Aluminum electrolytics have very high volumetric efficiency, indicating a high capacitance times voltage (CV) factor for their physical size in comparison to ceramic, mica and the various polymer film capacitors. Tantalum and niobium electrolytics are other parts with somewhat higher volumetric efficiency but are more expensive, generally have had longer lead times, and some will say lower reliability. This article will concentrate on aluminum electrolytic parts due to their low cost, wide and consistent availability, and their use in very large numbers in consumer, industrial, automotive and even military electronics.
Despite these benefits, aluminum electrolytics are sometimes shut out of some applications such as accurate timing circuits and low current consumption devices because they leak. The reasons for their size advantage are closely linked to the reasons they exhibit relatively high leakage current.
Why is the volumetric efficiency so high and how does this relate to leakage?
Consider that a capacitor is two plates separated by a dielectric and the equation for capacitance: (C) = permittivity (ε) of the dielectric times surface area (A), divided by distance between the plates (d). The distance between the plates in our capacitor is the thickness of the dielectric plus “tissue paper” soaked in electrolyte. In the construction of an aluminum electrolytic capacitor, the dielectric is a very thin layer of anodization which is aluminum oxide. Prior to anodizing, the aluminum is etched, effectively increasing the aluminum’s surface area.
The dielectric is thin at well under one micron, the permittivity is high (about 10) and the surface area is high due to the coil-wound nature of construction. Each term in the equation is thus optimum to attain high capacitance in a small package.
Why electrolytic capacitors are leaky
Put simply, the plates are extremely close together and their surface area is large.
United Chemi-Con is a very large aluminum electrolytic capacitor manufacturer. Their description of the causes of leakage is here:
“The dielectric of a capacitor has a very high resistance which prevents the flow of DC current. However, there are some areas in the dielectric which allow a small amount of current to pass, called leakage current. The areas allowing current flow are due to very small foil impurity sites which are not homogeneous, and the dielectric formed over these impurities does not create a strong bond. When the capacitor is exposed to high DC voltages or high temperatures, these bonds break down and the leakage current increases. Leakage current is also determined by the following factors:
- Capacitance value
- Applied voltage versus rated voltage
- Previous history
"The leakage current is proportional to the capacitance and decreases as the applied voltage is reduced. If the capacitor has been at elevated temperatures without voltage applied for an extended time, some degradation of the oxide dielectric may take place which will result in a higher leakage current. Usually this damage will be repaired when voltage is reapplied.”
A capacitor’s leakage current is usually expressed as a proportion of capacitance multiplied by the voltage applied to it, with a maximum current also listed. For example, “Leakage current equals .01 CV or 3uA, whichever is greater.” The square root of CV multiplied by a factor depending on temperature has also been used.
Measuring leakage current
Capacitor leakage testing can be done with an electrometer with a built-in power supply. A Keithley 6517A is a typical example. These meters will measure very low currents. In fact, this meter’s lowest range is only 20pA full-scale. The electrometer type meter is expensive, and for electrolytic capacitors is entirely unnecessary. However, one should be considered if leakage testing of ceramic, mica or film capacitors will be required.
Accurate measurements of leakage current of aluminum electrolytics can be accomplished with a regulated adjustable power supply, a calibrated digital multimeter (DMM) and a sensing resistor. In deciding the value of the resistor, the value should be small enough so the capacitor will fully charge in less than about a minute. The value should be large enough so the DMM can read the voltage.
For capacitors under about 5,000 uF the leakage will be some number of microamps. Above this value it may be in the milliamp range. For a 100 uF capacitor, a 10 K to 20 K ohm resistor would be appropriate and readable with most DMMs using the millivolts range. Connect the resistor and capacitor in series, connect them to the power supply, and connect the meter across the sensing resistor using high quality test leads. Keep the leads clean to ensure there are “zero ohm” connections that cannot develop a small voltage of their own.
With the power supply set to zero, turn it on then turn its output voltage to the voltage required for testing; for example if one is using a 25 V capacitor in a 15 V circuit, use 15 V. After a period of time (manufacturers often use one, two, three or five minutes), read the meter and use Ohm’s law to calculate the leakage current. Normally the input resistance of the DMM can be ignored but this should be verified. If as usual the input resistance is 10 megohms, it can be ignored.
Measurement variations need to be mentioned here. Leakage current will drop with time, in the short term. This is, in part, because the capacitor stops charging asymptotically, reaching “full” charge after many time constants. Another reason is as mentioned above, the capacitor will “repair” some small defects in the dielectric oxide layer that have built up, especially after long periods of storage and non-use. If the capacitor has been in storage, it is appropriate to use a period longer than five minutes if the leakage is dropping. Finally, leakage current will rise with temperature. If the capacitor is being tested for use in a circuit where it will be warmer than the test lab temperature, this needs to be considered. A Nippon Chemi-Con technical note indicates that from 20° C to 85° C, the leakage will increase by a factor of about four.
About the author:
Terry Conrad has performed 32 years of research, product design and technology management. He has been employed in quality and reliability engineering, design engineering, project management and as president in consumer electronics and military acoustic products design and production firms. Terry is currently an independent consultant, primarily in the fields of acoustics, batteries and component engineering. His clients have included General Dynamics, Molex, Lockheed Martin, Thomson-RCA and many small companies. He can be reached at tconrad@sealandinnovations.com
