VK5JST's ESR Meter project  - Page 1
Forum section: electronics / beginners

author: Gcsillaz
author_rank: Contributor
timestamp: April 10, 2021, 05:02:47 pm
content: Hello everyone!I'm trying to build the VK5JST's meter(On the forum there is a topic on the schematics) on my breadboard but i can not see the 6V swing on the colector of T3.I only see a ~100mV square wave.Schematic attached.I'm using the same components and everything seems to be ok. Can someone help me troubleshoot the problem?Thanks!

author: SuzyC
author_rank: Frequent Contributor
timestamp: April 10, 2021, 09:35:00 pm
content: Most likely reversed backwards wired C-E of T3 , or maybe T3 is not the correct transistor/PNP transistor.Also possible R6 is not 150-ohm as it should be. Check with ohm meter all resistors in this oscillator.

author: Gcsillaz
author_rank: Contributor
timestamp: April 10, 2021, 09:49:30 pm
content: Quote from: SuzyC on April 10, 2021, 09:35:00 pmHello SuzyC,I've looked all of them and T3. I'm even using 1% resistors and changed T3. All the same :/In the document i'm attaching now it explains how the circuit works. On the second paragraph it says that without the DUT connected there is a 110mV swing on the bridge... This made me just continue to build until everything finishes to test.Also, when connecting the 9V battery the -4.4V rail goes to 0V, when i disconect it jumps to -4.4V and begins discharging... Whats that all about?Thanks in advance!

author: Gcsillaz
author_rank: Contributor
timestamp: April 10, 2021, 10:06:28 pm
content: And just o it gets more "visual"Follow below what i'm seeing on the colector of T3.

author: SuzyC
author_rank: Frequent Contributor
timestamp: April 10, 2021, 10:18:39 pm
content: If you are seeing the correct waveform(squarewave)..and you are, the osc. circuit is working perfectly.I can't tell the time/div or Zero-V baseline or V/Div from this photo, but the waveform shape is 100% correct.This leaves wrong R8-R9 resistor values else not a connection of T3 collector to only pin9 of the 74HC04 and the junction of R8-R9 as likely causing trouble.Are you counting pins wrong on 74HC04?Check R8=R9 are 270-ohms ea. Their junction must only connect to collector of T3. Check the resistance of this T3 collector junction to ground(Power Off!), should read approx. 140 ohms.If you somehow put in 27-ohm resistors instead of 270-ohms you would get the incorrect waveform voltages you now see.The minus voltage burp you see is correct upon turnon.

author: SuzyC
author_rank: Frequent Contributor
timestamp: April 10, 2021, 10:48:26 pm
content: You should see 110mV at the DUT connection point, but 110mV at the input pin 9 of the 74HC04 would not work!

author: Gcsillaz
author_rank: Contributor
timestamp: April 10, 2021, 11:17:40 pm
content: Quote from: SuzyC on April 10, 2021, 10:18:39 pmI can see the correct waveform on the input to the differential amplifer... the 100mV(pin 5 and 6). But strangely the output(pin7) is show a +-5V outpout waveform attached. This brings L2 to always being lit, even tough nothing is connected...Regarding the picture before it is with .2mV/divI checked both r7 and r8, they are fine, pins on the 74hc04 is right also...

author: Gcsillaz
author_rank: Contributor
timestamp: April 10, 2021, 11:20:02 pm
content: Also what i'm not understaind is why i'm not getting the -4.4Vdc on the lower rail and the 0-6Vdc swing at collector t3.This is harder than i imagined xD

author: SuzyC
author_rank: Frequent Contributor
timestamp: April 10, 2021, 11:44:47 pm
content: If you are getting a +/- 5V peaks at the output of IC3 pin 1,that is impossiblewithout a -5V lower rail.No negative rail..no negative op-amp output seen on scope.Are you using AC-coupling on the scope? Is the middle line of the graticle your zero-V reference lilne?At most, even when working right, the output pin1 should swing low but might only reach a few hundred mV less negative than -5V peak due to spec'd output voltage limitations of the op-amp and the negative rail measuring only -4.4 DC when everything is working right.What is the +6volt supply measuring? What is the -supply measuring?What voltages do you see at 74HC04 pin 1 and at pin2 and pin14 and pin 7?

