Author Topic: VK5JST's ESR Meter project  (Read 1170 times)

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Offline Gcsillaz

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VK5JST's ESR Meter project
« on: April 10, 2021, 05:02:47 pm »
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!

 

Offline SuzyC

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Re: VK5JST's ESR Meter project
« Reply #1 on: April 10, 2021, 09:35:00 pm »
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.
« Last Edit: April 10, 2021, 09:40:21 pm by SuzyC »
 

Offline Gcsillaz

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Re: VK5JST's ESR Meter project
« Reply #2 on: April 10, 2021, 09:49:30 pm »
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.

Hello 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!
 

Offline Gcsillaz

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Re: VK5JST's ESR Meter project
« Reply #3 on: April 10, 2021, 10:06:28 pm »
And just o it gets more "visual"

Follow below what i'm seeing on the colector of T3.

[attachimg=1]
 

Offline SuzyC

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Re: VK5JST's ESR Meter project
« Reply #4 on: April 10, 2021, 10:18:39 pm »
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.
« Last Edit: April 10, 2021, 10:46:41 pm by SuzyC »
 

Offline SuzyC

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Re: VK5JST's ESR Meter project
« Reply #5 on: April 10, 2021, 10:48:26 pm »
You should see 110mV at the DUT connection point, but 110mV at the input pin 9 of the 74HC04 would not work!
 

Offline Gcsillaz

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Re: VK5JST's ESR Meter project
« Reply #6 on: April 10, 2021, 11:17:40 pm »
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.

I 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/div

I checked both r7 and r8, they are fine, pins on the 74hc04 is right also...

 

Offline Gcsillaz

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Re: VK5JST's ESR Meter project
« Reply #7 on: April 10, 2021, 11:20:02 pm »
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
 

Offline SuzyC

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Re: VK5JST's ESR Meter project
« Reply #8 on: April 10, 2021, 11:44:47 pm »
If you are getting a +/- 5V peaks at the output of IC3 pin 1, that is impossible without 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?
« Last Edit: April 10, 2021, 11:52:32 pm by SuzyC »
 

Offline SuzyC

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Re: VK5JST's ESR Meter project
« Reply #9 on: April 11, 2021, 12:04:51 am »
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.
« Last Edit: April 11, 2021, 02:15:03 am by SuzyC »
 

Offline Gcsillaz

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Re: VK5JST's ESR Meter project
« Reply #10 on: April 11, 2021, 12:08:55 am »
If you are getting a +/- 5V peaks at the output of IC3 pin 1, that is impossible without 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?

You 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! xD

Everything 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!
 

Offline SuzyC

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Re: VK5JST's ESR Meter project
« Reply #11 on: April 11, 2021, 12:14:48 am »
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 a 100uA meter movement? Do you get full-scale deflection?
« Last Edit: April 11, 2021, 12:30:21 am by SuzyC »
 

Offline Gcsillaz

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Re: VK5JST's ESR Meter project
« Reply #12 on: April 11, 2021, 12:29:47 am »
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.

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 a 100uA meter movement? Do you get full-scale deflection?


Thanks for the support Suzyc xD

I'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!
 

Offline SuzyC

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Re: VK5JST's ESR Meter project
« Reply #13 on: April 11, 2021, 12:32:03 am »
Yep. But 100umA is 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?

« Last Edit: April 11, 2021, 02:15:52 am by SuzyC »
 

Offline Gcsillaz

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Re: VK5JST's ESR Meter project
« Reply #14 on: April 11, 2021, 01:30:25 am »
Yep. But 100umA is 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?



Hi 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...
 

Offline SuzyC

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Re: VK5JST's ESR Meter project
« Reply #15 on: April 11, 2021, 01:52:52 am »
With no capacitor or resistor on the test points, the bridge is balanced and the difference amplifier output 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 unbalanced by 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.


 
« Last Edit: April 11, 2021, 02:04:19 am by SuzyC »
 

Offline Gcsillaz

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Re: VK5JST's ESR Meter project
« Reply #16 on: April 11, 2021, 02:15:56 am »
With no capacitor or resistor on the test points, the bridge is balanced and the difference amplifier output 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 unbalanced by 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.


 

Nope. 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-p

What do you think is the problem?
 

Offline SuzyC

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Re: VK5JST's ESR Meter project
« Reply #17 on: April 11, 2021, 02:22:10 am »
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.[/b]
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?
« Last Edit: April 11, 2021, 03:11:05 am by SuzyC »
 

Offline Gcsillaz

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Re: VK5JST's ESR Meter project
« Reply #18 on: April 11, 2021, 03:05:07 am »
Two-volts P-P looks like the correct P-P amplitude, but the waveform is unexpectedly different than a 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.

Looks to me like C9 is the 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.

Does the -4.4 volt rail look stable, meas. -4.4, clean, flat? If not, is C5 connected properly and the correct value?

What is the waveform at IC3 pin 3?

Rechecked C9 - Its ok both value and polarity
Checked D5-D8 all are ok, connected correctly and working ok

See 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
 

Offline Gcsillaz

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Re: VK5JST's ESR Meter project
« Reply #19 on: April 11, 2021, 03:08:44 am »
Sorry, forgot to mention.

The reading was 1Vdc/div

Changes the pot for 1k res, same issue.

Replace amp-op, same issue
« Last Edit: April 11, 2021, 03:15:26 am by Gcsillaz »
 

Offline SuzyC

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Re: VK5JST's ESR Meter project
« Reply #20 on: April 11, 2021, 03:14:15 am »
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 be positive voltage only waveform 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.
« Last Edit: April 11, 2021, 03:31:46 am by SuzyC »
 

Offline Gcsillaz

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Re: VK5JST's ESR Meter project
« Reply #21 on: April 11, 2021, 03:37:19 am »
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 be positive voltage only waveform 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.


Yes. It is 2V P-Pdc... C9 is not open, changed it just in case and same...
Timescale is 5uS/Div

Changed all diodes just in case also... same issue.. Damn...

Rebuild the diode bridge but no sucess
« Last Edit: April 11, 2021, 03:58:38 am by Gcsillaz »
 

Offline SuzyC

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Re: VK5JST's ESR Meter project
« Reply #22 on: April 11, 2021, 10:37:00 am »
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  have bogus TL-082 IC's?
 

Offline Gcsillaz

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Re: VK5JST's ESR Meter project
« Reply #23 on: April 11, 2021, 04:18:02 pm »
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  have bogus TL-082 IC's?

Hi Suzy! Morning,

I measured it today with my multimeter, it shows 0.2mVac

Rebuild 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_T

Thanks!
 

Offline SuzyC

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Re: VK5JST's ESR Meter project
« Reply #24 on: April 11, 2021, 06:33:10 pm »
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 IC3a
2) pin 7 of IC3a
3) pin 6 of IC3a
4) pin 5 of IC3a
5) pin 4 of IC3a
6) pin 3 of IC3a
7) pin 2 of iC3a
8) pin 1 of IC3a
9) junction of 270-ohm  to 270-ohm and T3 collector
10) Waveform at T3 base
11) Waveform at Meter + terminal
12) Waveform at Meter - terminal 
13)  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
« Last Edit: April 11, 2021, 07:16:31 pm by SuzyC »
 


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