On Mon, 10 May 2010 16:53:58 -0700 (PDT), saber850
wrote:
I'm looking for a cap w/ a lower ESR for the C107 cap (47uF, 50V). I
don't see 'ESR' in the data sheet. What do I look for?
If the ESR isn't specified, then look for loss tangent or dissipation
factor (tan sigma). Dissipation factor does NOT directly translate to
ESR, but it's proportional when comparing caps at the same frequency.
The lower number is better.
http://en.wikipedia.org/wiki/Dissipation_factor
ESR = DF / (Pi * freq * C)
You can calculate ESR, but there's a problem. The dissipation of the
dielectric varies with frequency where the ESR number specified is
only valid at the test frequency. When DF or loss tangents are
specified, it's sometimes at 120Hz, where the capacitor is intended
for a linear type AC power supply filter. When ESR is specified, it's
usually at 100KHz which implies that it's for a switching power
supply. The Panasonic EEU-FC series you included specifies ESR as
impedance at 100KHz. For comparison, a 47uF/50v Panasonic EEU-FC
series cap as below shows 0.6 ohms max ESR.
Would this
one be better:
http://www.mouser.com/ProductDetail/...Hyu04R92 c%3d
Well, for 47uf/50v the Panasonic EEU shows 0.15 DF.
The previous Chemicon KMQ series cap shows 0.12 DF.
I would call the Chemicon capacitor slightly better.
Unfortunately, the sanity check doesn't quite work. The ESR at 100KHz
grinds out to:
ESR = DF / (2Pi * freq * C)
ESR = 0.15 / (6.28 * 0.1*10^6 * 47*10^-6)
ESR = 0.15 / 29.8 = 0.0005
which is about 1000 times too small to be for real. When my brain
recovers from this cold or flu, I'll try to figure out what I've done
wrong. (I hate it when that happens).
Most of these caps were near, if not at, the physical size limit. All
of the 25V caps have = 2mm of space to a neighboring component, and
in 2 cases, it's another cap (so only one could be bigger). So there
isn't much room for these (one will actually be a very tight fit as
is).
Looking at the photos of the board, some of the caps look fairly
tight. I guess you should probably leave it at 25V.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558