On Sat, 7 May 2016 18:18:16 -0700 (PDT), Phil Allison
wrote:
Jeff Liebermann wrote:
Phil Allison
** You went to lot of trouble to see the obvious.
Cap-B was one that had failed with high ESR. There was plenty of data
and anecdotal evidence on how a normal capacitor would act. I wanted
to see how a defective capacitor acted.
** Well, that depends how *defective* it actually is !!!
Well, cap-B was labeled 2200uF 10v and was bulging at the top.
Room temp ESR was measured at about 2.3 ohms:
http://802.11junk.com/jeffl/Electrolytic-cap-test/Cap-B.jpg
and should have been about 0.1 ohms according to the latest chart for
my ESR meter:
http://802.11junk.com/jeffl/crud/ESR.txt
It's the 2nd from the left in:
http://802.11junk.com/jeffl/Electrolytic-cap-test/caps.jpg
and shows no signs of leakage except for the bulging top.
An electro that has lost nearly all of its fluid and reads
off-scale on Bob Parker's meter is not likely to come good
with a bit of heat.
True. However at 100C, the ESR dropped to 0.25ohms which is not
perfect but would probably be adequate for most purposes.
http://802.11junk.com/jeffl/Electrolytic-cap-test/Cap-B.jpg
I also wanted to see how the
ESR meter functioned with small value and size (0.22uF 50v) caps,
which was Cap-D. The change in ESR with temp was far less radical
than the others.
** Even a 0.22uF film cap reads 7 ohms on the Bob parker meter -
IOW the impedance of the thing at 100kHz. Bob's meter does not
read actual ESR values, rather it reads impedance vales at 100kHz.
Right. The impedance is the vector sum of the resistive (ESR)
component and the capacitive reactance of the capacitor. At 100KHz:
Xc = 1 / (2*Pi*MHz*uF) = 1 / 2*Pi * 0.1 * 0.22 = 7.23 ohms
If the ESR were the typical less than 1 ohm, the bulk of what the
meter is reading is from the capacitive reactance.
The meter was reading exactly 7.00 ohms, I can get a rough idea of the
resistive ESR from:
R = sqrt(Xc^2 - Z^2) = sqrt(7.2^2 - 7.0^2)
= sqrt(52.3-49) = 1.8 ohms
However, let's pretend that this capacitor was defective and it
exhibited a higher ESR, such as 6 ohms. What would the ESR meter
read? Same formula:
Z = sqrt(R^2 + Xc^2) = sqrt(6.0^2) + 7.2^2)
= sqrt(36+52.3) = 9.4 ohms
You would be able to see the difference between a good 7.0 ohm reading
and a bad 9.4 ohm reading, but only if you did the calculations in
advance. I once played with an HP/Agilent LRC meter that did all this
automatically. it would be nice if the next generation of ESR meters
could also do that.
Raising the frequency to 1MHz would reduce the capacitive reactance to
0.723 ohms, which would produce something closer to the real ESR.
However, the ESR changes with frequency, so accuracy will suffer.
This *fact* is pointed out in the instructions and the lowest
value electro you can reliably test is about 1uF - as shown on
the front panel table.
A man has gotta know the limitations of his ESR meter.
I just hate reading the instructions. However, the 1uF lower limit is
printed on the front of almost every ESR meter. My Capacitor Wizard
has it boldly displayed on the front panel.
http://www.howardelectronics.com/images/thumbnails/1138/1138/detailed/1/CAP-WIZARD.jpg?t=1440164290
All I did was try a few 450V electros, heat them with a hot air
gun until they were darn uncomfortable to hold and note that in
every case the ESR reading had plumeted by 5 to 10 times.
I wasn't interested in high voltage electrolytics.
** They have higher ESR values than low voltage caps, so there is
plenty of room for ESR readings to come down without approaching
the low reading limit of Bob's meter.
I used a value of 47uF, so its reactance at 100kHz was negligible.
Pays to think ahead, you know.
Yeah, but I think you missed my point. I don't do tube amplifiers and
therefore do not run into too many high voltage capacitors. The one's
I see most of are low voltage and high capacitance electrolytics as
found on PC motherboards, ATX power supplies, LCD monitors, wall
warts, and various consumer electronic devices.
Incidentally, speaking of tubes (valves), you might be amused by this:
https://www.kickstarter.com/projects/1815752970/hybrid-tube-amp-for-the-raspberry-pi
.... Phil
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558