author: SuzyC
author_rank: Frequent Contributor
timestamp: April 11, 2021, 12:04:51 am
content: Maybe Jay-Diddy got it right even with less different types of components, less costly components, easier to find components, and perhaps even using fewer components.This circuit is not that complicated, it just seems so to a noobie, but the point is getting the point to point wiring right.

author: Gcsillaz
author_rank: Contributor
timestamp: April 11, 2021, 12:08:55 am
content: Quote from: SuzyC on April 10, 2021, 11:44:47 pmYou are right. I was on AC coupling.But giving an update. I did not placed C5. Now i'm getting the 0-6Vdc swing at 62.5kHz! xDEverything seems ok... except the ammeter that is not fully deflecting to the right(i even tried reversing the poles) when placing a capacitor... it just moves slightly... the pot also does a little fine adjustment...jeez.. if i'm having this trouble building on the breadboard i think this is going to be impossible on the perf board xD Someone have tips regarding soldering and building on perfboard?Thanks Suzy for all inputs!

author: SuzyC
author_rank: Frequent Contributor
timestamp: April 11, 2021, 12:14:48 am
content: Breadboard, Schmedboard..practice makes perfect. (wiring a perf-board or a breadboard requires good eyesight and paying very close attention to making each connection right.) Make a perf-board wiring drawing, before wiring, before making mistakes. You need a plan.You don't need anything but a pencil and paper to make your layout of your component placements and wiring, but it is wise to double-check each wire, each component value, each pin number.My mother said, "It takes at least twice as long to get out of trouble than to get into it!"With circuits, you get it wired perfect or it likely no-workee.If the capacitor has a very low ESR then you would expect the reading to read near zero.Are you sure you have a100uA meter movement?Do you get full-scale deflection?

author: Gcsillaz
author_rank: Contributor
timestamp: April 11, 2021, 12:29:47 am
content: Quote from: SuzyC on April 11, 2021, 12:14:48 amThanks for the support Suzyc xDI'm 100% sure it is 100uA meter. It shows fs= 100umA on the display and also, i tested it before building everything.What I meant is that the "display" which i'm supposed to print and put on the ammeter starts at the right side and mine on the left side... is it possible that they come in variations like this?See pic attached!

author: SuzyC
author_rank: Frequent Contributor
timestamp: April 11, 2021, 12:32:03 am
content: Yep. But 100umAis a very-very sensitive meter!!!I don't like the way your circuit sets the zero because it also changes the gain of the metering circuit, the zero-ohm offset voltage of the meter op-amp and the meter op-amp gain is also dependent on the internal resistance of the 100uA meter, which could vary between mfg's.You can always use small valued resistors to check for correct ESR readings, but first you need full-scale "zero" deflection.If all values of resistors are as specified and wired correctly, it should work "out of the box".It is easy enough to change the gain of this meter-amplifier section of this circuit by varying the matched R16/R17 39K resistors. In this case, more (ohms) means more meter deflection.How far off are you from full-scale?

author: Gcsillaz
author_rank: Contributor
timestamp: April 11, 2021, 01:30:25 am
content: Quote from: SuzyC on April 11, 2021, 12:32:03 amHi Suzy,I just tried it. I shorted the inputs. The LED lights up and the scale goes to the right juuuust a little... i'd say 1/10 of the full scale...Tried some resistance too but there is very little deflection on the needle...

