Introduction

The world of embedded computing is filled with a menagerie of highly versatile and inexpensive components like resistors and diodes.  These systems are well-documented in textbooks and have wide availability on the open market.  Some components such as the integrated circuit are relatively new innovations.  Others—such as the capacitor—have been known for centuries.

But there is a growing list of components that exist in relative obscurity, despite having a lot of potential in the digital age.  This may be because of their novelty, their rising cost, or their rarity.  As a result, these are often overlooked, underutilized, or unexplained in technical literature.

This list will hopefully shed some light on how these underrated devices might function where they have the most potential.  Knowing their strengths and limitations can provide some insight as to how more popular alternates function.  We hope this sparks your curiosity to learn more about the untapped potential that these systems may hold for emerging applications.

Piezoelectric Transformers

These miniaturized transformers are luckily climbing their way out of obscurity thanks to the rise of compact fluorescent lighting in the late 1990s and early 2000s.

Its predecessor—the magnetic transformer—is an effective way to convert low voltage to high voltage, or vice versa.  The technology is well-established, and they are even useful safety devices for isolating parts of a system from each other.  But their large size and weight due to their materials put them at a disadvantage wherever miniaturization is the intent.  These units also generate heat and related efficiency losses due to effects such as eddy currents, flux leakage, and hysteresis within their core magnetic materials.

The piezoelectric transformer is a lightweight and compact alternative to the magnetic variety.

Like the magnetic transformer, piezoelectric transformers contain input (or primary) leads, and output (secondary) leads.  But rather than use electromagnetic coils to transfer energy from input to output; they use electric fields to invoke the piezoelectric effect in a non-conducting, non-magnetic ceramic material.

A piezoelectric transformer will convert an electric signal to mechanical pressure.  To use one, apply an AC waveform to the input of the piezoelectric transformer.  The electrical signal will reach a pair of electrodes that are bonded to the piezoelectric material.  The voltage will create a small acoustic wave that travels longitudinally down the length of the transformer.  This conversion from electricity to mechanical stress is the “converse piezoelectric effect [1].”  The wave continues moving forward to be received by one or more receiving electrodes that convert it back to electricity.  This stage is called the “direct piezoelectric effect.”  This essentially makes it an electrical-to-mechanical-to-electrical converter [2].

The type of piezoelectric transformer featured here is the Rosen type.  It is constructed from a single rectangular piece of piezoelectric ceramic material: possibly Pb(Zr,Ti)O₃ (called lead-zirconate-titanate, or simply PZT).  It may also be a lead-free alternative such as lithium niobate.  When used optimally, the voltage leaving it will be 50 times greater than the voltage entering.  This correspondingly reduces the current by the same factor of 50, just like a magnetic transformer.  There are some small energy losses along the way.  Still, the system is more efficient than a comparable magnetic transformer.  The multiplication factor is roughly proportional to the ratio of the ceramic thickness to its length.

The piezoelectric transformer can achieve higher power density and higher efficiency than its magnetic counterpart [2].  They also provide high levels of isolation due to the insulating ceramic material that is responsible for their function.  These characteristics mean they can outright replace magnetic devices in certain applications.

Electrically, the Rosen type transformer performs at peak efficiency when the signal entering them matches the transformer’s resonant frequency [2].  Typically, this happens around 100 kHz.  So unlike a piezoelectric buzzer, humans will not hear the device operating.  In-circuit simulation frameworks like SPICE, the whole device can be modeled using a transformer and other basic components [3].  Other types such as the cylindrical “Transoner” variation uses radial-mode vibrations rather than longitudinal vibrations, but feature a similar equivalent circuit model.

The piezoelectric transformer is actually popular enough in devices like laptops where high voltages drive cold-cathode fluorescent backlighting for displays [2].  They are also being used in certain switched power supplies and in ion generators.

Still, these devices are relatively obscure and harder to find on the open market.  Optimizing these systems is a multidisciplinary process and only a relatively small population of researchers are active in the field [4].  Integrating them into new or existing designs is also mathematically challenging due to effects such as resonance and load dependence.

