The Pearson–Anson effect, discovered in 1922 by
Stephen Oswald Pearson
Stephen or Steven is a common English first name. It is particularly significant to Christians, as it belonged to Saint Stephen ( grc-gre, Στέφανος ), an early disciple and deacon who, according to the Book of Acts, was stoned to death; h ...
[Stephen Oswald Pearson,''Dictionary of Wireless Technical Terms'' (London: Iliffe & Sons, 1926).] and
Horatio Saint George Anson
Horatio St George Anson (1 August 1903 – 30 June 1925) was a British electrical engineer who in collaboration with Stephen Oswald Pearson discovered the Pearson–Anson effect, inventing the neon lamp relaxation oscillator.
He was the son of A ...
,
is the phenomenon of an
oscillating
Oscillation is the repetitive or periodic variation, typically in time, of some measure about a central value (often a point of equilibrium) or between two or more different states. Familiar examples of oscillation include a swinging pendulum ...
electric voltage produced by a
neon bulb
A neon lamp (also neon glow lamp) is a miniature gas discharge lamp. The lamp typically consists of a small glass capsule that contains a mixture of neon and other gases at a low pressure and two electrodes (an anode and a cathode). When suffi ...
connected across a
capacitor
A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals.
The effect of ...
, when a direct current is applied through a
resistor
A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias active el ...
.
This circuit, now called the Pearson-Anson oscillator, neon lamp oscillator,
or sawtooth oscillator,
is one of the simplest types of
relaxation oscillator.
It generates a
sawtooth output waveform.
It has been used in low frequency applications such as blinking warning lights,
stroboscopes,
tone generators in electronic organs
and other electronic music circuits,
and in time bases and deflection circuits of early
cathode-ray tube
A cathode-ray tube (CRT) is a vacuum tube containing one or more electron guns, which emit electron beams that are manipulated to display images on a phosphorescent screen. The images may represent electrical waveforms (oscilloscope), pictur ...
oscilloscope
An oscilloscope (informally a scope) is a type of electronic test instrument that graphically displays varying electrical voltages as a two-dimensional plot of one or more signals as a function of time. The main purposes are to display repetiti ...
s.
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
The 555 timer IC is an integrated circuit (chip) used in a variety of timer, delay, pulse generation, and oscillator applications. Derivatives provide two ( 556) or four ( 558) timing circuits in one package. The design was first marketed in 197 ...
.
Neon bulb as a switching device
A
neon bulb
A neon lamp (also neon glow lamp) is a miniature gas discharge lamp. The lamp typically consists of a small glass capsule that contains a mixture of neon and other gases at a low pressure and two electrodes (an anode and a cathode). When suffi ...
, often used as an
indicator lamp
A check engine light or malfunction indicator lamp (MIL), is a tell-tale that a computerized engine-management system uses to indicate a malfunction. Found on the instrument panel of most automobiles, it usually bears the legend , , , , , o ...
in appliances, consists of a glass bulb containing two electrodes, separated by an inert gas such as
neon
Neon is a chemical element with the symbol Ne and atomic number 10. It is a noble gas. Neon is a colorless, odorless, inert monatomic gas under standard conditions, with about two-thirds the density of air. It was discovered (along with krypton ...
at low pressure. Its
nonlinear
In mathematics and science, a nonlinear system is a system in which the change of the output is not proportional to the change of the input. Nonlinear problems are of interest to engineers, biologists, physicists, mathematicians, and many other ...
current-voltage characteristics ''(diagram below)'' allow it to function as a switching device.
When a
voltage
Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to m ...
is applied across the electrodes, the gas conducts almost no
electric current
An electric current is a stream of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is measured as the net rate of flow of electric charge through a surface or into a control volume. The moving pa ...
until a threshold voltage is reached ''(point b)'', called the ''firing'' or ''breakdown voltage'', ''V''
b.
[Dance, 1967](_blank)
p.6-7 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
Ionization, or Ionisation is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged atom or molecule i ...
, starting a
glow discharge
A glow discharge is a plasma formed by the passage of electric current through a gas. It is often created by applying a voltage between two electrodes in a glass tube containing a low-pressure gas. When the voltage exceeds a value called the str ...
, 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'' ''V''
m. The bulb will continue to conduct current until the applied voltage drops below the ''extinction voltage'' ''V''
e ''(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 ''V''
b is higher than its "turn off" voltage ''V''
e. This property, called
hysteresis
Hysteresis is the dependence of the state of a system on its history. For example, a magnet may have more than one possible magnetic moment in a given magnetic field, depending on how the field changed in the past. Plots of a single component of ...
, 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,
which is a property of all
gas-discharge lamp
Gas-discharge lamps are a family of artificial light sources that generate light by sending an electric discharge through an ionized gas, a plasma.
Typically, such lamps use a
noble gas (argon, neon, krypton, and xenon) or a mixture of thes ...
s.
Up until the 1960s sawtooth oscillators were also built with
thyratrons.
These were gas-filled
triode
A triode is an electronic amplifying vacuum tube (or ''valve'' in British English) consisting of three electrodes inside an evacuated glass envelope: a heated filament or cathode, a grid, and a plate (anode). Developed from Lee De Forest's 19 ...
electron tubes. These worked somewhat similarly to neon bulbs, the tube would not conduct until the cathode to anode voltage reached a breakdown voltage. The advantage of the thyratron was that the breakdown voltage could be controlled by the voltage on the grid.
This allowed the frequency of the oscillation to be changed electronically. Thyratron oscillators were used as time bases in oscilloscopes.