author: SuzyC
author_rank: Frequent Contributor
timestamp: April 11, 2021, 01:52:52 am
content: With no capacitor or resistor on the test points, thebridge is balancedand thedifference amplifieroutput IC 3A pin 7 should be very close to 0V P-P on your scope and the meter should read zero(using a scale with "0" on the leftmost side of the meter.) This is because the resistor bridge circuit is applying exactly the same voltage/waveform to the difference op-amp inputs and a balanced bridge means no-difference. Adj. the "zero" trimmer should have little or no effect.On the other hand, when the DUT capacitor test points are shorted,the bridge is unbalancedby 5-ohms and this creates a large square wave output(approx 2.2V P-P) at IC3A pin 7 that feeds into the IC3B input to be amplified, rectified and fully deflect the meter. When the meter is therefore at full-scale, the "zero" adjustment can set this point to read zero near the meter scale's full rightmost deflection.Do you get the correct scope readings at IC3A pin 7, when under these two conditions?Be sure to check C9 is installed correctly across the meter.

author: Gcsillaz
author_rank: Contributor
timestamp: April 11, 2021, 02:15:56 am
content: Quote from: SuzyC on April 11, 2021, 01:52:52 amNope. Something is wrong...See pic attached of what I see when inputs are open(.2mV/div - DC coupled).When shorted is just the same waveform but magnified to 2Vdcp-pWhat do you think is the problem?

author: SuzyC
author_rank: Frequent Contributor
timestamp: April 11, 2021, 02:22:10 am
content: Two-volts P-P looks like the correct P-P amplitude, but the waveform is unexpectedly different than the osc. square wave, and the distortion seen would not likely match at both the high and low output voltage conditions, should be most noticeable at the high output level, or more likely not be seen at all.In this case, looks like a bad TL-082 op-amp or bad rail voltages.Does the -4.4 volt rail look stable, meas. -4.4, clean, flat? If not, is C5 connected properly and the correct value?[bPossible C9 wrong value or not connected across the meter.Alternatively, one or more of the meter's bridge rectifier diodes (D5 - D8) are connected reverse or open circuited?Put the scope leads directly across the meter. You should see almost perfect flat, but small DC voltage when DUT connections are shorted. Your scope should see about a plus 1 to 2volt DC pulse at the top of the 6.8k R21 connected pin2 of the op-amp.You also could have the wrong values for C8, R20 or R21.What is the waveform at IC3 pin 3?

author: Gcsillaz
author_rank: Contributor
timestamp: April 11, 2021, 03:05:07 am
content: Quote from: SuzyC on April 11, 2021, 02:22:10 amRechecked C9 - Its ok both value and polarityChecked D5-D8 all are ok, connected correctly and working okSee attached what i see when scope is across the meter.There is a strange waveform coming from the middle pin of the potentiometer... i'll try to replace it with a resistor and see if it changes.-4.4V is stable and flat.Pin3 is exactly the same as pin 7 which i sent in the previous reply

author: Gcsillaz
author_rank: Contributor
timestamp: April 11, 2021, 03:08:44 am
content: Sorry, forgot to mention.The reading was 1Vdc/divChanges the pot for 1k res, same issue.Replace amp-op, same issue

author: SuzyC
author_rank: Frequent Contributor
timestamp: April 11, 2021, 03:14:15 am
content: The middle pin of the "zero" pot is grounded. Should be 0-V p-p, flat, only noise pickup from room EMI seen with >20 MHz scope BW.You fail to mention time/div when showing waveforms.If you are measuring w/scope 2V P-P directly across the meter it should bepositive voltage onlywaveform only and the meter should be slammed FS and it should be flat D.C. unless C9 is open.Your battery powered multimeter should read 0-Volt AC across the meter.Something's very wrong.

author: Gcsillaz
author_rank: Contributor
timestamp: April 11, 2021, 03:37:19 am
content: Quote from: SuzyC on April 11, 2021, 03:14:15 amYes. It is 2V P-Pdc... C9 is not open, changed it just in case and same...Timescale is 5uS/DivChanged all diodes just in case also... same issue.. Damn...Rebuild the diode bridge but no sucess

author: SuzyC
author_rank: Frequent Contributor
timestamp: April 11, 2021, 10:37:00 am
content: To observe this waveform directly across the meter is impossible without the meter slam-pegging to the right.Check your scope readings across the meter using your digital multimeter on the AC scale.Something is wrong with the way you are using the scope/probes to get the waveform you show across the meter.If everything measures up to the schematic by value and connection, the way you describe, this circuit will absolutely work as well as expected.Is is possible you havebogus TL-082 IC's?

