October 1970 Popular Electronics
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
Reading this kind of article takes
me back to my days of building prototype circuits as an electronics technician at
Westinghouse Electric's Oceanic Division in Annapolis, Maryland. It was without a
doubt the best technician job I ever had. In fact, working with über engineer Jim Wilson
and a few others is what really drove my determination to get an electrical engineering
degree of my own. Laying out perf boards for resistors, capacitors, inductors, LEDs, switches,
etc., and soldering all the point-to-point connections, then testing (and sometimes fixing
miswirings) was a great gig. Drilling and labeling the project box with the dry transfer rub-ons
in an attempt to make everything look as "real" as possible was always the goal. Prior to
the engineering tech assignment, I spent a couple years building low volume (sometimes just
one or two) specialty systems for the U.S. Navy. It involved a lot of PCB assembly (all through-hole
components), wire-wrapping digital circuit boards, building cable harnesses on nail boards,
and lots of other cool stuff. There were only about a dozen guys on each of the two shifts
doing that work, making the days fly by as random discussions between everyone were held on
just about any topic. The two inspectors were Westinghouse employees and good friends that
also joined in, but man, they never let anything slide for any reason - every solder joint
and W/W had to be perfect, wire bundles exactly routed per drawings with specified strain
relief, and God help you if you accidently scratched the surface finish on anything ;-) Sometimes
I wish I had just stuck with Westinghouse (now Northrop Grumman).
Build an Electrolytic Restorer
Prevent High-Voltage Capacitor Breakdown
By George J. Plamondon
This project is used to restore (reform) the dielectric
in electrolytic capacitors that have not been in use for an extended period of time. Half-wave
rectified ac is switch-selected and applied to the capacitor. As the dielectric re-forms,
the voltage increases indicating a reduction in current flow through the capacitor. Various
reforming rates are available to the builder, as well as various applied voltages from 100
to 600 volts.
When a high-voltage electrolytic capacitor has been unused for too long a time, it is customarily
looked upon as a possible troublemaker. Too often, when power is applied to such units, the
dielectric punctures, destroying the capacitor and probably the associated circuit. Unfortunately,
many people have some of these capacitors in their junk boxes (they were quite common in power
supplies for vacuum-tube circuits), but hesitate to use them. Since they are fairly expensive,
it behooves the electronics experimenter or service man to salvage such capacitors by restoring
the dielectric so that there is no chance of its breaking down when put to use.
However, before finding out how to restore an electrolytic, let's be sure we know the exact
nature of the trouble.
Fig. 1. Although not shown in the photographs, isolation transformer T1
should be used for safety. The voltage range of the reformer is sufficient to effect repairs
on a wide range of electrolytics.
Perf board construction may be used with the operating controls and jacks
mounted on the front panel of the selected cabinet. A TV-type "cheater" connector is used
to make the power connection. Mount the perf board on suitable spacers and be sure that components
on the board do not make electrical contact with any of the front-panel elements.
Be careful when drilling the holes in the plastic front panel as it will
chip easily. The neon indicator lamp is cemented in the hole, other components use hardware.
Insulated wiring is used to make interconnections. If a metal case is used,
make sure spacers keep connections from touching the case.
The finished front panel should be labeled as shown here. A coating of
transparent plastic spray keeps lettering from becoming smeared.
What Is an Electrolytic Capacitor?
An electrolytic capacitor usually consists of two flexible sheets of aluminum foil separated
by gauze impregnated with an electrolyte. Leads are connected to each foil section. The foil
connected to the positive lead has an oxide coating which serves as the capacitor's dielectric.
It is the thickness of this coating that determines the working voltage of the capacitor.
While the capacitor is being used, the oxide coating is preserved by chemical processes
resulting from the voltage impressed across the terminals. Unfortunately, when it is in storage,
time and ambient heat take their toll and the oxide deteriorates. When the full working voltage
is applied to a capacitor whose oxide is weak, the latter breaks down and a short circuit
is placed across the circuit.
Reforming the Dielectric. The dielectric of a suspect capacitor can be reformed by connecting
a low dc voltage across the capacitor and slowly increasing the voltage until the rated value
is reached. This must be done over a long period of time to allow the oxide to reform properly.
The "Electrolytic Restorer" described here does this job automatically, and requires only
an occasional look at a dc voltmeter to check progress. The cost of the project is about $14
if all parts are bought new.
Construction. The prototype shown in the photos was housed in a conventional plastic case
although any type of arrangement will suffice. The schematic of the circuit is shown in Fig.
1. Exact placement of parts is not given since dimensions are not critical and the control
locations can be changed depending on personal preferences.
Most of the circuit can be assembled on perforated board. The front panel controls and
jacks are mounted directly on the case cover, making sure that an leads are long enough to
reach the electronics board. For safety, a 1:1 ac line isolation transformer should be used,
though this is not shown in the prototype.
Operation. The electrolytic capacitor to be reformed is connected to the output jacks,
making sure that the polarities are observed. The positive side of the capacitor is connected
to either J1 or J2 and the negative side to either J3 or J4. The dc voltmeter for checking
the reforming action is connected to the remaining two jacks. Make sure that the polarity
and voltage range are correct. The voltmeter can be disconnected and reconnected at any time
without affecting the operation.
Place S3 in the Discharge position, ping the unit in, and turn on the power. Neon indicator
lamp I1 should glow. Set the desired forming rate on S2 and then rotate S3 to the working
voltage of the capacitor. If the capacitor is unformed, the voltmeter will indicate a much
lower voltage than that set on S3.
Note that the voltmeter indication starts to increase quickly at first, then slows down
as the dielectric forms. The rate of increase is determined by the condition of the capacitor
and the setting of S2. When the Slow setting is used, the operation takes longer but the oxide
formed will be of better quality The opposite is true for the Fast setting. Use the Normal
position for most cases.
When the voltage across the capacitor is approximately equal to the set on S3, put the
switch on Discharge and remove the capacitor. No harm will be done if the capacitor is left
connected longer than required, so it is not necessary to check progress constantly.
To use the unit as a high-voltage, low-current power supply, set the forming rate switch
(S2) to Direct. A current of 4 mA may be drawn continuously, and somewhat higher currents
for a short period of time. (A load current of 10 mA causes a dissipation of 3 watts in the
The Electrolytic Restorer can also be used for a quick go-no-go check of voltmeters. Comparison
of voltage switch settings and voltmeter readings will reveal any gross inaccuracies.
Theory of Circuit Design
Diodes D1 through D4 and capacitors C1 through C4 form a half-wave voltage quadrupler rectifier
with a dc output of approximately 600 volts. Resistors R7 through R16 form a voltage divider
network and S3 selects the desired voltage and applies it to the parallel-connected positive
output jacks J1 and J2. The negative side of the power supply is connected through a switch-selected
resistor network consisting of R4 through R6 to the parallel-connected negative jacks J3 and
J4. The use of S2 determines the forming rate, The Direct position permits the unit to be
used as a high-voltage, low-current power supply. This position can be eliminated if desired.
The Discharge position of W3 places R17 across the output to discharge the formed capacitor,
while resistors R2 and R3 keep a small load on the power supply and discharge the power supply
During the forming process, the capacitor's resistance is low so most of the voltage is
dropped across the limiting resistor. As the oxide coating in the capacitor is re-formed,
less current flows through the capacitor, causing the voltage across it to increase. When
this voltage equals the preset voltage on S3, the reformation is complete.
Posted September 18, 2017