Spark-Gap Sawtooth Oscillator

A capacitor charges slowly until a spark gap ignites, then discharges rapidly until the arc extinguishes.

Circuit operation

RT = 100 kΩ charges CT = 10 nF from a 500 V supply. SG1 ignites near 300 V and quickly discharges CT. Its 10 mA holding current exceeds the approximately 5 mA available through RT, allowing the arc to extinguish and the cycle to repeat.

Expected behavior

$$\begin{gathered}T\approx R_TC_T\ln a\\a=\frac{V_s-V_{ext}}{V_s-V_{break}}\end{gathered}$$ $$V_{ext}\approx V_{arc}+R_{on}I_{hold}$$

Expect a slow exponential rise to about 300 V followed by a fast drop toward 20 V. The period is approximately 0.88 ms. The blue gap-current trace contains short discharge pulses. Use a narrower time window to inspect a pulse; its duration is much shorter than the charging interval.

Reading the results

The first voltage probe is green; the second, when present, is red. The first current probe is blue. The schematic opens fitted to its extents. The saved transient spans 5 ms with a maximum step of 0.2 µs. The chart uses separate voltage and current lanes, and the table retains the same probe order.

Try a change

Double RT to 200 kΩ; charging takes roughly twice as long while the ignition voltage remains near 300 V. Restore the saved values after comparison.

Model limits

The spark gap is a symmetric functional model: voltage-triggered ignition, current-held conduction, a constant arc drop, finite resistance, and finite switching delay. It does not model plasma dynamics, gas pressure, electrode geometry, heating, insulation, or physical construction.

Reference: spark-gap relaxation behavior.