Glow lamp voltage and current

When the gas is not ionized the lamp is almost an open circuit. There is, of course some stray capacitance (in the pF range) and a dark current which can be modeled with a parallel resistor in the MΩ range. The lamp will remain in this state as long as the voltage is below the ionizing voltage (also called striking voltage or breakdown voltage) which is usually between 50 to 100 V depending on many factors: the type of lamp, its age, the surrounding illumination,...

When the gas is ionized (the lamp glows) the voltage across the lamp is almost constant and depends only slightly on the current. This voltage is called maintaining voltage or sustaining voltage. It’s always lower than the ionizing voltage, usually by about 10 to 20 V. Even if glow lamps can show negative resistance for some low currents, in normal applications (the current is above the 100 μA range) the voltage across the lamp increases with increasing voltage showing a dynamic resistance in the kΩ range.

The following plot (left) shows the nonlinear characteristics of a NE-2 lamp. Voltage is on the horizontal (X) axis, current is on the vertical (Y) axis. As one can see, the current is almost zero until the voltage reaches the striking voltage (here 70 V), the voltage drops then to the maintaining voltage (here 51 V) and only slightly increases with current (it goes from 51 V at 0 mA to 57 V at 3 mA, giving a dynamic resistance of 2 kΩ).

Glow lamps are not perfectly symmetric: this lamp has a striking voltage of 71 V a marinating voltage of 52 V and a dynamic resistance of 1.6 kΩ if reversed.

The same data is plotted as a function of time (right). Here the lamp is connected to the alternating mains voltage (at 50 Hz in this case). Lamp voltage is yellow (CH1) and lamp current is blue (CH2). One can see that after the voltage crossed zero, the current stays zero until the striking voltage is reached; then the voltage suddenly decreases to the sustaining voltage causing the narrow spikes: the lamp is now lit. The current follows the sinusoidal form of the mains voltage (not shown), but the voltage across the lamp only changes slightly. As soon as the voltage drops below maintaining voltage the current is zero and the lamps switches off. It remains in this state until the voltage reaches the striking voltage again.

The following plots show the same kind of measures as before but for a green glow lamp. These kinds of lamps have a fluorescent layer inside the bulb that glows green when hit by UV radiation produced by mercury vapor inside the bulb. Here the striking voltage is 107 V (102 V when reversed), the maintaining voltage is 57 V (64 V) and the dynamic resistance is 2 kΩ (3.2 kΩ).

The following circuit diagram and pictures show how the measurements were made.