This is due to the fact that incandescence is heat-driven light emission, thus a large portion of the electric energy put into an incandescent bulb is converted into heat. In comparison with incandescent light bulbs, neon lamps have much higher luminous efficacy. A photoionization effect can also be observed here, as the electrode area covered with the discharge can be increased by shining light at the lamp. Convective currents make the glowing areas flow upwards, not unlike the discharge in a Jacob's ladder. When there is not enough current to ionize the entire volume of the gas around the electrodes, only partial ionization occurs and the glow forms around only part of the electrode surface. The potential needed to strike the discharge is higher than what is needed to sustain the discharge. The flickering effect is caused by the differences of the ionization potential of the gas, which depends on spacing of the electrodes, temperature, ambient radiation, and the pressure of the gas. However, while too low a current causes flickering, too high a current increases the wear of the electrodes by stimulating sputtering, which coats the internal surface of the lamp with metal and causes it to darken. This may be a sign of aging of the indicator bulb, and is exploited in the decorative "flicker flame" neon lamps. When the current through the lamp is lower than the current for the highest-current discharge path, the glow discharge may become unstable and not cover the entire surface of the electrodes. Larger neon sign sized lamps often use a specially constructed high voltage transformer or ballast to limit the available current, usually by introducing a large amount of leakage inductance in the secondary winding. For indicator-sized lamps, a resistor is conventionally used to limit the current. voltage graph.) Because of this characteristic, electrical circuitry external to the neon lamp must provide a means to limit current through the circuit or else the current will rapidly increase until the lamp is destroyed. (This behavior occurs between the points labeled A and B on the lamp's current vs. Once lit, a neon lamp has a negative resistance characteristic: increasing the current through the device increases the number of ions, thereby decreasing the resistance of the lamp and allowing even more current. Graph showing the relationship between current and voltage across a neon lamp. Higher power devices, such as mercury-vapor lamps or metal halide lamps use a higher current arc discharge. Neon lamps operate using a low current glow discharge.
#PUT NEON TUBE TEXT ON IMAGE ONLINE SERIES#
These attributes make neon bulbs (with series resistors) very convenient as the basis for low-cost testers they determine whether a given voltage source is AC or DC, and if DC, the polarity of the the points being tested. When driven from an AC source, both electrodes will glow (each during alternate half cycles). When driven from a DC source, only the negatively charged electrode ( cathode) will glow. Once lit, the voltage required to sustain operation is significantly (~30%) lower. The applied voltage must initially reach the striking voltage before the lamp can light. The exact formulation of the gas is typically the classic Penning mixture, 99.5% neon and 0.5% argon, which has lower striking voltage than pure neon. A small electric current, which may be AC or DC, is allowed through the tube, causing it to glow orange-red.