Michael A. Covington    Michael A. Covington, Ph.D.
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When to discard old but unused electrolytic capacitors
Moon and Jupiter
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Edward Van Peenen II, 1940-2019


Word has reached me that one of my early mentors has died. When I was in 7th to 9th grades, 1968-71, Edward Van Peenen helped me use laboratories, the observatory, and the planetarium at Valdosta State College, where he taught physics. This was my first extended acquaintance with a professional scientist, and it was instrumental in launching my academic career.

To see his Valdosta obituary, click here. The color picture above was taken in 1976, when, on a visit to Valdosta, I encountered him bicycling through the town in his usual way. I last saw him in 1985, and my understanding is that by minimizing his cost of living, he had been able to retire in his forties and continue living the solitary, orderly life that he preferred.

May his memory be eternal.


When to discard old but unused electrolytic capacitors


Semiconductors and resistors last forever when not used, but electrolytic capacitors deteriorate with non-use. How fast? Should I throw away my old ones? It used to be standard practice for repair shops to throw them away after two years on the shelf.

The question that came up while I was cleaning up and modernizing my electronics lab. I asked it on two electronics forums and got very divergent answers. So I did some of my own testing.

What follows is a ramble through a lot of information, some of it uncertain. First, some background facts:

  • Many of us have 50-year-old equipment that still works with its original capacitors. The catch is, it hasn't been sitting unused for 50 years. Even charging a capacitor once every 5 years will greatly extend its life.
  • Electrolytic capacitors aren't used as much as they used to be. Nowadays they're used almost exclusively for power-supply filtering. Those big audio coupling capacitors have been superseded by op-amp circuitry, and large capacitors in RC timing circuits have been superseded by digital counters. Even for power supply filtering, there are now 20- and 30-microfarad film capacitors.
  • In the industry, electrolytics are not sold as fresh if they are more than about 3 years old. Since the new equipment built with them also has to have a shelf life, I figure they're actually good for maybe 6 to 10 more years, at least, and this agrees with the specifications I can find, which are short and vague.
  • Of course, you're not going to find measured 10-year reliability for what's being made now; improvements to the process and materials come along every few years. At best, you'll find accelerated aging tests. 1000 hours at 105 C are comparable to 30 years or more at room temperature, depending your preferred version of Arrhenius' equation and the fact that at 105 C, the capacitor is at its thermal limits and experiencing a stress it would not experience at room temperature no matter how much time went by.

    And if the process and materials are continuing to improve, that's actually another reason to distrust old capacitors — not only have they aged, they weren't as well-built in the first place.
  • Some of the best electrolytics now have no date codes, or at least none in the conventional format. That suggests they may have a longer shelf life than the manufacturers actually want to announce, when they are stored at the factory or distributor under controlled conditions.

    Unfortunately, dirt-cheap electrolytics also often lack date codes so they can be sold forever...
  • The industry became afraid of cheaply made electrolytics when a lot of computers failed around 2003-2006 and the problem was traced to cheap manufacturing based on a mis-copied stolen formula. (Strange but true. I'm not making this up!) Nowadays, premium brands such as Panasonic, Cornell-Dubilier, Nichicon, and Rubycon are highly respected.
  • Good electrolytics no longer cost much, partly because of a better supply chain. Instead of $2 at Radio Shack, think 20 cents at Digi-Key or Mouser. I faced a cost of about $15 to completely restock — which I gladly paid — one bad capacitor would cost me more than that in time and frustration.

With all that in mind, I tested about thirty capacitors of various ages. Here is what I now believe, based on those tests:

  • Deterioration of old capacitors is slow and unpredictable. Basically, the percentage of bad ones goes up gradually. I have a lot of vintage-1997 capacitors (22 years old), and most of them pass all tests (after re-forming) as if they were new. A percentage of them do not, and that percentage increases with age. Individual capacitors deteriorate suddenly, while others, the same age, don't show any aging effects.
  • Leakage (electrical) is the most valuable test for old-but-unused capacitors. The deteriorating ones have much more leakage than the healthy ones. The capacitance and ESR don't change much. ESR and capacitance testing are for capacitors that may have deteriorated in use.

    To test leakage, connect 4 things in series: an adjustable power supply (set to the rated voltage of the capacitor), a microammeter, a 1k resistor, and the capacitor itself. You'll see a burst of high current as the capacitor charges, and then, if the capacitor is fresh, the current will settle down within about a minute. If the capacitor is older, it will drop more slowly and continue to improve for five minutes or so. Re-forming the capacitor in this way is important whenever you use an older capacitor. It renews the electrolytic layers.
  • Many capacitors perform much better than specified. For example, Panasonic says that on low-voltage capacitors (100 V or less), maximum leakage is 3 µA or 0.01×C×V, whichever is larger. That means a 100-µF, 20-volt capacitor can leak 20 µA. I seldom saw that much, even in older capacitors; 5 µA was more typical.
  • In my experience, brand is a better predictor of bad capacitors than age. Many capacitors, even major brands, no longer have date codes. But if a capacitor looks old, grungy, or untrustworthy, it is untrustworthy. The most unreliable batch of capacitors that I've encountered was a cheap assortment marketed to hobbyists.

So the bottom line? My current practice is to freely use capacitors up to 10 years old, use them with caution up to 20 or 25 years old, and distrust those that are older. There is certainly no need to toss them when they're just 2 or 3 years old. But note that their appearance has not changed much, and capacitors from the 1980s may look just like new ones if you don't look too closely.

I expect people to write in and tell me I'm wrong. But I expect to be told I'm wrong in both directions (too cautious or not cautious enough), so it will balance out.


Happy birthday, Melody!

Happy birthday, Melody! And thank you for bringing us some mild weather. 100 F on the 3rd, 99 F on the 4th, and 75 F today. How did you do it?

Impeachment is not a football game

Time out for some political advice. Expanding on what I said on Facebook:

Impeachment of the President is not a football game. You are not supposed to pick a side in advance and cheer for it unconditionally. What is going on right now is fact-finding, and you should be ready to adjust your opinion in response to the facts that come out.

I see people doing several unreasonable things:

(1) Making up a story in which their side wins, and believing the made-up story rather than paying attention to reality.

(2) Simply refusing to believe facts that don't support their side, especially if some guy somewhere — quoted somewhere in the news media — makes the same refusal. To a person in this state of mind, one person with a fringe opinion outvotes a hundred who are well-informed.

(3) Assuming that one side can do no good and the other side can do no wrong, or that the slightest hint of wrongdoing on one side makes everything on that side entirely worthless.

Do not be like them.

If you actually have enough information to know for sure how the impeachment investigation should come out, you shouldn't be posting on Facebook — you should be testifying before Congress.


Happy July 96th!

We have started giving the date as July 96th rather than October 4th because July's weather never stopped. October 3 was the hottest October day Athens, Georgia, has ever had, with a high of 100 F. Today is supposed to be the last day of it, thank goodness.

The night sky has been clear with scattered clouds, an odd combination, but last night I grabbed a quick shot of Jupiter next to the moon, and the bright satellites of Jupiter in a straight diagonal line. The bright side of the moon is very overexposed; you can see the earthlit side. You can also see a few stars; leaves and branches of a tree; and a lot of streaks and glare from reflections and diffraction in my lens.


Canon 60Da, ISO 400, Sigma 105/2.8 DG EX lens, 1 second at f/4 on a fixed tripod.

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