On Tue, 25 Jan 2011 13:40:21 +1100, "Phil Allison"
wrote:
"Jeff Liebermann"
If you have 1 volt of output swing, and an assumed function generator
output impedance of 50 ohms, the short circuit current will be about
20ma. Shove that into a 0.5 ohm electrolytic cazapitor, and you'll
measure maybe 10mv across the cazapitor. That's kinda hard to see.
** What kind of POS scope do you have ???
Tek 2247A. I have several others, but that's the one I like to use.
http://802.11junk.com/jeffl/pics/home/slides/BL-shop6.html
10mV rms = 28.3 mV p-p.
Read what I wrote again. I said "1 volt of output swing" which means
1 volt peak to peak. Sorry for not being absolutely clear. Also, all
the ESR test circuits I've seen use a square wave, which is normally
not measured in RMS units.
My scope goes down to 5mV per division - then has a 5X mag switch if
needed.
Mine goes down to 2mv/div. About 1/3 of that is noise. My guess(tm)
is that anything I measure has a built in 1mv error due to this noise.
I think the X5 or whatever magnification on your unspecified model
scope usually refers to the horizontal sweep, not the vertical gain.
Of
course you could supply more than 1 volt but then you will have some
problems trying to use this technique without removing the cazapitor
from the PCB.
** Utter ******** !!!
As your own example proves.
How so? If the cazapitor happens to appear across a semiconductor
junction, as will happen with the output of an integrated switching
regulator with a reverse protection diode on its output, the
protection diode is going to conduct if fed more than about 0.6V peak
to peak. I think (not sure) that the original Bob Parker meter ran at
about 80mv peak to peak to avoid this manner of problem.
For what it's worth, I did it this way until I bought an ESR meter.
Later, I decided to actually measure the output impedance of my
function generator. The data sheet said 50/75 ohms, but it was
anywhere between 30 and 100 ohms depending on the position of the dial
and output step attenuator.
** Just set it to max and that's it for ESR testing.
Lets just say I don't like running my test equipment flat out into an
AC short circuit (the cap under test). My cheapo Leader LFG-1300S
function generator has some kind of overcurrent protection circuit on
the output, that limits the square wave output swing, but also mangles
the waveform.
If you look carefully at the schematics supplied below, most of them
put a known series resistance at the output of the generator section
to the cazapitor. You could do the same thing with the function
generator. 100 ohms in series with an alleged 50 ohms will not
magically make the method more accurate, but it will reduce the error
to a tolerable level.
** More ********.
The vast majority of electros have less than 1 ohm ESR values - so any
error in the calculated drive current is TINY in comparison to a 50 ohm
source.
True, except that the variation between what's considered a good and a
bad electrolytic is sufficiently small that source impedance makes a
difference.
http://members.ozemail.com.au/~bobpar/2003esrchart.txt
(Notice the odd variations across the voltage ranges).
I find myself working at the bitter edge of some of the recommended
maximum ESR values all too often. I sometimes have to flip a coin to
decide if the cap is good or bad. The last thing I need is a blurry
oscilloscope trace or an undefined generator source resistance.
The variation in ESR values between new electros of the same type is far
more.
Huh? I've taken bags of brand new electrolytic caps and tested them
to see if there was any way to predict which ones would fail and which
ones would survive. For a given value, voltage, and temperature,
they're all about the same. I see wide variations in value (i.e. +80%
to -20%), but the initial ESR just follows the value. However, after
running a crude accelerated lifetime test on the caps (high ripple
current and high temperatures), I find the good and the bad caps
diverging in ESR.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558