Ok Alan,

What do you imagine is happening when you reform an
old electrolytic?

Let's go back once again to an old industry reference on electrolytics.

The 1968, "Reference Data For Radio Engineers", published by ITT. The
articles in it are authored by experts from academia, and industry.

First, a useful definition that provides a clue:
-----------------------------------------------

Forming Voltage (electrolytics): The voltage at which the anodic oxide
has been formed. The thickness of the oxide layer is proportional to
this voltage.

Next a description:
------------------

Aluminum Electrolytics

This is the most widly know electrolytic capacitor and is used
extensively in radio and television equipment. It has a space factor
about 6 times better than the equivalent paper capacitor. Types of
improved reliability are now available using high-purity (better than
99.99%) aluminum.

Conventional aluminum electrolytic capacitors which have gone 6
months or more without voltage applied may need to be reformed. Rated
voltage is applied from a dc source with an internal resistance of 1500
ohms for capacitors with a rated voltage exceeding 100 volts, or 150
ohms for capacitors with a rated voltage equal to or less than 100
volts. The voltage must be applied for one hour after reaching rated
value with a tolerance of +/- 3 percent. The capacitor is then
discharged through a resistor of 1 ohm/volt. [by rated value, they
mean, of course, rated voltage - cfh]

Now a description of the construction of electrolytic capacitors:
-------------------------------------------------------------------------

ELECTROLYTIC CAPACITORS

Electrolytic capacitors (Fig 11) employ for at least one of their
electrodes a "valve metal". This metal when operated in an electrolytic
cell as the

[My attempt at an ascii figure 11 - cfh]

- +
O-----|C|. . . . . . . .|O|A|-----O
|A| . . . . . .|X|N|
|T| Conducting .. .. .. .|I|O|
|H| Electrolyte . . .|D|D|
|O|. . . . . . . .|E|E|
|D| . . . . . .| | |
|E|. . . . . . .| | |

O---------SERIES-R---------+---)|-------+---O
| |
+-Leakage R--+

Note the series R is caused by the leads and electrolyte, etc.
the Leakage R is caused by the oxide layer.

FIG. 11

anode, forms a layer of dielectric oxide. The most commonly used
metals are aluminum or tantalum. The valve metal behavior of these
metals was know about 1850. Tantalum electrolytic capacitors were
introduced in the 1950's because of the need for highly reliable
miniature capacitors in transistor circuits over a wide temperature
range. These capacitors were made possible by improved refining and
powder metallurgy techniques.

The term "electrolytic capacitor" is applied to any capacitor in
which the dielectric layer is formed by an electrolytic method. The
capacitor does not necessarily contain an electrolyte.

The oxide layer is formed by placing the metal in a bath containing a
suitable forming electrolyte, and applying voltage between the metal
as anode and another electrode as cathode. The oxide grows at a rate
determined by the current flowing, but this rate of grwoth decreases
until the oxide has reached a limiting thickness determined by the
voltage. For most practical purposes it may be assumed that the
thickness of the oxide is proportional to the forming voltage.

[ a figure showing some characteristics of oxides...]

The structure of these oxide layers plays an important part in
determining their performance. Ideally they are amorphous but
aluminum tends to form two distinct layers, the outer one being porous.
Tantalum normally forms an amorphous oxide which, under conditions of
a high field strength of the oxide layer, may become crystaline.
Depending on the forming electrolyte and the surface condition of the
metal, there is an upper limit of voltage beyond which the oxide breaks
down. The working voltage is between 25 and 90 percent (according to
type) of the forming voltage at which stable operation of the oxide
layer can be obtained.

To produce a capacitor it is necessary to make contact to the oxide
layer on the anode, and there are two distinct methods of doing this.
The first is to use a working electrolyte that has sufficient
conductivity over the temperature range to give a good power factor.
There are many considerations in choosing the working electrolyte, and
the choice is usually a compromise between high and low temperature
performance. The working electrolyte also provides a rehealing feature
in that any faults in the oxide layer will be repaired by further
anodization.

In aluminum electrolytic capacitors the working electrolyte must be
restricted to those materials in which aluminum and its oxide are inert.
Corrosion can be minimized by using the highest possible purity of
aluminum. This also reduces the tendency of the oxide layer to dissolve
in the electrolyte, giving a better shelf life.

------------------------------------------------------------------------

Notice three things about the above dissertation:

1) The oxide layer is formed before assembly, and its thickness
determines both the working voltage, and the capacitance.

2) The working electrolyte makes contact to the oxide layer, and
also performs a rehealing feature that repairs any faults in
the oxide layer by reanodizing the layer.

3) The working electrolyte will dissolve the oxide layer over time
if no voltage is applied to the capacitor. This is the cause of
shelf life.


I believe this discription. I have personally witnessed it many many
times with old aluminum electrolytics (with wet working electrolytes).

You can continue to think it a myth, but it isn't.

-Chuck Harris, WA3UQV


Alan Douglas wrote:
Hi,
Frank wrote:


I think we can both say with confidence that Ducati got the electrolyte
formulation right, way back two or three decades ago. I didn't measure
any significant change in capacitance or ESR (except ESR went up on a
couple on the last test). If they had a +/- 20% tolerance, they were in
spec at the start and stayed in spec at every step of the test. But I
never noticed a big change when I've checked electrolytics before.


I didn't find any big changes either, measuring high-voltage
electrolytics from as far back as 1947. Looking at your data, within
the limits of experimental error I don't see any changes at all. That
pretty much kills the myth about capacitors reforming to a new working
voltage and changing the oxide-layer thickness.

However I'm sure we haven't seen the last of it. I believe there's
still a website with quotes from Deeley's book on electrolytic
capacitors. What it doesn't say is that this book was published in
1939.

73, Alan