Pearson–Anson effect

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Pearson-Anson oscillator circuit

The Pearson–Anson effect, discovered in 1922 by Stephen Oswald Pearson[1] and Horatio Saint George Anson,[2][3] is the phenomenon of an oscillating electric voltage produced by a neon bulb connected across a capacitor, when a direct current is applied through a resistor.[4] This circuit, now called the Pearson-Anson oscillator, neon lamp oscillator,[5][6] or sawtooth oscillator,[7] is one of the simplest types of relaxation oscillator.[8] It generates a sawtooth output waveform.[7] It has been used in low frequency applications such as blinking warning lights,[9] stroboscopes,[9] tone generators in electronic organs[8][10] and other electronic music circuits,[11] and in time bases and deflection circuits of early cathode ray tube oscilloscopes.[8][12] Since the development of microelectronics, these simple negative resistance oscillators have been superseded in many applications by more flexible semiconductor relaxation oscillators such as the 555 timer IC.

Neon bulb as a switching device[edit]

NE-2 neon lamp

A neon bulb, often used as an indicator lamp in appliances, consists of a glass bulb containing two electrodes, separated by an inert gas such as neon at low pressure. Its nonlinear current-voltage characteristics (diagram below) allow it to function as a switching device.[13]

When a voltage is applied across the electrodes, the gas conducts almost no electric current until a threshold voltage is reached (point b), called the firing or breakdown voltage, Vb.[7][14] At this voltage electrons in the gas are accelerated to a high enough speed to knock other electrons off gas atoms, which go on to knock off more electrons in a chain reaction. The gas in the bulb ionizes, starting a glow discharge, and its resistance drops to a low value. In its conducting state the current through the bulb is limited only by the external circuit. The voltage across the bulb drops to a lower voltage called the maintaining voltage Vm. The bulb will continue to conduct current until the applied voltage drops below the extinction voltage Ve (point d), which is usually close to the maintaining voltage. Below this voltage, the current provides insufficient energy to keep the gas ionized, so the bulb switches back to its high resistance, nonconductive state (point a).

The bulb's "turn on" voltage Vb is higher than its "turn off" voltage Ve. This property, called hysteresis, allows the bulb to function as an oscillator. Hysteresis is due to the bulb's negative resistance, the fall in voltage with increasing current after breakdown,[7][14] which is a property of all gas discharge lamps.

Operation[edit]

IV curve of neon bulb (right)[7] showing the oscillator hysteresis loop (abcd), the load line (blue), and output waveforms (left). The load line must lie within the shaded region for the circuit to oscillate. v is the voltage across the neon bulb, and i is the current through it.

In the Pearson-Anson oscillator circuit (top) a capacitor C is connected across the neon bulb N[7] The capacitor is continuously charged by current through the resistor R until the bulb conducts, discharging it again, after which it charges up again.[15] The cycle is illustrated by the hysteresis loop abcd on the current-voltage diagram at right:[4][7][10]

  • When the supply voltage is turned on, the neon bulb is in its high resistance condition and acts like an open circuit. The current through the resistor begins to charge the capacitor and its voltage begins to rise toward the supply voltage.
  • When the voltage across the capacitor reaches b, the breakdown voltage of the bulb Vb, the bulb turns on and its resistance drops to a low value. The charge on the capacitor discharges rapidly through the bulb in a momentary pulse of current (c). When the voltage drops to the extinction voltage Ve of the bulb (d), the bulb turns off and the current through it drops to a low level (a). The current through the resistor begins charging the capacitor up again, and the cycle repeats.

The circuit thus functions as a low-frequency relaxation oscillator, the capacitor voltage oscillating between the breakdown and extinction voltages of the bulb in a sawtooth wave. The period is proportional to the time constant RC.

The neon lamp produces a brief flash of light each time it conducts, so the circuit can also be used as a "flasher" circuit. The dual function of the lamp as both light source and switching device gives the circuit a lower parts count and cost than many alternative flasher circuits.

