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November 2, 2011
Howard Anthony
2024 Hawthorne Avenue
St. Joseph, MI. 49085
Dear Mr. Anthony:
My name is Tucker Leavtt; I am a high school Junior at Animas High School in Durango,
Colorado. I am a novice electrical hobbyist and until recently have been using your model IP-17
DC power supply in my projects and ventures into high-power electronics. I am writing to you to
suggest a change in your product’s design that would make it more suitable for applications in
this area and in other high-stress situations.
I bought your IP-17 power supply intending to use it to power a homemade Marx
generator- a DC circuit consisting of a large capacitor/spark-gap ladder, used to generate short,
high voltage pulses- but was unable to due to a failure in the power supply’s rectifying circuit
during a test of the Marx generator. The failure was caused by a leak in one of the supply’s
electrolytic filter capacitors. The leakage current led to an increase in the voltage across the
adjacent capacitors, resulting in a cascading dielectric breakdown across the whole capacitor
bank. This shorted the output signal to ground and rendered the supply useless.
Such failures are not uncommon among electrolytic capacitors. Their design allows for
high capacitance, relative to plate size, in a small volume, but limits reliability and tolerance. The
power supply used eleven 450 volt, 47 microfarad aluminum electrolytic capacitors in its filter
rectifier, which should ideally function equivalently to a single 4950 V, 4.3 µF capacitor. There
are many other arrangements of real capacitors that approximate these ideal properties, and many
other types of capacitors that behave differently than aluminum electrolytics under strenuous
conditions. Thus, by changing the types of capacitors used and modifying their arrangement, the
reliability of you power supply in strenuous situations can be improved.
The dielectric used in aluminum electrolytic capacitors, aluminum oxide (Al2O3), is an
ionic, crystalline solid with an octahedral geometry. It has a relative permittivity of 10 and a
dielectric strength of 13.4 MV/m.
When aluminum is anodized, a thin layer of aluminum oxide (usually several
micrometers in thickness) is deposited onto its surface. This anodized aluminum can be wrapped
in an electrolyte-saturated paper film (usually, a solution of boric acid or sodium borate is used
as the electrolyte) to form a large, compact capacitor, with the anodized aluminum strip as the
anode and electrolyte-soaked paper as the cathode. The thinner the oxide layer on the anode, the
larger the capacitor, but the smaller its working voltage.
The properties of electrolytic capacitors are very dependent on their operating
temperature. At high temperatures (i.e. higher than 50° C), the water in the electrolyte solution
may evaporate and result in an increase in the capacitor’s effective series resistance. At low
temperatures (i.e. lower than 10° C), the conductance of the salts in the electrolyte declines,
raising the capacitor’s effective series resistance, and the surface tension of the electrolyte
increases, reducing contact between the electrolytic cathode and the alumina dielectric and
lowering the capacitor’s capacitance. These fluctuations in capacitance and resistance due to
temperature severely limit the reliability of electrolytic capacitors.
Additionally, the series arrangement of the electrolytic capacitors in your power supply
exacerbates their disadvantages. Electrolytic capacitors can deviate significantly from their
nominal capacitance ratings. When multiple such capacitors are connected in series, the smaller
capacitors tend to hog more of the voltage across the bank. This leads to an increased risk of
dielectric breakdown and makes capacitors more prone to leakage currents. Generally, series
capacitor networks are eschewed so as to avoid such risks.
Capacitors with plastic dielectrics (e.g. polycarbonate and polypropylene capacitors) on
the other hand, generally have a much higher tolerance and operate more reliably under strain.
The dielectric strengths of polycarbonate and polypropylene range from 15 to 67 MV/m and
19.7-26.0 MV/m, respectively, depending on the plastics’ composition.
Such organic polymers are very stable, and capacitors made with these polymers can be
used over a wide range of temperatures without de-rating. Plastic capacitors have very high
tolerances (less than ± 5%), and their capacitance varies minimally with time and applied
voltage. The dielectrics of plastic capacitors are formed through a solvent casting process in
which the plastics are dissolved in an organic solvent, the resultant solution set into thin strips,
and the solvent is evaporated, leaving the plastics cast in the desired shape. The aluminum
electrodes are then vacuum-deposited onto the cast plastic dielectrics to form the capacitor.
Plastic capacitors are generally available in sizes ranging from 1 nF to 1 µF and voltage
ratings ranging from 300 V to 6 KV, whereas electrolytic capacitors commonly exhibit a larger
capacitance value and a lower nominal voltage rating. Capacitors with plastic dielectrics are
generally cheaper than their electrolytic counterparts due to the ubiquity of polymerized plastics
in the packaging industry; polypropylene capacitors cost around 30% less than electrolytic
capacitors of comparative rating ($3.99 for a 3000 V, 1 μF polypropylene capacitor, $5.49 for a
400 V, 10 μF electrolytic capacitor).
Eleven 400 nF, 5000 V polypropylene capacitors connected in parallel would function
equivalently to the eleven aluminum electrolytic capacitors found in each Heathkit IP-17 power
supply, but using polypropylene capacitors would increase the power supply’s reliability and
decrease its manufacturing cost. I would suggest switching capacitor types and configurations, as
it would help your products better meet the varied needs of your customers, and would be
keeping with Heathkit’s prolific “we won’t let you fail” attitude.