Ideal Diodes

It’s fair to say the diode isn’t an obscure component.  These devices have been around for generations.  Today they exist in over 20 different forms [4] and different chemistries: ranging from vacuum diodes, light-emitting diodes, diodes made from silicon, germanium, and selenium just to name a few.  Even raw minerals mined from the earth were used as diodes in early 20th-century radio.

But all of these variations feature a few properties that make them less than ideal.  Namely their forward voltage drop.  When a diode is in forward conduction, the electric charge carriers it conducts will lose a portion of their potential energy.  This wastes electrical power.

Additionally, using diodes alone will not optimally rectify signals whose amplitudes are less than the threshold voltage for the diode itself (approximately 0.7 volts for silicon diodes, and 0.3 volts for germanium diodes) [5].

For example, a vehicle’s alternator may contain high-current silicon diodes arranged in a full wave bridge configuration.  This diode bridge converts AC to DC, but it also reduces the voltage available from the alternator by around 2.2 volts [6].  At these levels, around 15 % of the alternator’s output will be lost to the diode bridge alone.

Luckily, there exists a class of devices called ideal diodes.  The ideal diodes are actually integrated circuits that allow transistors to operate as if they were diodes with an extremely low forward voltage drop.  Using metal oxide field effect transistors (MOSFETs) is efficient and cost-effective thanks to the relatively low voltage drop these devices experience (denoted as the voltage from drain to source v_DS).  This advantage can make them an attractive option for battery charging and other efficiency-sensitive applications.

Dust Filtration

Environmental factors like dust and other airborne contaminants are often an afterthought in the circuit design process.  But these can all wreak havoc on even the most well-designed circuits.

Electronic equipment has known vulnerabilities to dust-induced failures and pollutants [7].  Dust is always present in the atmospheric air but is particularly plentiful in industrialized urban environments [8] [9].  Due to the small size of many of these particles (0.5 µm for particle size centers), even indoor electronics protected by industrialized air-handling equipment may not be fully protected.  The dust problem is especially pernicious for forced-air applications, where cooling fans are constantly blowing air across electronics and heatsinks.  But even devices without cooling fans can collect dust over time through small openings or in their connectors.

Once the dust is able to settle it can potentially absorb moisture from the atmosphere and form a solution.  This process is deliquescence. 

Depending on the types of pollutants present, it could result in the dusty material changing from an electrical insulator to a conductor.  The exact effects depend on a litany of environmental factors such as humidity, temperature, and the type of airborne pollutants present.  Therefore, the exact level of risk is difficult to generalize, and should probably be considered on a case-by-case basis.

Unijunction Transistor

Bipolar transistors contain three layers of silicon: each having different levels of impurities intentionally added.  The three layers each have one ohmic contact connected to three metal leads: emitter, base, and collector.  This type of transistor can act as a simple on/off switch.  They can also amplify by multiplying a signal by an arbitrary amount.

Unijunction transistors have three leads that operate much differently.  One lead operates as an emitter.  The remaining two leads share opposite sides of one single silicon layer.  Effectively, this gives it 2 bases (typically labeled B1 and B2).  One of the bases will be physically closer to the emitter than the other: as indicated by its schematic symbol.  This earned the unijunction transistor the occasional nickname of: “double-base diode.”

Not to be confused with the unipolar junction transistor (a type of field-effect transistor [11] [12]), unijunction transistors have useful properties thanks to a characteristic called negative resistance. Correctly arranged, the unijunction transistor can form a type of signal generator called a relaxation oscillator [13].  Relaxation oscillators can produce a variety of repeating signals from a very small number of inexpensive components.

In the schematic above, the emitters are connected to resistors, and the base is connected to a resistor-capacitor pair.  When the circuit is switched on:

1. The capacitor will slowly charge up through the resistor above it.  The charging time is controlled by the value of the resistor and capacitor.
2. When the capacitor charges up to a threshold voltage, the unijunction emitter switches on and pulls the capacitor’s charge into the transistor and eventually back to the ground.
3. When the capacitor is nearly empty, the emitter will switch off to let the capacitor recharge just like before.
4. These steps continue in an indefinite loop.

