Neon (NE-2) Circuits You Can Build
by Lewis Loflin
The neon lamp has been around for many decades. Largely obsolete today, it's fun to play with and does some remarkable things. My purpose here is to introduce the student of today with and old friend. There are three NE-2s neon lamps being used above on my proto-typing board. One on the left is a power indicator, the two on the right are blinking back and fourth. Let's look at some simple circuits.
Pictured above are two NE-2 neon lamps with 100k dropping resistors attached. A NE-2 is a very low-current device and the resistor limits the current. These could be used for 120 VAC or with a 220K for 240 VAC.
In the above diagram these can be used with a pair of test leads to find the "HOT" side of an AC or DC circuit.
How do they work?
A neon lamp is simply a small glass tube filled with a low pressure such as neon or sometimes with additional small amounts of krypton, argon, etc. to change the color from the normal orange.
The NE-2 will fire at around 90-volts AC or CD, then the operating voltage will drop to about 60-volts. This produces an odd effect known as negative resistance. That means once it conducts the resistance will drop and the operating voltage is much lower than the ignition voltage. The lamp will stay on until the voltage drops below 50-volts or so.
This property allows neon lamps to be used with resistor/capacitor charging circuits to creating a relaxation oscillator. All of the schematics below take advantage of that property.
In the picture above shows how the NE-2 will operate with direct current versus alternating current. Like all gas discharge tubes the ignition voltage (about 90 volts) produces positively charged neon ions and free electrons. Looking at this from the standpoint of DC, the negative electrons flow to the positive side of the circuit while the positive ions move to the negative side.
The circuit above is the one illustrated in the photo at the beginning of this page. I used 1 mega-ohm resistors and a 170-volt power supply in my circuit.
Added July 31, 2011
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