author: Gcsillaz
author_rank: Contributor
timestamp: April 11, 2021, 04:18:02 pm
content: Quote from: SuzyC on April 11, 2021, 10:37:00 amHi Suzy! Morning,I measured it today with my multimeter, it shows 0.2mVacRebuild the wheatstone bridge still nothing...Would you be willing to build this circuit and try it? Maybe show some scope pictures as of how it should appear...T_TThanks!

author: SuzyC
author_rank: Frequent Contributor
timestamp: April 11, 2021, 06:33:10 pm
content: I have my own garden to attend to. so I just ask you to carefully send me evidence and we might just certainly get to the heart of the problem. I only ask you to please send me some waveforms.Don't re-send those already sent, but carefully make all new scope pics to be sure of accuracy.Waveforms requested!1) pin 8 of IC3a2) pin 7 of IC3a3) pin 6 of IC3a4) pin 5 of IC3a5) pin 4 of IC3a6) pin 3 of IC3a7) pin 2 of iC3apin 1 of IC3a9) junction of 270-ohm to 270-ohm and T3 collector10) Waveform at T3 base11) Waveform at Meter + terminal12) Waveform at Meter - terminal13) Meter reading.If I have all this info, I can certain the problem's cause.I am not sure yet, but it certainly looks like the TL082 is not behaving, not showing the high-freq. slew rate it is spec'd at.That's not possible unless the TL082 is not really a TL082 but another low-freq op-amp (like a LM358).Where did you purchase the TL082's?Be sure to show V/div, 0-volt baseline (center?) time/div for all measurements

author: Gcsillaz
author_rank: Contributor
timestamp: April 11, 2021, 07:41:48 pm
content: Quote from: SuzyC on April 11, 2021, 06:33:10 pmPlease follow attached all the pictures requested.I tried as best as possible to show de V/div reading on each picture. All pictures were taken with 5uS/Div and centered base line with DC coupling. I'm using a 10x probe. Tektronix P6139BObs:+ and - terminal reading are exactly the same

author: Gcsillaz
author_rank: Contributor
timestamp: April 11, 2021, 07:42:42 pm
content: The remaing pins.Thanks Suzy!

author: SuzyC
author_rank: Frequent Contributor
timestamp: April 11, 2021, 08:02:48 pm
content: You still haven't answered where you got these TL082 op-amps.Slew Rate VS = ±15V, TA = 25°C =13 V/μs, this is the spec for the TL082: 13V/uSecThis means the output will swing 13V per uSec. which also means the output will reach 6V in .5 uSecWave form on IC3 pin 7 shows a slew rate of approx 3V over 7 uSec, or slew rate <.5V/uSecYour op-amp is not a TL082. Looks rather like a LM358.That's your problem!Have you any other op-amps on hand?

author: Gcsillaz
author_rank: Contributor
timestamp: April 11, 2021, 08:05:08 pm
content: Quote from: SuzyC on April 11, 2021, 08:02:48 pmI got them from a retailer online shop in Brazil. Surely they come from Ebay or something

author: SuzyC
author_rank: Frequent Contributor
timestamp: April 11, 2021, 08:20:12 pm
content: If you can get a good picture, post a clear pic to show the top of the op-amp. Might show phoney-baloney markings.In any case, you could easily modify the circuit to match Jay-Diddy's and no op-amp is required.

author: Gcsillaz
author_rank: Contributor
timestamp: April 11, 2021, 08:28:58 pm
content: Quote from: SuzyC on April 11, 2021, 08:02:48 pmJeez!!!Quote from: SuzyC on April 11, 2021, 08:20:12 pmThese are the best two I could take...I SUPPOSEDLY now have a ua741 and a Lf356...Which do you reckon would be better?Thanks a lot!