Conditions for oscillation[edit]

The supply voltage VS must be greater than the bulb breakdown voltage Vb or the bulb can never conduct.[7] Most small neon lamps have breakdown voltages between 80 and 150 volts. If the breakdown voltage is close to the supply voltage, the capacitor voltage will be in the "tail" of its exponential curve by the time it reaches Vb, so the frequency will depend sensitively on the breakdown threshold and supply voltage levels, causing variations in frequency.[15] Therefore the supply voltage is usually made significantly higher than the bulb firing voltage.[7] This also makes the charging more linear, and the sawtooth wave more triangular.[15]

The resistor R must also be within a certain range of values for the circuit to oscillate.[7] This is illustrated by the load line (blue) on the IV graph. The slope of the load line is equal to R. The possible DC operating points of the circuit are at the intersection of the load line and the neon lamp's IV curve (black) In order for the circuit te be unstable and oscillate, the load line must intersect the IV curve in its negative resistance region, between b and d, where the voltage declines with increasing current.[7] This is defined by the shaded region on the diagram. If the load line crosses the IV curve where it has positive resistance, outside the shaded region, this represents a stable operating point, so the circuit will not oscillate:

  • If R is too large, of the same order as the "off" leakage resistance of the bulb, the load line will cross the IV curve between the origin and b. In this region, the current through R from the supply is so low that the leakage current through the bulb bleeds it off, so the capacitor voltage never reaches Vb and the bulb never fires.[7] The leakage resistance of most neon bulbs is greater than 100MΩ, so this is not a serious limitation.
  • If R is too small, the load line will cross the IV curve between c and d. In this region the current through R is too large; once the bulb has turned on, the current through R will be large enough to keep it conducting without current from the capacitor, and the voltage across the bulb will never fall to Ve so the bulb will never turn off.[7]

Small neon bulbs will typically oscillate with values of R between 500kΩ and 20MΩ.[7] If C is not small, it may be necessary to add a resistor in series with the neon bulb, to limit current through it to prevent damage when the capacitor discharges.[10] This will increase the discharge time and decrease the frequency slightly, but its effect will be negligible at low frequencies.

Frequency[edit]

The period of oscillation can be calculated from the breakdown and extinction voltage thresholds of the lamp used.[6][7][10][16] During the charging period, the bulb has high resistance and can be considered an open circuit, so the rest of the oscillator constitutes an RC circuit with the capacitor voltage approaching VS exponentially, with time constant RC. If v(t) is the output voltage across the capacitor

v(t) = V_S(1 - e^{-t/RC}) \,

Solving for the time

t = RC\ln\Bigl[{V_S \over {V_S - v}}\Bigr]  \,

Although the first period is longer than the others because the voltage starts from zero, the voltage waveforms of subsequent periods are identical to the first between Ve and Vb. So the period T is the interval between the time when the voltage reaches Ve, and the time when the voltage reaches Vb

T = t(V_b) - t(V_e) \,
T = RC\ln\Bigl[{V_S \over {V_S - V_b}}\Bigr] - RC\ln\Bigl[{V_S \over {V_S - V_e}}\Bigr]\,
T = RC\ln\Bigl[{{V_S - V_e} \over {V_S - V_b}}\Bigr]\,

This formula is only valid for oscillation frequencies up to about 200 Hz;[7] above this various time delays cause the actual frequency to be lower than this.[8] Due to the time required to ionize and deionize the gas, neon lamps are slow switching devices, and the neon lamp oscillator is limited to a top frequency of about 20 kHz.[7][8][10] The breakdown and extinction voltages of neon lamps may vary between similar parts;[15] manufacturers usually specify only wide ranges for these parameters. So if a precise frequency is desired the circuit must be adjusted by trial and error.[15] The thresholds also change with temperature, so the frequency of neon lamp oscillators is not particularly stable.[10]

Forced oscillations and chaotic behavior[edit]

Like other relaxation oscillators, the neon bulb oscillator has poor frequency stability, but it can be synchronized (entrained) to another oscillator by applying an oscillating signal in series with the neon bulb. Even if the external signal's frequency is different from the natural frequency of the oscillator, the peaks of the applied signal can exceed the breakdown threshold of the bulb, discharging the capacitor prematurely, so the period of the oscillator becomes locked to the applied signal.

