C.5 – Neon-Tube Relaxation Oscillator Circuit
The capacitor C charges through the resistor R. When the circuit is closed, the voltage across the capacitor (and the neon tube) rises from 0 to Vb, which is the breakdown voltage of the neon gas in the tube (about 80 volts). At that voltage, the gas is ionized and begins to conduct electricity and the tube lights up. When the tube is in a conducting state, the capacitor discharges through it. But when the voltage across the discharging capacitor can no longer sustain the tube discharge, the tube stops conducting. During this short time, the voltage across the capacitor and the tube drops from Vb to Vm, which is called maintaining voltage. The voltage then rises from Vm to Vb, and the cycle begins again. The continual repetition of this cycle causes the tube to blink on and off. The period of the oscillator, or the time between flashes, depends on the RC time constant. The time between flashes of the neon tube, or the period of the oscillator circuit, is the time it takes for the voltage to rise from Vm to Vb.
T = tb – tm = R C ln[(Vo – Vm) / (Vo – Vb)]
where Vo is the maximum applied voltage in the circuit.
These neon tubes are about the sizes of miniature Christmas tree lights.
C = 0.5 µF
R = 64 x 104 ohms
t = 0.32 sec
