why are electrolytic caps so much worse than in 1981?

Post1: Chunk of CB's text from EW+WW April 1995, page 287.

+---||---+ tsr L | C | o---/\/\---///---+ +---o | Rp | +--/\/\--+

Reduces to:-

esr C o--/\/\-------------||--------o

Rp comprises. a) Dielectric loss due to molecular and interfacial polarisation. b) Leakage resistance measured at dc.

tsr is the true series resistance cause by actual metallic resistances in the component makeup.

2 esr = tsr + Rp/[1 + (w.Rp.C) ].

esr = tan(delta) x Xc.

esr = cos(phi) x |Z|.

Equivalent series resistance tends to reduce with increasing frequency, but by considerably less than the theoretical halving for each doubling of frequency. Ultimately attaining a minimum value when Xc = Xl.

Also in that article is a listing and graphs of the power loss due to harmonics in a 22nF used to snub a rectangular wave of about 320V peak-peak amplitude. It appeared to suffer about 7.4W losses. CB notes that an X7R ceramic capacitor failed quickly.

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Tony Williams.
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Tony Williams
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Post2: Chunk of CB's text from EW December 1997, page 1002.

I have stressed that esr is frequency dependant, but does it really change by a significant amount, or am I simply being pedantic?

Consider the case of a high quality 10nF polystyrene foil/film capacitor. I selected such a device as one of the standard capacitors when building my capacitance bridge. All measurements were taken using a Wayne Kerr precision component analyser. with the test voltage set to 1V. Table 1.

These results show clearly how esr values do change significantly with frequency, for this capacitor. Many writers on this topic have confused these esr and tsr terms. Obviously they differ substantially, except at that frequency when the capacitor is self resonant.

Since correct understanding of esr is essential to avoid over-stressing capacitors, I make no apology for labouring the point.

CB's Table 1, page 998. ~~~~~~~~~~~~~~~~~~~~~~~ Frequency Capacitance (nF) Tan(delta) 'Q' ESR(ohms)

100Hz 9.9982 0.00010 9000 17.0 1KHz 9.9988 0.00005 20000 0.8 10KHz 9.9986 0.00015 6000 0.26 100KHz 10.0000 0.0005 3000 0.05
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Tony Williams.
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Tony Williams

I read in sci.electronics.design that Tony Williams wrote (in ) about 'why are electrolytic caps so much worse than in 1981?', on Wed, 9 Mar 2005:

.... but both ESR and C values in this series circuit are frequency- dependent ....

.... as this shows for ESR. The C value normally varies only very slightly with frequency because Rp is very much larger than Xc.

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Regards, John Woodgate, OOO - Own Opinions Only. 
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