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A neon lamp is a gas discharge lamp containing neon gas at low pressure. A small electric current, which may be AC or DC, is passed through the tube, causing it to glow orange-red. In AC-excited lamps, both electrodes produce light, but in a DC-excited lamp, only the negative electrode glows. This simple fact can be used to distinguish between AC and DC sources using a neon lamp and to distinguish the polarity of DC sources.
Small neon lamps are used as indicators in electronic equipment. Larger lamps are used in neon signage. Because of their comparatively fast response time, in the early development of television, neon lamps were used as the light source in many mechanical-scan TV displays.
Most small neon (indicator-sized) lamps start conducting at a fairly consistent 60 to 80 volts, so they were used as very simple voltage regulators. They were also used for a variety of other purposes; since a neon lamp can act as a relaxation oscillator with an added resistor and capacitor, it can be used as a simple flashing lamp or audio oscillator. In the 1960s General Electric (GE), Signalite, and other firms made special extra-stable neon lamps for electronic uses. They even devised digital logic circuits, binary memories, and frequency dividers using neons. Such circuits appeared in electronic organs of the 1950s, as well as some instrumentation.
Neon lamps are negative resistance devices where increasing the current flow through the device increases the number of ions, thereby decreasing the resistance of the lamp, thereby allowing increased current flow. Because of this, the electrical circuitry external to the neon lamp must provide a means to limit the current flow in the circuit or else the current will increase until the neon lamp destroys itself. For indicator-sized lamps, a resistor is conventionally used to limit the current flow. For sign-sized lamps, the high voltage transformer usually limits the available current, often by its having a large amount of leakage inductance in the secondary winding.
Indicator-sized lamps can also be filled with argon rather than neon. While most operating characteristics remain similar, the lamps light with a bluish glow (including some ultraviolet) rather than neon's characteristic reddish-orange glow.
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