Rogowski Coil Redeux

Way back when I was building the capacitor bank, I built a inductive current sensor called a rogowski coil so that I could have some instrumentation to see just what kind of voltage and current I was dealing with when I used my capacitor bank. Sadly I wasn’t able to get it to work.

But first a little bit of introduction, the rogowski coil is essentially an inductive current sensor similar to a current transformer, but different in that it is wound on a flexible coilform that allows temporary installation. Additionally the can be tailored to suit a wide range of currents and are ideal for short high intensity transient pulses (such as capacitive discharge). The coil itself when placed around a conductor has a voltage proportional to dI/dt, however this by itself is fairly useless since anyone carrying out an experiment wants to know I over time.

Figure 1 – Graph of data captured from Rogowski coil integrator using a Tektronix 2012B, arbitrary scaling used. Calibration unknown on current data. dV/dt smoothing by moving average over 4 data points.

In order to extract current over time from dI/dt, you must integrate the voltage. This is done using an opamp as an integrator. By itself not terribly challenging; the hard part is data capture/acquisition, and calibration. Note that because the coil itself gives dI/dt, the peak rate of change of the circuit voltage should coincide with the peak current after integration, which is a pretty good indicator of whether you have the right configuration for your integrator.

Originally I had tried to use my sound card with an integrator + buffered circuit, however with the level of AC decoupling present, terribly ineffective with a very slow response. The trick is to avoid extra AC decoupling, and simply correct for voltage offset, which in itself is a good idea, however the challenging part was compensating for the voltage offset. If you don’t correct for voltage offset using opamps, and simply AC decouple, you introduce a RC network at every junction, and blur your current waveform over a longer period of time.

Last week I set up my old original coilgun with the rogowski coil, and a DSO that I have access to and began capturing data to test a circuit, with some success. I finalized the circuit topology and laid out the remaining tasks:

• Prototype rogowski coil integrator with attenuator, integrator, gain stage, and line driver
• Build current shunt reference for DSO
• Build DC amplifier to provide current waveform
• Calibrate stepped attenuator, gain stage to match current reference
• Build final version, calibrate, package in metal enclosure

2.5 kJ Capacitor Bank Complete!

After plenty of delays due to more important projects, I have finally completed the capacitor bank control system and construction.

The capacitor bank is a 22,500uF, 450V capacitor bank with a 10kA surge current capacity using solid state switching. A custom built charger was created so that I could use 12VDC (ie: car batteries) to charge the capacitor bank outdoors for more energetic tests. Along with the charger, a control system to monitor bank voltage, and provide safety interlocks to prevent discharge during firing, automatic voltage control, and discharge load control to abort high-current tests were key aspects of the charge controller.

Tesla Coil Audio Redeux

So I’ve carried out further testing by removing a good portion of the bass present in the audio signal to great effect. I got Daft Punk’s Aerodynamic, and Axel F (original) and they both turned out pretty good.

Daft Punk – Aerodynamic

Axel F

Audio Modulated Tesla Coil part 2!

So over the last few days I’ve been busting my balls getting the modulator complete, a few hours before midnight I met my personal deadline of plugging in the modulator board to the main circuit board. The result is a half-passable audio modulation. My future plan is to replace the entire mainboard with a PLL circuit, fullbridge, and current transformer feedback giving much better audio quality.

The modulator is a bit of a rats nest, I’d mount all the pots to a board and have fewer connectors, but I don’t have the ability to drill super-accurate holes into heavy steel (thus any board would be useless).

The plugin modulator board

There is also video! For anyone with time I’d suggest taking a look at my Youtube channel, I have more videos there, but for anyone that wants the quick sample, look below.

Note: audio quality has since been improved from the time of the filming as the enclosure was not closed (and therefore not shielded) at the time of filming.

Audio Modulated SSTC

So a project of mine that has been ‘in limbo’ for quite some time is finally starting to come to fruition. My intention is to convert my current SSTC to an audio modulated SSTC, I’ve already done up schematics for the modulator and receiver, and have acquired most of the parts needed.

Below you can see the first and second revisions of the modulator box’s front panel, since there is too much to cram onto one small panel, I’m going to have to use both sides, but on the whole it should be an efficient use of space.

I’ll post updates over the next 2-3 weeks (my winter break) as I make some progress on this, so stay tuned.