File:Neon bulb relaxation oscillator hysteresis curve.svg

English: The IV curve (characteristic curve) of a neon bulb relaxation oscillator (right) showing the hysteresis loop (abcd) of the oscillation, and the resulting voltage and current waveforms (left). A neon bulb relaxation oscillator consists of a capacitor which is charged from a voltage source V₀ through a resistor R, with a neon bulb across the capacitor which discharges it once it reaches the bulb's breakdown voltage V_b. Ths hysteresis curve (abcd) shows the detailed cycle:

• a-b: The voltage across the bulb rises slowly from zero as the capacitor charges, through a to b, where it reaches the breakdown voltage V_b of the bulb. The gas in the bulb ionizes and it become conductive.
• b-c: The charge from the capacitor begins to flow through the bulb, instantly increasing the current to point b in the "abnormal glow" region of the curve. The bulb turns on.
• c-d: The voltage across the bulb and capacitor drops quickly to the extinguishing voltage V_e at which the voltage is insufficient to continue ionizing the gas. The bulb turns off.
• d-a: The current through the bulb drops to a very low level, allowing the current from the resistor R to begin charging the capacitor up again from a to b and the cycle repeats.

The circuit generates a sawtooth wave, (left) alternating between V_e and V_b. The blue line is the load line, with a slope equal to the resistor R. In order for the circuit to oscillate, the load line must intersect the IV curve in it's negative resistance region (shaded region) the segment from b to d, where it is declining to the right.

• If the resistor is too large value, so the load line intersects the curve in section a-b, the resistor current will be so low that the bulb leakage current will bleed the capacitor charge away and the voltage will never reach V_b.
• If the resistor is too small, so the load line intersects the curve in section c-d, the current through the resistor will be large enough to keep the bulb turned on, and it won't reach the extinguishing voltage V_e.

Information from C. R. Daugherty, et al, General Electric Glow Lamp Manual, 2nd Ed., General Electric, Cleveland, Ohio, 1966