Interesting behavior can result from applying different amplitudes and frequencies of external signal to the forced oscillator. One is submultiplication or demultiplication; the oscillator can produce frequency components that are submultiples of the natural oscillator frequency and the external frequency, lower than both of them.[17][18] This is because the pattern of sawtooth waves can go many cycles before it repeats. Another is "aperiodic" oscillations in which the pattern of sawtooth waves never repeats. This is called chaos or chaotic behavior and is an active area of research, called chaos theory.[18]

The forced neon bulb oscillator was the first system in which chaotic behavior was observed.[19] In 1927 Balthasar van der Pol, investigating demultiplication phenomena in a neon oscillator, was listening to the output with a telephone receiver to identify the submultiple tones. He wrote[17][18]

Often an irregular noise is heard in the telephone receivers before the frequency jumps to the next lower value. However this is a subsidiary phenomenon, the main effect being the regular frequency demultiplication

Any periodic oscillation would have produced a musical tone; only aperiodic, chaotic oscillations would produce an "irregular noise". This is thought to have been the first observation of chaos, although van der Pol didn't realize what it was.[19][18]

See also[edit]

Notes[edit]

  1. ^ Stephen Oswald Pearson,Dictionary of Wireless Technical Terms (London: Iliffe & Sons, 1926).
  2. ^ Pearson, S. O.; H. St. G. Anson (December 1921). "Demonstration of Some Electrical Properties of Neon-filled Lamps". Proc. of the Physics Soc. London (London: Physics Society of London) 34 (1): 175. doi:10.1088/1478-7814/34/1/435. 
  3. ^ Pearson, S. O.; H. St. G. Anson (August 1922). "The Neon Tube as a Means of Producing Intermittent Currents". Proc. of the Physics Soc. London (London: Physics Society of London): 204–212. doi:10.1088/1478-7814/34/1/341. 
  4. ^ a b Kinoshita, Shuichi (2013). "Introduction to Ionequilibrium Phenomena". Pattern Formations and Oscillatory Phenomena. Newnes. p. 17. ISBN 012397299X. Retrieved February 24, 2014. 
  5. ^ Morris, Christopher G. (1992). Academic Press Dictionary of Science and Technology. Gulf Professional Publishing. p. 1453. ISBN 0122004000. 
  6. ^ a b Pollack, Dale (December 1932). "Neon Tube Oscillators, Part 1". Radio News (New York: Teck Publications) 14 (6): 342–343. Retrieved April 14, 2014. 
  7. ^ a b c d e f g h i j k l m n o p q Daugherty, C. L.; et al (1965). G.E. Glow Lamp Manual, 2nd Ed.. Cleveland, Ohio: General Electric. pp. 14–19. 
  8. ^ a b c d e Bauman, Edward (1966). Applications of Neon Lamps and Discharge Tubes. USA: Carleton Press. p. 18. 
  9. ^ a b Burton, Walter E. (February 1948). "Magic with neon glow lamps". Popular Science (New York: Popular Science Publishing Co.) 152 (2): 194–196. ISSN 0161-7370. Retrieved April 14, 2014. 
  10. ^ a b c d e f Dance, J. B. (1967). Cold Cathode Tubes. London: Iliffe Books. pp. 25–27. 
  11. ^ Holmes, Thomas B. (2002). Electronic and Experimental Music: Pioneers in Technology and Composition. Psychology Press. p. 66. ISBN 0415936446. 
  12. ^ Wahl, Horst D. (2005). "Tutorial Oscilloscope". Phys4822L Advanced Lab-Experiment 11: Studies of electrons with a CRT. Prof. Horst D. Wahl, Physics Dept.,Florida State Univ. Retrieved April 14, 2014. 
  13. ^ Gottlieb, Irving M. (1971). Understanding oscillators. H. W. Sams. p. 69. ISBN 0672208377. 
  14. ^ a b Dance, 1967, p.6-7
  15. ^ a b c d e Miller, Walter G. (1969). Using and Understanding Miniature Neon Lamps. New York: Howard W. Sams. pp. 37–39. 
  16. ^ Hill, Eric (2014). "Lecture 7a – Waveform Generators & Timers". Course Notes Physics 310: Electronics Applications. Prof. Eric Hill website, Physics Dept., Univ. of Midlands. Retrieved April 14, 2014. 
  17. ^ a b van der Pol, Balthasar; Jan van der Mark (September 10, 1927). "Frequency Demultiplication". Nature (MacMillan) 120 (3019): 363–364. doi:10.1038/120363a0. Retrieved April 16, 2014. 
  18. ^ a b c d Sprott, Julien C. (2010). Elegant Chaos: Algebraically Simple Chaotic Flows. World Scientific. pp. 234–235. ISBN 9812838821. 
  19. ^ a b Eisencraft, Marcio , Romis Attux, Ricardo Suyama; Romis Attux; Ricardo Suyama (2013). Chaotic Signals in Digital Communications. CRC Press. p. 87. ISBN 1466557222. 

References[edit]