Tantalum Capacitors, what I've learned

[Kostas Kritsilas]

(05-07-2021 04:42 PM)Dreato Wrote:  I’ve been an electronics enthusiast for > 50 years, and inhaled a lot of solder flux fumes! Over the years I’ve learned a lot about both passive and active components. Specifically, electrolytic capacitors. I know if you install a conventional electrolytic backwards and apply the rated voltage, they will explode. Like many of you, I found this out the hard way! When installed properly, they have a limited lifetime. In my experience, after about 30-40 years, they can ‘dry out’ or otherwise fail. I’ve always assumed that tantalum electrolytics had a much longer lifetime and were immune to this type of failure. I’ve learned that this is not the case. Full disclosure – I am an electrochemist by training. I am NOT an electrical engineer, and I claim no special knowledge. Some of what I state below may be incorrect. Feel free to make corrections and to relieve me of some of my ignorance!

I assumed that all tantalum electrolytics were ‘dry’, ie: did not contain a liquid electrolyte and were not subject to drying out. This was not true. There are 2 types of tantalum caps, one with dry electrolyte and one with wet electrolyte. I assumed that the wet electrolyte tants would be subject to ‘drying out’ and would have a limited lifetime. This is true. I assumed that the dry electrolyte tants would have a longer lifetime. This is NOT true. My experience has shown me that they have the same 30-40 lifespan that all the other types of electrolytics do.

HP uses both dry and wet electrolytics in their calculators. They cost more that traditional electrolytics, but can pack more capacitance in a smaller space. Tants have a lower series resistance, too. This is important in switching power supplies and digital circuits.
Recently I have been working on some HP25 calculators and replacing the dead ACT chips with the 25E ACT from Panamatik (highly recommended). Even after replacement, the calcs were still dead or intermittent. A quick check showed that the internal switching power supplies were not working properly. I actually found a cracked/broken resistor in one! (replaced and now works 100%), but most often I found bad tantalum capacitors. Rather than replace them one by one, I ordered the full complement of tants from eBay sellers (60uF 10V axial, 22uF 15V axial, 2.2uF 10V radial) and replaced them all. This revived each calculator. In EVERY case, the tants I removed tested good in my capacitance meter. EVERY case! I should note that some HP25C calculators use a different style capacitor (radial instead of axial) on the board, and later woodstocks use a completely different power supply daughterboard, which uses a couple of the old style, cheaper, electrolytics.

In another case, I had 2 HP97s with intermittent or bad card readers. One gave intermittent read errors, the other would stop pulling the cards part way through, both showing ERROR. CLX cleared the error and complete the card feed process. Teenix, a very helpful forum member, suggested checking the tantalum caps on the card reader board. I ordered these (22uF 10V radial, 6.8uF 10V radial, 3.3uF 10V radial) and swapped them all. Both readers started working 100%. Again, all the removed tants tested 100% GOOD on my capacitance meter. Actually, I used 2.2uF caps rather than 3.3uF, since I had them from my HP25 power supply adventure.

Why did these tantalum caps test good but still fail in the circuit? I don’t know. I do know that next time I open a classic or Woodstock series or HP97 calculator, the tants are all gonna be swapped.

Questions, comments or corrections are welcomed.
Best Regards,
Dave

Capacitance testers do what they say they do, they test capacitance. What they don't do, is test whether a capacitor will work in a circuit. To be more specific, they test at one frequency (usually), and with one applied voltage. That is most often not reflective of what the circuit the capacitor will be used in. What is not really mentioned often, is that varying the frequency and applied voltage (ac or dc) will produce a different capacitance value, and that may be significantly different from what the capacitance meter shows. Most of the lower end capacitance meters also do not test for leakage, which is a pretty good indicator of how a polarized capacitor is doing.

Most tantalum capacitors, whether liquid electrolyte or solid, are spectacularly intolerant of reverse voltage, to a far higher extent than other electrolytic capacitors. While it may take a few seconds for a regular electrolytic capacitor to fail when reverse biased, the failure on a tantalum is instant, and sometimes quite noticeable.

Many times these days, you can find multi-layer surface mount capacitors that are in the lower range of tantalum and electrolytic capacitors (up to about 10u), and are in a smaller form factor, and often at a higher working voltage. You do have to work out the connection type, but with the smaller form factor, this is not a big issue.