Neon lamps
I put these in 13A plugs to indicate power is on so even if the lamp in the machine fails there is still an indication of power. Simply wire across live and neutral terminals. Don't forget the resistor! See below, they are really interesting devices
To calculate the series resistor R required to operate this neon from a 240V ac supply, you need the "maintain" or "maintaining" voltage (region B to C in diagram) and the max. working current for you neon from the data sheet. If for example the maintain is 60V and the current 2.0mA. You need to provide a voltage divider 240-60v= 180V across R, 60V across the lamp.
When the gas ionizes it becomes a conductor, and an external series resistor is required to limit the current.
R=180V/2mA which gives a value of 90Kohms (pick the nearest value available eg 100k = 1.8mA)
I²R can be used to calculate the dissipation in the resistor
P=I²R=0.002A x 0.002A x 90000ohms=0.36watts
As such a 1/2 watt resistor is suitable
Little current flows until the striking voltage has been reached. As ionisation of the gas occurs, the voltage appearing across the neon drops to the "maintaining voltage" and current flows requiring an external series resistor to limit the current to the designed value. This is the normal region of operation for the lamp. Any increase in voltage across the lamp (past region C in diagram) will cause an increase in the maintain voltage accompanied by an abnormal glow in the tube followed by formation of arcing. Click on the NE2 lamp for more info
A neon lamp is constructed by mounting two electrodes within a small glass envelope. Two wire leads made of tin plated Dumet metal are brought out of the lamp to make electrical connection to the electrodes. Standard brightness lamps are filled with a neon/argon gas mixture, and high brightness lamps are filled with pure neon gas. When a starting voltage (usually 55-110 volts AC, or 90-140 volts DC) is applied, the gas ionizes and starts to glow permitting a very small current to travel from one electrode to the other. Once ionized, a lower voltage will maintain the operation of the lamp. The maintaining voltage is usually 10-20 volts below the starting voltage, depending on the lamp and the operating current. For lamps operating on AC voltages of 60 Hz or higher frequency, the light output will appear to the eye as continuous. When the gas ionizes it becomes a conductor, and an external series resistor is required to limit the current. Typical resistance values range from 10 Ohms to 220 Ohms. The power dissipation of the required resistor is small, usually less than 1/4 watt, but should be checked for high voltage applications. Typical current for neon lamps is between 0.5 and 3.0 mA.
Neon lamps gradually decline in light output as electrodes evaporate and condense on the inside of the glass envelope. This situation is gradual with failure defined as a 50% decrease from the original brightness. As neon lamps age, the firing voltages slowly increase until reaching the value of the supply voltage. At this point the lamp flickers and becomes erratic, indicating the end of useful lifetime. Life expectancy of a neon lamp increases considerably as operating current is decreased. For high brightness lamps, there is a 4 to 5 power inverse relationship between current and life, whereas for standard brightness types, the lamp life varies inversely as the 3.5 power of the current. With light output exponentially proportional to current, large increases in rated life can be obtained with small reductions in current and only a small reduction in brightness. For neon lamp applications requiring life ratings of more than 50,000 hours the use of a higher value resistor will reduce lamp current and achieve longer life. For DC operation, the life of a high brightness lamp is about 50% of a neon lamp operating at the same RMS AC voltage. The life of a standard brightness lamp at DC is about 60% of the life value at AC. Typical light output color for clear glass neon lamps is in the orange-red range of 600 to 700 nanometers. Other emitted colors as green, yellow and blue are available through secondary emission by coating the inside surface of the envelope with phosphor.
