[SI-LIST] Re: Capacitor surge current
- From: steve weir <weirsp@xxxxxxxxxx>
- To: jrbarnes@xxxxxxxxx, fasig.jonathan@xxxxxxxx,si-list@xxxxxxxxxxxxx
- Date: Wed, 17 Mar 2004 09:20:54 -0800
John, that's good information. In any event, a 63V electroytic cap is far
too low a rating for a 48V supply that is nominally 52V, can run at I
believe 70V for extended times ( NEBS 72 hours as I recall ), and surge to
80V. So applying the more current and generous Mil 198 ratings we get:
70V/80% = 90V for operating, which covers the 80V surge. So, a 100V rating
seems the minimum reasonable value.
Regards,
Steve.
At 12:01 PM 3/17/2004 -0500, John Barnes wrote:
>Jonathan,
>A quick search through 8" of articles/papers on capacitors, and through
>some of the more likely books in my personal electronics library,
>yielded:
>
>* Arsenault, J. E., and Roberts, J. A., Reliability & Maintainability
> of Electronic Systems. Computer Science Press, 1980, page 284--
> maximum voltage stress ratio for aluminum electrolytics is 40% of
> rated voltage.
>
>* Kaiser, Cletus J., The Capacitor Handbook, 2nd Edition. Olathe, KS:
> CJ Publishing, 1995, page 71--"...the sum of the peak AC plus the
> applied DC voltage should never exceed the DC rating."
>
>* MIL-HDBK-198, Capacitors, Selection and use of. Department of
> Defense, 14 July 1999, page 24--"The thickness of the oxide film ...
> determines the maximum peak or surge voltage which may be applied.
> ... For maximum reliability and long life, the dc working voltage
> should not be more than approximately 80% of full rating..."
>
>* MIL-STD-198E, Capacitors, Selection and Use of. Department of
> Defense, 29 May 1984, page 704.1 and 704.5--"For polarized
> capacitors, ... the sum of the applied ac peak and dc voltages should
> never exceed the dc working voltage. ... For maximum reliability and
> long life, the dc working voltage should not be more than
> approximately 80% of full rating..."
>
>* Technical Notes on Aluminum Electrolytic Capacitors. Schaumburg, IL:
> Nichicon, page 10--"Please pay attention so that the peak voltage,
> which is DC voltage overlapped by ripple current, will not exceed the
> rated voltage."
>
>* Understanding Aluminum Electrolytic Capacitors. Rosement, IL: United
> Chemi-Con, 1987, pages 5 & 6--"When in service at voltages equal to
> or below the rated value, the life of electrolytic capacitors is
> affected less by applied voltage than by operating temperature. ...
> Continuous application of excessive operating voltage will rapidly
> increase the leakage current. (See Fig. 6) Internal heating and gas
> generation caused by increased leakage may destroy the capacitor."
>
>* "EEE Parts Derating," NASA Preferred Reliability Practices, No.
> PD-ED-1201. download from
>http://klabs.org/DEI/References/design_guidelines/design_series/1201.pdf
> page 2--"Capacitors...RECOMMENDED DERATING LEVEL Max. of 60% of rated
> voltage"
>
>So commercial practice seems to be not to exceed the rated voltage under
>any circumstances. Military/aerospace practice seems to be to derate
>the voltage rating to 40 to 80% of the manufacturer's rating--and not to
>use aluminum electrolytics at high altitudes or in space because of
>leakage of the electrolyte past the seals.
>
> John Barnes KS4GL, PE, NCE, ESDC Eng, SM IEEE
> dBi Corporation
> http://www.dbicorporation.com/
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