Capacitors - Application Guides for Aluminum Electrolytics

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Capacitors - Application Guides for Aluminum Electrolytics

Post by mediatechnology » Thu Jul 17, 2008 10:07 pm

Here are a couple of my favorite aluminum electrolytic application guides. These always come in handy for non-polar vs. polar discussions.

This is from Cornell Dubilier: ... _Guide.pdf

From Nichicon: ... _Guide.pdf

The I-V characteristics of a polarized capacitor are characterized in this image from Nichicon:

Capacitor I-V Characteristics

I did an experiment a couple of years ago where I took a 100 uF polarized coupling capacitor and bridged it with back-to-back diodes. I don't remember what the load resistance was, probably 1-10K or so, and I could not measure any difference in distortion at audio frequencies with or without the diodes in parallel. Why? Because the terminal voltage developed across the cap was so small that it completely shunted the diodes. The diodes did not conduct.

The above figure is the terminal voltage across the capacitor. In a coupling application this is not the AC voltage applied to it, which, when the reactance is small, is essentially the same voltage on the other terminal. It's only when the reactance of the capacitor becomes large relative to the load that AC terminal voltage develops in coupling applications. The 1V or so reverse bias where leakage current begins to flow would be due to a large DC offset, not the negative (or positive) peak AC potential. The point is we can use polarized aluminum electrolytic* capacitors to AC couple from op amp outputs (with bipolar supplies) in non-critical applications without worrying too much about the capacitor's I-V curves.

*Tanatulms, which are poor performing to begin with, do have high THD and far greater non-linearity.
See: "Picking Capacitors," Part 1, Audio, February 1980 ... tors_1.pdf
And: "Picking Capacitors," Part 2, Audio, March 1980 ... tors_2.pdf

I write this because in a pinch, or when cost is a concern, non-polarized capacitors can be used without major penalty. Where performance vs. cost is no object, non-polarized capacitors are a better choice.

Safety Warning: With non current-limited DC applied across them they darn sure ought to be pointed the right way. Otherwise you'll be picking aluminum foil out of your teeth. Always check for correct polarity in a power supply.

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