engineering

Pearson–Anson effect

Pearson–Anson effect is a engineering topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Pearson–Anson effect rather than just read about it. In short: The Pearson–Anson effect, discovered in 1922 by Stephen Oswald Pearson and Horatio Saint George Anson, 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. This circuit, now called the Pearson-Anson oscillator, neon lamp oscillator, or sawtooth oscillator, is one of the simplest types of relaxation oscillator.

Key takeaways

Pearson–Anson effect belongs to engineering; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Pearson–Anson effect to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Pearson–Anson effect from memory before moving on to harder problems.

Reference excerpt

The Pearson–Anson effect, discovered in 1922 by Stephen Oswald Pearson and Horatio Saint George Anson, 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. 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 base generators and deflection circuits of early cathode-ray tube oscilloscopes. Since the development of microelectronics, these simple negative resistance oscillators have been superseded in many applications by more flexible semiconductor relaxation oscillators such as the 555 timer IC.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

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. 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. 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, which is a property of all gas-discharge lamps. Up until the 1960s sawtooth oscillators were also built with thyratrons. These were gas-filled triode 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 base generators in oscilloscopes.

Operation

In the Pearson-Anson oscillator circuit (top) a capacitor 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 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 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 The supply voltage VS must be greater than the bulb breakdown voltage Vb or the bulb can never conduct. Most small neon lamps have breakdown voltages between 80 and 150 volts, so they can operate on 120 Vrms mains voltage, which has a peak voltage of about 170 V. 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 Vb, 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:

… excerpt ends here. Continue reading the full article.

Illustrations

Pearson–Anson effect: NE-2 neon lamp — NE-2 neon lamp

Worked examples

Example 1 — a first encounter with Pearson–Anson effect

Start with the simplest possible case. Write down what Pearson–Anson effect claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In engineering, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Pearson–Anson effect before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Pearson–Anson effect ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Pearson–Anson effect

In research: Pearson–Anson effect appears in engineering research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Pearson–Anson effect in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Pearson–Anson effect is common in secondary-school and first-year university syllabi. It links to neighbouring topics Analog circuits, Electronic oscillators, so understanding it makes those chapters shorter.
In everyday life: Look for Pearson–Anson effect outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Pearson–Anson effect in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Pearson–Anson effect means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Pearson–Anson effect out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Pearson–Anson effect in simple terms?

Why does Pearson–Anson effect matter?

Because it connects several engineering ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Pearson–Anson effect?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Pearson–Anson effect.