Re: Problems with building an ESR meter, part II


"Simoc" <simo.kaltiainen@xxxxxxxxxxxx> schreef in bericht
news:1154023558.750050.220610@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
petrus bitbyter wrote:
"Simoc" <simo.kaltiainen@xxxxxxxxxxxx> schreef in bericht
Btw, you advised to measure AC voltage at the C2/T1 junction, but isn't
it just pulsating
DC voltage there?

No. Have a look at the schematic. A DC current, pulsed or not, should
come
from some place and go to another. Neither the capacitor nor the
tranformer
will let through DC. (That's to say you can't get DC from primary to
secundary in the transformer.)

Pulsed DC goes through a transformer. Just steady doesn't. In some
simple SMPSs there is just a pulsed DC fed to the primary. As the opamp
oscillator outputs just pulsed dc (although it gets connected to gnd
when not voltage there), is it the cap which makes it ac, by
discharging itself through the opamp output when it's low? Took a time
to realize that :-) Am I right now?



Yes and no. Pulsed DC is not only steady DC but can be considered to have a
"pure" DC part and an AC part. The AC part can pass through a capacitor or a
transformer, the DC part can't. So you'll never find a DC voltage on the
C2/T1 junction. Only AC.


I haven't more TL062 chips, nor I have a component store nearby, so
it'd take less
or more time to get a new one. But I have a working TLC272CP in my junk
box. Would

Download the datasheets of both op-amps and compare.

Yes, but I don't know (I haven't skills to tell from the schematic :-)
that what are the requirements of the opamp in this case. Do you know
how to tell? Then I could check them from the datasheets. Those opamps
have, of course, many differences.

You can give it a try anyway.

Yes, but then I'm unable to tell if the opamp is incompatible or if
there is other fault in the circuit :-( Is there some way then to find
out by measurements if the inopereability is due to an unsuitable
opamp?


At first glance the TLC looks like good enough though I can't go too deep
into the datasheets at the moment. But you can try it out without problems I
think. Best instrument for looking at these circuits is an o'scope, alas I
guess you have none available :) But... You can listen! Replace C1 by a 10nF
cap and you can hear the 5kHz by an ordinary cheap earphone. You only may
need to use a series resistor of somewhere between 100 Ohm and 10k to avoid
blowing it. If everything works fine, you can go ultrasone again. Remind,
haste is an enemy. Make sure the power supply is ok. Then, build only the
oscillator, including C2. If you can hear your oscillator, build the Test
leads part, including C3. Sound will be week but usless your earphone is
very insensitive, you will hear it on C3 (test leads shorted.) Next part is
the amplifier followed by the rectifier. Don't forget to replace C1 before
you start ESR measurements.

Can't be sure. The true-RMS may be a disadvantage in this case as it may
contain electronics that does not work at 50kHz at all. A dumb analog
meter
using only a rectifier and adjusted for 50-60Hz sine wave will give some
reading up to in the MHz range. (At least mine does.) Totaly inaccurate
but
enough to find out whether or not a 4MHz X-tal oscillator works.

Thanks for the info. I haven't an analog multimeter, but maybe I could
build an ACV meter from a panel meter and rectifier, sometime when i've
time.

But like John said, there should be about 200mV according to the text
on the Homo Ludens homepage.

200mVdc across R6? I'll eat my hat if someone can make me clear how that
has
been done....

I didn't say DC! :-) (Sorry I quoted too much, that messed up on you)
It's AC, of course, but I just meant to say that it should be 200mV, so
8mV is far too low, unlike you stated...


The oscillator produces some 5Vpp/50kHz triangle. It has an RMS value of
about 1.5V (AC part only, so at the C2/T1 junction.) The transformer ratio
is 20:1 so at the T1/R5 junction I'd expect a 75mVrms which will become
lower when loaded with a 5 Ohm resistance. Measured with a multimeter that's
usually adjusted for 50/60Hz sine, an 8mV looked like acceptable. But I can
of course be mistaken. I guess the 200mV mentioned in the Homo Ludens
homepage is the peak-peak value as seen on an o'scope.


Thanks for the reply.
Simoc


petrus bitbyter


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