In mid-April, Peterson submitted the following inquiry to Technet
     

> Does anyone have an opinion, comments or data to support a rumor I heard?
     
> I'm told that tantalum caps can fail shorted if required to supply a     
> current surge.  To prevent this failure aluminum electrolytics are       
> preferred on digital boards.
     
> I thought Aluminum electrolytics weren't very stable in capacitance over 
> temp and were not preferred to tantalum for that reason as well as size  
> (more Capacitance per volume in Ta than Al).
     
> Gary P.
---
>                                        Gary D. Peterson
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     Ed Nieckula, of Rockwell - Autonetics' Components Engineering Group 
     has kindly submitted the following in response.
          
     I'm sure what you are referring to, or what you heard, is a 
     shortcoming of Solid Electrolyte Tantalum Capacitors. They have 
     historically been known to dislike low impedance circuits where they 
     are subjected to fast rising charge or discharge currents. The 
     problem is somewhat compounded if they are used in parallel. When one 
     in the chain temporarily shorts, it cannot heal the fault site in 
     time to absorb the charge. All the energy in the circuit including 
     the source will then discharge into the failing device and cause a 
     catastrophic failure.
          
     The solution to this low impedance circuit problem has been tantalum 
     foils or wet slug tantalum devices. The wet devices can heal the 
     fault sites quickly and prevent catastrophic shorting. 
          
     Today, unfortunately, tantalum foils are no longer available. The 
     last major source, Sprague Electric, no longer builds tantalum foils. 
     The foil market has been declining for years and Sprague finally 
     decided to toss the towel in. Although somewhat controversial, wet
     slugs are the best bet for low impedance circuit applications. Some 
     applications are using switch mode ceramic multi-stack capacitors. 
     Primarily in high(er) frequency circuits. They are still rather new 
     and have little field history on major programs.
          
     You are correct about aluminum electrolytic capacitors. They need to 
     be vented to prevent overpressure from Hydrogen gas buildup and are 
     therefore subject to loss of water in the electrolyte and eventually 
     dry out. This leads to high dissipation factor(losses) and unstable 
     capacitance. We do not use Aluminum devices in airborne equipment.
          
     A NASA study is underway trying to improve the performance of 
     Aluminum devices primarily for weight savings but I have not heard 
     any encouraging news.
          
     Hope this helps,
          
     Ed Nieckula
     714/762-0702  

    
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     Name:      David T. Novick
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