Neon-lamp relaxation oscillator
The capacitor charges through a large resistor. At about 90 V the neon gas ionises, the lamp flashes and drains the capacitor down to about 60 V, where the glow goes out. Then it charges again: a flasher with one lamp, one resistor and one capacitor.
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How it works
The capacitor swings between the maintaining voltage (60 V) and the striking voltage (90 V):
T = R·C·ln((V − 60) ÷ (V − 90)), f = 24.7 Hz
It only oscillates if the resistor cannot keep the lamp lit: (V − 60) ÷ R must be below the lamp’s extinction current.
Try it on the bench
- Lower the supply to 100 V: the flashes slow right down.
- Change R to 220 kΩ: now the resistor can keep the lamp lit, so it glows steadily instead of flashing.
Parts on the bench
- Bench power supply
- Resistor
- Film capacitor
- Neon lamp
In the course
Electronics Fundamentals, module “Switching with transistors”.