why are electrolytic caps so much worse than in 1981?

[Tony Williams]

EW April 1995, page 287. "Power dissipation in capacitors".

[/QUOTE]

can you please scan & post it to ABSE? or provide a summary?

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.

 

[Tony Williams]

can you please scan & post it to ABSE? or provide a summary?

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

 

[John Woodgate]

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

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

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

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) ].

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