Besides being used to switch lights on and off, UJTs were used successfully to provide clock and reset signals to digital state machines and counters [18].

Despite being very good at this application, unijunction transistors never became as popular as ordinary bipolar junction transistors (BJTs) or the various field-effect transistors (FETs).  Many college-level textbooks on semiconductors will only mention them in passing, if at all.  There are a few possible reasons for this:

1. FET and BJT devices are able to switch on and off.  They can also reach active states that are part-way between on and off, meaning they can amplify.  Unijunction transistors can only switch on and off.
2. Relaxation oscillators can be built from other devices, such as the 555 timer.

The market for unijunction transistors is apparently much smaller.  Therefore, the selection of unijunction transistors is restricted.  Still, these devices are still being used to control the switching of more popular devices at regular intervals.

References

[1]	R. E. Hummel, “Electrical properties of polymers, ceramics, dielectrics, and amorphous materials,” in Electronic Properties of Materials, 4th ed., Springer Science+Business Media, LLC, 2011, p. 209.
[2]	E. L. Horsley, M. P. Foster and D. A. Stone, “A frequency-response-based characterisation methodology for piezoelectric transformers,” in 2nd Electronics System-Integration Technology Conference, Greenwich, UK, 2008.
[3]	W. Huang, D. Chen, E. M. Baker and et. al., “Design of a power piezoelectric transformer for a PFC electronic ballast,” IEEE Transactions on Industrial Electronics, vol. 54, no. 6, pp. 3197-3204, Dec. 2007.
[4]	B. M. Wilamowski, “Semiconductor diode,” in The Industrial Electronics Handbook, Fundamentals of Industrial Electronics, 2nd ed., B. M. Wilamowski and D. J. Irwin, Eds., Boca Raton, FL, CRC Press, 2011, pp. 1-10.
[5]	A. Virattiya, B. Knobnob and M. Kumngern, “CMOS precision full-wave and half-wave rectifier,” in IEEE International Conference on Computer Science and Automation Engineering, Shanghai, 2011.
[6]	A. Van den Bossche, S. Haddad, D. Petrov and et. al., “Mosfets used in ideal diode circuits for Lundell alternator rectifiers,” in Thirteenth International Conference on Ecological Vehicles and Renewable Energies (EVER), Monte-Carlo, Monaco, 2018.
[7]	J. D. Sinclair, L. A. Psota-Kelty, C. J. Weschler and H. C. Shields, “Deposition of airborne sulfate, nitrate and chloride salts as it relates to corrosion of electronics,” Journal of the Electrochemical Society, vol. 137, pp. 1200-1205, 1990.
[8]	J. D. Sinclair, L. A. Psota-Kelty, C. J. Weschler and H. C. Shields, “Measurement and modeling of airborne concentrations and indoor surface accumulation rates of ionic substances,” Atmospheric Environment, vol. 24, no. 3, pp. 627-638, 1990.
[9]	M. Tencer, “Deposition of aerosol (“hygroscopic dust”) on electronics – Mechanism and risk,” Microelectronics Reliability, vol. 48, no. 4, pp. 584-593, 2008.
[10]	U.S. Environmental Protection Agency, “Selenium Compounds,” Jan. 2000. [Online]. Available: https://www.epa.gov/sites/production/files/2016-09/documents/selenium-compounds.pdf. [Accessed 28 Sept. 2019].
[11]	R. Ahrons, “Industrial Research in Microcircuitry at RCA: The Early Years, 1953–1963,” IEEE Annals of the History of Computing, vol. 34, no. 1, pp. 60-73, 2011.
[12]	W. Shockley, “A unipolar “field-effect” transistor,” Proceedings of the IRE, vol. 40, no. 11, p. 1365, 1952.
[13]	J. J. Vandemore, I. Geneseo and D. E. Henry, “Timing circuit method and apparatus”. U.S. Patent 3,466,472, 9 Sept. 1969.
[14]	J. R. Anderson, “Unijunction transistor oscillator circuit”. U.S. Patent 3,202,937, 25 Mar. 1963.

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