to get the best out of the components the coils are driven for short periods of time, at high currents, quite often exceeding the continuous rating of the drivers. During this short ‘on’ period, power is stored as in the primary and secondary coil. As soon as the power is turned off the stored energy is transformed into a high voltage high frequency pulse. It is this pulse that you see as the discharge from the top of the coil.

Creating hot plasma in the atmosphere has an effect, the air heats up to thousands of degrees C in a microsecond or so. This expanding air causes a sound wave that you hear as a click. (or a bang with real lightning, its just a matter of scale.)

So, if you repeat this process at say 440 times a second, you will hear an audio tone at middle C

Music can be played by controlling the length and frequency of the pulses you send to the coil. With these coils getting the frequency and length information is relatively simple from a MIDI audio source via a suitably programmed microprocessor.

Due to the pulsed nature of the coil real audio (e.g speech,singing etc) cannot be rendered.

OK, I mentioned CW (Constant or Carrier wave) Tesla coils
With CW coils the power isn’t interrupted they are driven constantly. Their output is essentially silent (You do get a hiss at lower frequencies due to variations in the spark output). To get music from these coils you vary the power going in to them in a very similar way to an AM radio transmitter. There is two ways of achieving this.
1) You control the voltage to the driver therefor controlling the power
2) You vary the frequency of the drive (FM modulation), The tuned circuit formed by the secondary gives maximum output only at the central frequency, so the effective output power is varied as the frequency is varied.

As the power is continuous, you can’t gain any power advantage by short pulses and neither do you get the voltage gain by the release of the stored power in the secondary so the sparks are smaller than a pulsed coil.

But, you can render speech, singing and any instrument can be played due to the almost radio style modulation. Plasma speakers are made using this technique.

First light of a Tesla Coil tuner.

So I needed to get a better Tesla coil Tuner. I have been using this NE555 pinger for 10+ years.

Always said I’d put it in a box.. Never happened…

With some of the tuning problems I’ve had recently I needed a better solution.

So..

PIC powered USB controlled Tesla Coil Tuner using AD9850 DDS Frequency Synthesis.

Built on a PIC low pin count, USB development board (hence redundant RS232 connector)

Direct connection to the Primary Coil with current measuring.

Three ways of measuring.

Primary and cap on its own(Secondary removed)

Secondary on its own (driven by primary with no Primary cap)

Primary Cap and Secondary (across spark gap) – Should see both current dips.

First plot of a small Secondary coil, using VB to drive the USB.

Plots at ~15 seconds for 1000 points (e.g will scan from 500Khz to 1.5Mhz in 1khz steps in ~15 seconds.)

Range 50Khz to 4Mhz (with some droop at each end)

I’m having some issues getting a plot with just a primary coil, not sure why at the moment. Also my driver (tc428) is damaged, its not driving correctly to ground. Which may be causing me some issues. But not bad for a first go.

More developments

Added an Ariel input (B)

Improved the display, and removed a whole load of noise on the traces.

Also, added the ability to take snapshots of the traces (pale colours) so you can see any change from scan to scan

This shows a current scan in Dark blue, with a snapshot in light blue and the Ariel input in dark green with a light green snapshot. The current scan was done with my hand near the topload hence the lower resonant frequencies.

This clearly shows the primary (C & L) as a large dip on the left and the secondary / topload as the smaller dip on the right. I’m feeding the primary coil and primary cap in parallel as this gives the best indication of the current change in the primary.  For some reason driving the coil with Cpri in series shows a lumped resonance (or doesn’t show the primary resonance up at all)