PSN-L Email List Message

Subject: Re: VBB displacement / capacitors
From: ChrisAtUpw@.......
Date: Sun, 6 Feb 2000 12:11:51 EST

In a message dated 05/02/00 20:32:13 GMT Standard Time, sean@........... 

<< The biggest problem in using a large Cp is that non-polarized 
electrolytics are not acceptable for the feedback components. They are very 
temperature sensitive in regard to value and leakage. Polypropylene or 
polystyrene are needed. The leakage of these is 10^5 megohm-microfarads or 
more; electrolytics have a value as little as 1, and being electro-chemical 
cells, are very temperature dependent. And since the dielectric layer is 
formed by the imposition of voltage, it degrades with time, or the actual 
capacitance varies with the voltage and frequency.... Years ago, I got into a 
discussion with Mr. Guralp about using tantalum capacitors in the feedback.  
There was a problem with instability with 
 temperature change as the Cp change made the feedback unstable.

    You and Mr.Guralp may have been using different components. The original 
'Wet Tantalum Electrolytics' that were marketed in the UK had virtually NO 
leakage. They were sealed metal cans with conc. Sulphuric acid inside. I 
charged a 100 muF up to 10 V, measured it with an Keithley Electrometer and 
left it overnight. It read over 9.9 V next morning. Then I believed it! We 
used them in long period integrators in glider flight computers. Then solid 
tantalums came along which had leakage's comparable to Al. foil 
electrolytics, but were much less costly and the wet tantalums became 

    Regards Chris Chapman 


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Larry Cochrane <cochrane@..............>