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Relaxation Oscillator

Update History
Created 01 January 2010

I first built a relaxation oscillator with a neon lamp around 1960, so it's a fitting project to duplicate 50 years later in 2010.

It's difficult to build a simpler oscillator than a neon lamp RC circuit. As illustrated below, it consists of a DC power source, a resistor, a capacitor and a neon lamp.

Neon bulb relaxation oscillator
The circuit values above are for the standard neon lamp component in LTspice. The circuit we'll look at in more detail uses a NE2 lamp and an 80 volt DC supply. R1 is 1 meg and C1 is 1uF just as in the SPICE simulation.

What is it about a neon lamp that makes it special? The figure below—a voltage versus current sweep of a NE2 neon lamp—provides the answer. The horizontal axis is voltage across the lamp (10 volts/division) and vertical axis (50 uA/division) is current through the lamp. 

As the voltage increases from 0, the curve tracer shows no current flow until point A is reached. At the striking voltage, about 70 volts for this lamp, the lamp switches on and current flows, which causes the voltage across the lamp to drop. At point B, 500uA current is seen through the lamp and the voltage drops below a point sufficient to keep the lamp illuminated and the current rapidly drops back to zero at point C. The breakdown occurs so rapidly that the trace is difficult to see so I've added a green line from A to B. If you look carefully around point A, however, you can see part of the conduction cycle.

 


Consider what happens when the neon lamp is connected across C1 in our circuit. At some time, t0, the power supply is switched on. C1 charges from zero volts through R1, with a time constant of 1 second. At some time t1 the voltage across C1 and the neon lamp will exceed the striking voltage and the lamp will fire. When the lamp fires, it forms a voltage divider with R1 and reduces the  voltage across C1 below the lamp's extinguishing voltage. C1 then charges through R1 until the striking voltage is achieved a second time and the cycle repeats, forming an oscillator.

The SPICE simulation below shows the voltage across C1 and the lamp. The RC nature of the charging voltage should be apparent from the plot.


The photograph shows the relaxation oscillator I built.
 

The oscilloscope capture below shows the voltage across C1 and the lamp. The lamp fires at 67 volts and extinguishes at a lower voltage. The oscilloscope's measurement reports 60.6 volts as the extinguishing voltage, but with a cautionary note that the analysis is unstable. Looking at the waveform, the difference between peak and valley is about 14 volts, which implies an extinguishing voltage of 53 volts or so.

Both these values match the curve trace measurement of the neon lamp within a reasonable tolerance.

The oscillation frequency is rather modest, about 1.6 seconds. But, with suitable choice of R1 and C1, the oscillation frequency can be increased into the audio range. The upper frequency limit is a function of how fast the neon gas can de-ionize, with a typical maximum of 20 KHz for lamps such as the NE2. (See E. Bauman, "Applications of Neon Lamps and Gas Discharge Tubes," Carlton Press, New York, 1966.)
 


The fall time is a second RC discharge, in this case C1 discharging through the neon lamp's resistance. The discharge fall time is 2.7 ms.
 
I also looked at the current though the neon lamp during the discharge cycle, using a Hall effect current probe. The bottom trace (blue) is current, with a scale of 1mA/division.

The peak current is approximately 1.8 mA, with a peak voltage of 67 volts, which implies the NE2 has an equivalent series resistance of 37 KΩ.