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Electronics Forum / Basics / July 2006Problems with building an ESR meter, part II
Sorry for reposting, so I posted earlier a thread "Problems with building an ESR meter", http://groups.google.fi/group/sci.electronics.basics/browse_thread/thread/a2a2bc 5b2520bed9/909e2ffec4e9986f?lnk=arm (Question is about a home-made ESR meter, documentation here: http://www.ludens.cl/Electron/esr/esr.html ) John O'Flaherty wrote: > If the oscillator is running you should have an average voltage of > about 1/2 Vcc on the plus input, the minus input, and the output. Well, now I measured those and got: Output: 2.65 V - Input: 2.65 V + Input: 2.48 V Vcc: 4.95 V So it seems to be oscillating, although I'm a little confused about that all of them are a little above 1/2 Vcc...but might that just be an error of DMM in measuring average of pulsating DC? > If the oscillator and transformer are ok, and you > short the test leads, you should get (per the article) around 200 mV > across R6. On DC range, DMM read just 0 mV...but is it AC or DC across the R6? On AC range, the DMM read 8 mV. Across the R5 I got the same readings. And btw, if it's AC voltage across the R6, does the DMM give any reasonable readings, as it's high frequncy square wave? (as AFAIK usually the AC range of DMM's is optimized for 50/60Hz sine wave) How could I continue troubleshooting now? Thanks in advance. Simoc > Sorry for reposting, so I posted earlier a thread "Problems with > building an ESR meter", [quoted text clipped - 33 lines] > Thanks in advance. > Simoc Well, The readings of the oscillator fall in the expected range. You may try the to measure the AC-voltage at the C2/T1 junction. The reading will not be accurate due to both frequency and waveform but I expect you to find some 2Vac. If you find only some mV at this point I'd become suspicious as to measure only 50/60Hz hum. You'll never find any DC across R6 (unless you made a grave wiring error, have a defective cap or some other serious howler). This part of the circuit only has very small AC voltages and the 8mV you found, is in the expected range. The amplifier, U1B, should have Vcc, so 4.95V as the op-amp share the power pins. Pin 5 should be at Vcc/2. Take or leave one, maybe two, tenth of a Volt depending on the accuracy of R9 and R10. For measuring the output voltage of this amplifier short the test leads and temporarely disconnect C6 at the diode side. Measure between the loose end of the cap and ground. Should be some 300mV I guess. If no voltage, most likely your amplifier has gone, but there are still possibilities like bad solder joints, shorts or other defective components. After reconnecting C6, you can measure the DC across C7. Should be some hundreds of mV. petrus bitbyter > The readings of the oscillator fall in the expected range. You may try the > to measure the AC-voltage at the C2/T1 junction. Thanks very much for your info. I measured there, and there wasn't any voltage. But one fault found...the opamp was bad now! As I took it apart and wired up for testing, I got readings of 2V - 3.3V on outputs and inputs when nothing of them was connected. So maybe they were just steady DC voltages that I measured on output and inputs of the oscillator section of the opamp. When I connected Vcc to plus input or gnd to minus input, the output went high, like it should, BUT when I connected Vcc to minus input or gnd to plus input, the output went high too! Btw, you advised to measure AC voltage at the C2/T1 junction, but isn't it just pulsating DC voltage there? 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 it work on this circuit? Although probably there is still some other mystic fault in the circuit as earlier when I was attempting to troubleshoot it, the opamp was ok when I tested it then. > The reading will not be > accurate due to both frequency and waveform but I expect you to find some > 2Vac. If you find only some mV at this point I'd become suspicious as to > measure only 50/60Hz hum. BTW, my DMM is true-RMS, so does that mean that it should give correct readings of the effective value regardlessly to the waveform? Although when I took a look on its manual, there was rated a frequency range on AC voltage measurement up to 1kHz...but do you think that it's readings will be reasonable enough to measure these 50kHz voltages? (although it of course won't be as accurate) > You'll never find any DC across R6 (unless you made a grave wiring error, > have a defective cap or some other serious howler). This part of the circuit > only has very small AC voltages and the 8mV you found, is in the expected > range. But like John said, there should be about 200mV according to the text on the Homo Ludens homepage. Thanks. Simoc >> The readings of the oscillator fall in the expected range. You may try >> the [quoted text clipped - 18 lines] > 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.) > 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 Don't know. Download the datasheets of both op-amps and compare. You can give it a try anyway. > it work on this circuit? Although probably there is still some other > mystic fault [quoted text clipped - 16 lines] > voltages? (although > it of course won't be as accurate) 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. >> You'll never find any DC across R6 (unless you made a grave wiring error, >> have a defective cap or some other serious howler). This part of the [quoted text clipped - 4 lines] > 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. More likely you'll find it across C7 although I'd expect somewhat more. Look at the schematic again. Test leads shorted, no battery involved. Where should the DC current come from to cause that voltage? Capacitors will not conduct DC so as far as DC is concerned, the circuit ends at C2 resp. C3. > Thanks. > Simoc petrus bitbyter > "Simoc" <simo.kaltiainen@sunpoint.net> schreef in bericht > > Btw, you advised to measure AC voltage at the C2/T1 junction, but isn't [quoted text clipped - 5 lines] > 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? > > 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? > 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... Thanks for the reply. Simoc >> "Simoc" <simo.kaltiainen@sunpoint.net> schreef in bericht >> > Btw, you advised to measure AC voltage at the C2/T1 junction, but isn't [quoted text clipped - 14 lines] > 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 [quoted text clipped - 14 lines] > 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 [quoted text clipped - 18 lines] > 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 > 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. Yes, I meant that in flyback-type switchings PSs and regulators, pulsed DC is used there, having "only the AC-component" being still DC as its polarity never gets reversed. But that's off-topic since I realized that the cap, by charging and discharging in turns, makes it "true" AC (going in reverse direction, too). Was I right? > So you'll never find a DC voltage on the > C2/T1 junction. Only AC. Agreed, if my cap-making-it-AC -theory was right :-) (So was it?) > At first glance the TLC looks like good enough though I can't go too deep > into the datasheets at the moment. I could look the datasheet myself, but the problem is just that the requirements of this circuit aren't told there ;-) > 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 :) Correct guess :-) > But... You can listen! Replace C1 by a 10nF > cap and you can hear the 5kHz by an ordinary cheap earphone....... Oh, I'm impressed! Very nice trick, I have to try it, thanks. > The oscillator produces some 5Vpp/50kHz triangle. Isn't it square? > I guess the 200mV mentioned in the Homo Ludens > homepage is the peak-peak value as seen on an o'scope. Maybe... Thanks for the reply. Simoc >> Yes and no. Pulsed DC is not only steady DC but can be considered to have >> a [quoted text clipped - 42 lines] > Thanks for the reply. > Simoc You're right for the AC going through the capacitor. You're right for the square as well. The triangle is on the minus input of U1A. Which makes my calculations worthless. Hope you nevertheless got the idea. petrus bitbyter |
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