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The interrupter: bangs per second, ontime and duty

Bangs per second is the frequency of the note in hertz. There is no best rate, only a note, and two settings on this box kill transistors.

CommonGnd4 August 20265 min read

The coil rings at hundreds of kilohertz and nobody can hear it. The interrupter switches it on and off at an audible rate. That is where the note comes from, and all the music with it.

What it is and why

Bangs per second is the frequency of the note in hertz: 200 bangs a second is 200 Hz. Ontime is how long each one lasts.

duty = ontime × bangs per second

The average power stays tolerable while the peak goes up many times over. At 5 per cent duty each bang can carry twenty times the power, and that is the whole reason a coil is interrupted rather than run flat out.

There is no best rate. It is a note.

What you decide

The rate

Criterion: average power, which is the rate times the ontime. A higher rate wants margin in RMS current and in heatsinking, and buys you a higher note and nothing else.

The ontime, and why too short is also a mistake

A doubly resonant coil is not a spark gap coil. The energy does not all leave in some transfer time; it is pumped in across the whole bang. So a longer ontime gives a longer spark, and it does so most clearly at modest peak currents. Setting the ontime short "to be safe" costs length for no protection.

The dead time between pulses

Dead time over about 30 µs is audible as noise. Set it to the fall time of the primary current, which is a thing to look at on a scope rather than guess.

Whether to run staccato

Tying the bangs to the mains zero crossing. Three practical notes:

Use a CMOS 555. A plain NE555 draws a shoot-through spike and makes a mess of its own supply.
The trigger wants a fast falling edge near zero, a couple of microseconds wide: a Schmitt trigger and a small capacitor.
With the bridge connected the rectified voltage never reaches zero and the detector stops working. Fit a separate half-wave diode with a bleeder and no capacitors, purely for the zero detection.

What will get you

Two settings kill transistors: too high a rate and too long an ontime. Both raise the die temperature faster than the heatsink can take it away. And it is not only the peak: every bang is a heating and cooling cycle, and the size of the swing tears bond wires over thousands of cycles, a long way below Tjmax.

And the link to the coil has one rule that is not about music at all: a broken fibre must turn the coil off, not leave it on, and the receiver does not do that by itself. That is the fibre and the fail-safe rule, and it is worth reading before the first MIDI file rather than after.

The numbers

A note: bangs per second in hertz. 200 bangs a second is 200 Hz.
MIDI ontime: typically no more than about 300 µs a note.
Dead time: under about 30 µs, or it is audible.
Duty: ontime times rate, summed over voices.
At 5 per cent duty: each bang can carry about twenty times the average power.

About the sound, honestly. It is a train of square pulses with a great many harmonics: recognisable, but metallic, and the range is narrow. The timbre follows the size of the coil, so a ten centimetre coil sounds like a wasp and a metre one sounds impressive on the same MIDI file. For real music out of plasma people go to HFSSTC.

What goes wrong

Devices die during a particular piece of music and not others. The summed duty of the voices. Scale the ontime in firmware.
Audible noise between notes. Dead time over about 30 µs.
The arc looks disappointing. Test in the dark. In daylight the arc is pale and you will undersell your own result.
A staccato trigger that stops working once the bridge is connected. The rectified voltage no longer reaches zero. Separate half-wave diode.
Shorter arcs after shortening the ontime for safety. Working as described. The energy is pumped across the whole bang.

Where next

The fibre and the fail-safe rule.
The bridge and what kills it, since the two settings above are the two that get it.
The shape of the ramp, where the same box does a different job.

On a QCW there is no interrupter as such. The box in that position tells the bridge when to run and tells the buck what voltage to hold while it does, and the second of those is the ramp. Bangs per second there is five to seven rather than hundreds, and there is no ontime of its own: the bridge runs for exactly as long as the ramp lasts. The interrupter is a box on the DRSSTC diagram with the two knobs under it that it actually sets; on the QCW page the same box is a modulator, with a line running up the outside to the buck.

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