Neon Tube Relaxation Oscillator Triangle Wave Timebase Circuit

OddMix.com - Technical Note - 090821 - by Karl Nagy
Picture 1 Neon lamp NE2 [4 KB]
Figure 1. Neon lamp NE2

Oscillators are frequently used circuits. Many of them are quite simple. Some oscillators are exceedingly complex. The neon tube Figure 1 oscillator belongs to the simple group as its entire circuit consists of four components. In TV [Television] sets one required function is to provide deflection voltages for a Cathode Ray Tube [CRT]. When TV sets used vacuum tubes for their circuits using a neon tube for deflection frequency or time base oscillator was a natural choice. In tube type circuits the required high voltage was already available and a triode or pentode vacuum tube served as an amplifier and process the wave shape to have it made more linear. Neon tube oscillators have long term stability because the neon lamp, when properly biased has extremely long life. With a minute amount of mercury added when the tube is made, the life extends into ten thousands hours.
Figure 2 Neon tube relaxation oscillator circuit [2 KB]
Figure 2. Neon tube relaxation oscillator circuit

In this relaxation oscillator circuit Figure 2 the B1 battery provides 80-100V DC potential. This voltage has to be only a bit larger than the ionization voltage of the NE neon bulb. When less distorted waveform required, higher voltage is necessary. The R1 resistor is limiting the to the C1 capacitor. C1 charges up to the B1 battery potential. The time the charge up is completed is a function of the R1C1 time constant T1. The NE neon tube is open circuit until the voltage across C1 exceeds the breakdown voltage of the tube. When the tube breakdown voltage is reached, a very abrupt ionization makes the tube a good conductor and provides a discharge path for C1 capacitor. Because the current can be damaging to the tube when it conducts, resistor R2 is necessary to limit the discharge current. The time to discharge is determined by the R2C1 time constant T2.

Picture 3 Neon tube relaxation oscillator circuit waveforms [2 KB]
Figure 3. Neon tube relaxation oscillator circuit waveforms

        Parts list for neon tube relaxation oscillator circuit

             C1 - Capacitor - 0.05 uF
             R1 - Resistor - 300 Kohm, 1/2 W, 10%
             R2 - Resistor - 120 Kohm, 1/2 W, 10%
             NE - Neon Lamp - NE2
             B1 - Battery or Power Supply - 80-100 Volt

By carefully selecting the two resistors and the capacitor a triangular or sawtooth waveform can be obtained Figure 3. The triangle is made with not quite straight lines but line segments from exponential charge discharge curves. If higher voltages used, the voltage waveform becomes a lot more ideal triangle. Increasing the value of the C1 capacitor or R1 resistor will lower the frequency of the circuit. When resistor R2 increases it lowers the frequency of the oscillator. The minimum value of R2 is the function of the permitted current for the neon lamp, usually between 0.5 and 1.5 mA. The frequency depends of the tube ionization and extinguishing voltage, on the R1C1 and R2C1 time constants. The formula to calculate the exact frequency is given here:

Picture 4 Neon tube relaxation oscillator frequency [1 KB]

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