author: SuzyC
author_rank: Frequent Contributor
timestamp: April 11, 2021, 08:41:00 pm
content: The LF356 would work fine, but you get only a single op-amp per package, so you need two of 'em

author: Gcsillaz
author_rank: Contributor
timestamp: April 11, 2021, 09:08:06 pm
content: Quote from: SuzyC on April 11, 2021, 08:41:00 pmI have exactly two of themI'll try to substitute and see if it works! I'm really glad for your help SuzyC! Has a friend of ours would say: "Trap for young players!" Never thought that fake amp-ops would get me on SlewRate :ßWill be back in a sec!Update:Ok, I placed them but believe it or not, still there is something fishy! xDWhen shorted the inputs the needle moves a lot more but still is not able to go FS.Follow attached the output at the meter (2V/div - 5uS/div - DC coupled - centered) and the output at pin 7. Also this is the max deflection I can get using the pot.

author: SuzyC
author_rank: Frequent Contributor
timestamp: April 11, 2021, 10:07:55 pm
content: more important, what is the output at pin 7 and input pin 3

author: Gcsillaz
author_rank: Contributor
timestamp: April 11, 2021, 10:15:52 pm
content: Quote from: SuzyC on April 11, 2021, 10:07:55 pmSorry I have written wrong the subtitle for picture before.The picture named "Pin1" is actually Pin7 output has I had written on the reply. Which is the same as Pin3 Input but with offset (~1Vdc)

author: SuzyC
author_rank: Frequent Contributor
timestamp: April 11, 2021, 10:30:47 pm
content: The LF356 has slew rate spec'd at +/- 15V supply, the specsheet shows normal operation not lower than +/- 15V. The scope shows slew rate here about 2.2V/2uSec so not quite up to par.Quick fix: change R21 to 2.7k or 3.3k

author: Gcsillaz
author_rank: Contributor
timestamp: April 11, 2021, 11:06:39 pm
content: Quote from: SuzyC on April 11, 2021, 10:30:47 pmYeeeey! Finally! With the change of R21 to lower value I was able to adjust for FS and even tried a capacitor, it seems to be working!Could you please explain what you meant with the slewrate quite not up to par?For what I understood was that because I'm powering the opamp with +6V and -4.4V the IC isnt speced for this operation and thus might not be operating has on the datasheet.I'll give the diagram a new study and try to understand why the slower slewrate of the fake TL082 caused all this trouble.The fix you purposed was because I'm going to get a higher gain with lower value R21 and thus would get "quicker" to the required voltage for the differential amplifier right?Nonetheless with all this trouble and IC not working in the specification they were meant to, I'll propably give it a try to the Jay-Diddy's variant.Can't stress enough how thankfull I'm for all your effort and sticking with me until the end SuzyC! Thanks a bunch my friend!

author: SuzyC
author_rank: Frequent Contributor
timestamp: April 11, 2021, 11:41:50 pm
content: Because of the LF356's design, lower rails voltages than +/- 15V cause it to have a lower slew rate than spec'd at normal not lower than +/- 15V operation.The slew rate affects the differentiator combination of C8 and R20. These two components react to rate of change of voltage, low slew rate =low rate of change. This two-component basic differentiator should be almost a perfect AC short to the fast-rise waveshape of a square wave with a fast rise time.What'sgoing on nowis the positive output swing of the op-amp is hitting the upper limit of output swing when running with reduced supply voltages than spec'd. You can see if this is a problem by putting known value resistors upon the DUT terminals. If your readings match the resistor values on the meter, you are good to go and nothing more needs to be done.You can easily fix this output voltage swing problem by connecting only the +Vdd power pins of both LF356's to the input side(R1) of the voltage regulator and this will give you several more positive volts for positive output swings and thus overcome this limitation. After fixing this voltage supply problem, you might have to change R21 back to 6.8K.Be sure to add an additional bypass capacitor of at least .1uF from Vdd to ground on the second LF356 and locate each cap as close as possible to the Vdd pins.