Operation
In the Pearson-Anson oscillator circuit ''(top)'' a
capacitor
A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals.
The effect of ...
''C'' is connected across the neon bulb ''N''
The capacitor is continuously charged by current through the resistor ''R'' until the bulb conducts, discharging it again, after which it charges up again.
The detailed cycle is illustrated by the
hysteresis loop
Hysteresis is the dependence of the state of a system on its history. For example, a magnet may have more than one possible magnetic moment in a given magnetic field, depending on how the field changed in the past. Plots of a single component of ...
''abcd'' on the current-voltage diagram at right:
*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 ''V''
b, 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 ''V''
e 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 sawtooth wave (or saw wave) is a kind of non-sinusoidal waveform. It is so named based on its resemblance to the teeth of a plain-toothed saw with a zero rake angle. A single sawtooth, or an intermittently triggered sawtooth, is called a ...
. 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
The supply voltage ''V''
S must be greater than the bulb breakdown voltage ''V''
b or the bulb can never conduct.
Most small neon lamps have breakdown voltages between 80 and 150 volts. If the supply voltage is close to the breakdown voltage, the capacitor voltage will be in the "tail" of its exponential curve by the time it reaches ''V''
b, so the frequency will depend sensitively on the breakdown threshold and supply voltage levels, causing variations in frequency.
Therefore, the supply voltage is usually made significantly higher than the bulb firing voltage.
This also makes the charging more linear, and the sawtooth wave more triangular.
The resistor R must also be within a certain range of values for the circuit to oscillate.
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 to 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.
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 ''V''
b and the bulb never fires.
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 ''V''
e so the bulb will never turn off.
Small neon bulbs will typically oscillate with values of ''R'' between 500kΩ and 20MΩ.
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.
This will increase the discharge time and decrease the frequency slightly, but its effect will be negligible at low frequencies.
Frequency
The period of oscillation can be calculated from the breakdown and extinction voltage thresholds of the lamp used.
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
A resistor–capacitor circuit (RC circuit), or RC filter or RC network, is an electric circuit composed of resistors and capacitors. It may be driven by a voltage or current source and these will produce different responses. A first order RC ci ...
with the capacitor voltage approaching ''V''
S exponentially
Exponential may refer to any of several mathematical topics related to exponentiation, including:
*Exponential function, also:
**Matrix exponential, the matrix analogue to the above
* Exponential decay, decrease at a rate proportional to value
*Exp ...
, with
time constant ''RC''. If ''v''(''t'') is the output voltage across the capacitor
:
Solving for the time
:
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 ''V''
e and ''V''
b. So the period ''T'' is the interval between the time when the voltage reaches ''V''
e, and the time when the voltage reaches ''V''
b
:
:
:
This formula is only valid for oscillation frequencies up to about 200 Hz;
above this various time delays cause the actual frequency to be lower than this.
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.
The breakdown and extinction voltages of neon lamps may vary between similar parts;
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.
The thresholds also change with temperature, so the frequency of neon lamp oscillators is not particularly stable.
Forced oscillations and chaotic behavior
Like other relaxation oscillators, the neon bulb oscillator has poor frequency stability, but it can be synchronized (
entrained) to an external periodic voltage applied in series with the neon bulb. Even if the external 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 that the period of the oscillator becomes locked to the applied signal.
Interesting behavior can result from varying the amplitude and frequency of the external voltage. For instance, the oscillator may produce an oscillating voltage whose frequency is a submultiple of the external frequency. This phenomenon is known as "submultiplication" or "demultiplication", and was first observed in 1927 by
Balthasar van der Pol and his collaborator Jan van der Mark.
In some cases the ratio of the external frequency to the frequency of the oscillation observed in the circuit may be a
rational number
In mathematics, a rational number is a number that can be expressed as the quotient or fraction of two integers, a numerator and a non-zero denominator . For example, is a rational number, as is every integer (e.g. ). The set of all ration ...
, or even an
irrational one (the latter case is known as the "
quasiperiodic
Quasiperiodicity is the property of a system that displays irregular periodicity. Periodic behavior is defined as recurring at regular intervals, such as "every 24 hours". Quasiperiodic behavior is a pattern of recurrence with a component of unpred ...
" regime).
When the periodic and quasiperiodic regimes overlap, the behavior of the circuit may become aperiodic, meaning that the pattern of the oscillations never repeats. This aperiodicity correspond to the behavior of the circuit becoming ''chaotic'' (see
chaos theory
Chaos theory is an interdisciplinary area of scientific study and branch of mathematics focused on underlying patterns and deterministic laws of dynamical systems that are highly sensitive to initial conditions, and were once thought to have co ...
).
The forced neon bulb oscillator was the first system in which chaotic behavior was observed.
Van der Pol and van der Mark wrote, concerning their experiments with demultiplication, that
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 and van der Mark didn't realize its significance at the time.
See also
*
Relaxation oscillator
*
Schmitt trigger
*
555 timer
*
Negative resistance
Notes
References
* S. O. Pearson and H. St. G. Anson,
Demonstration of Some Electrical Properties of Neon-filled Lamps', ''Proceedings of the Physical Society of London'', vol.34, no. 1 (December 1921), pp. 175–176
* S. O. Pearson and H. St. G. Anson,
The Neon Tube as a Means of Producing Intermittent Currents', ''Proceedings of the Physical Society of London'', vol. 34, no. 1 (December 1921), pp. 204–212
*
{{DEFAULTSORT:Pearson-Anson effect
Analog circuits