Re: Capacitor esr and impeadance?

"Hammy" <spamme@xxxxxxxxxxx> wrote in message
news:h9nv35hef6sud3ta4ch6geaa55mvijrrno@xxxxxxxxxx
How can you determine the esr of a capacitor if all that is speced in
the data *** is the impedance at a specific frequency? No graphs no
nothing.

Sure they do. In fact, they give it to you twice! -- Once in the "impedance"
columns, and secondly via the loss tangent. The impedance columns are
specified at 100kHz, and if you run through the numbers, you'll see that while
an ideal 1500uF capacitor at 100kHz would have an impedance of about a
milliohm, you're told that in actuality it'll be around 17 milliohms at 20C,
so you can assume that the vast bulk of that is a loss resistance (say, 16
milliohms).

As for loss tangent... see here for a discussion:
http://www.radio-electronics.com/info/data/capacitor/esr-df-loss-tangent-q-tutorial-basics.php .
In your case... the loss tangent numbers on that data *** are given at
120Hz; it's ~0.15 for a 25V capacitor. The reactance of a capacitor of 1500uF
@ 120Hz is -j0.884 ohms. Hence, the ESR should be ~0.884*0.15=133milliohms.
That's a lot more than the 16milliohms @ 100kHz, and while the ESR will tend
to increase at lower frequencies, I'm actually surprised by the number.

Perhaps someone who has more experience here can comment on the discrepancy.
:-)

---Joel


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