Sincerely,
Tucker Leavitt
Student, Animas High School
Durango, CO 81301
Enclosures: (1)
Technical Exposition of Related Concepts:
Electrolytic Capacitors
The dielectrics of Aluminum Electrolytic Capacitors are made by submerging a strip of
aluminum metal in a solution of weak acid and passing an electrical current through the liquid.
This oxidizes the aluminum at its surface, and forms aluminum oxide according to the following
half reactions:
2Al  2Al3+ + 6e2Al3+ + 6OH-  Al2O3 + 3H2O
The oxygen anions in alumina arrange themselves in a hexagonal close-packed lattice, and the
aluminum cations fill the lattice’s octahedral interstices (Figure 1). In terms of crystallography,
alumina forms in a trigonal Bravais lattice with a space group of R-3c.
Figure 1
Most often, a solution of sodium borate, or borax, is used as the electrolyte in the cathode. Borax
is the most widely produced salt of borate, and reacts readily with many substances to produce
boric acid. When suspended in solution, sodium borate becomes borax decahydrate and can be
described chemically as
Na2[B4O5(OH)4]∙8H2O
The [B4O5(OH)4]2- anion comprising borax salt is depicted below (Figure 2).
Figure 2
Electrolytic capacitors have self-healing properties; the alumina dielectric layer can be
replenished if partially deteriorated by applying a small voltage across the capacitor. The
capacitor will short through, and the leakage current will quickly re-oxidize the aluminum anode
at the area of the breach, replenishing the alumina dielectric.
Conversely, if voltage is applied in the reverse direction, i.e. if the anode goes positive to the
cathode, the alumina dielectric will be reduced and the capacitor will punch through. Significant
short-circuit current will lead to the rapid evaporation of the electrolyte, which will either leak
out of the capacitor’s casing or cause the capacitor to burst. Consequently, electrolytic capacitors
can only be used in DC circuits or for pulse power applications, as one electrode must always be
positive to the other.
Plastic Capacitors
As their names suggest, polypropylene and polycarbonate capacitors are made using
polypropylene and polycarbonate dielectrics. Both substances are comprised of large, organic,
thermoplastic polymers that form into various crystalline geometries. The structural monomers
of each are depicted below; the polymers themselves consist of large numbers of these
monomers covalently bonded to each other.
Polypropylene
Polycarbonate
To manufacture plastic capacitors, the plastics are cast into thin parallel strips or concentric
cylindrical shells. Then, very thin layers of aluminum (≈ 30 nm in thickness) are vacuumdeposited (sputtered) onto the insulating plastic films. The layers of sputtered aluminum serve as
the conducting electrodes, while the plastic film serves as the insulating dielectric (Figure 3).
Figure 3
Plastic capacitors are generally smaller than electrolytic capacitors; the dielectric constants of
polypropylene and polycarbonate are 2.3 and 2.9, respectively, whereas the dielectric constant of
aluminum oxide is around 10. The dielectric strengths of polypropylene and polycarbonate are
much higher than that of aluminum dioxide, however: aluminum oxide has a dielectric strength
of 13.4 MV/m; capacitor-grade polypropylene and polycarbonate have dielectric strengths of
approximately 26 MV/m and 55 MV/m, respectively. Thus, plastic capacitors generally have
higher nominal voltage ratings than do electrolytic capacitors.
Bibliography
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"Capacitors by Dielectric." Cornell Dubilier Capacitors. Cornell Dubilier, n.d. Web. 2 Nov.
2011. <http://www.cde.com>.
Egebo, Hans. "Electrolytic capacitors." Hans Egebo. N.p., 2011. Web. 27 Oct. 2011.
<http://www.hans-egebo.dk/Tutorial/electrolytic_capacitors.htm>.
"Metallized Construction." WIMA. N.p., 2002. Web. 1 Nov. 2011.
<http://www.wima.de/EN/metallized.htm>.
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<http://en.wikipedia.org/wiki/Polycarbonate>.
"Polypropylene." Wikipedia. Wikimedia, 2011. Web. 2 Nov. 2011.
<http://en.wikipedia.org/wiki/Polypropylene>.
"Polypropylene Specifications." Boedeker Plastics. Boedeker Plastics, 2011. Web. 1 Nov. 2011.
<http://www.boedeker.com/polyp_p.htm>.
"Solvent Casting and Particulate Leaching." Wikipedia. Wikimedia, 2011. Web. 2 Nov. 2011.
<http://en.wikipedia.org/wiki/Solvent_casting_and_particulate_leaching>.