Author Topic: VK5JST's ESR Meter project  (Read 1160 times)

0 Members and 1 Guest are viewing this topic.

Offline Gcsillaz

  • Contributor
  • Posts: 43
  • Country: br
Re: VK5JST's ESR Meter project
« Reply #25 on: April 11, 2021, 07:41:48 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

Please 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 P6139B

Obs:
+ and - terminal reading are exactly the same
« Last Edit: April 11, 2021, 07:44:09 pm by Gcsillaz »
 

Offline Gcsillaz

  • Contributor
  • Posts: 43
  • Country: br
Re: VK5JST's ESR Meter project
« Reply #26 on: April 11, 2021, 07:42:42 pm »
The remaing pins.

Thanks Suzy!
 

Online SuzyC

  • Frequent Contributor
  • **
  • Posts: 604
Re: VK5JST's ESR Meter project
« Reply #27 on: April 11, 2021, 08:02:48 pm »
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/uSec 
This means the output will swing 13V per uSec. which also means the output will reach 6V in .5 uSec
 
Wave form on IC3 pin 7 shows a slew rate of approx 3V over 7 uSec,  or slew rate <.5V/uSec
Your op-amp is not a TL082. Looks rather like a LM358.


That's your problem!

Have you any other op-amps on hand?
« Last Edit: April 11, 2021, 10:00:21 pm by SuzyC »
 

Offline Gcsillaz

  • Contributor
  • Posts: 43
  • Country: br
Re: VK5JST's ESR Meter project
« Reply #28 on: April 11, 2021, 08:05:08 pm »
You still haven't answered where you got these TL082 op-amps.

I got them from a retailer online shop in Brazil. Surely they come from Ebay or something
 

Online SuzyC

  • Frequent Contributor
  • **
  • Posts: 604
Re: VK5JST's ESR Meter project
« Reply #29 on: April 11, 2021, 08:20:12 pm »
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. 

« Last Edit: April 11, 2021, 08:25:11 pm by SuzyC »
 

Offline Gcsillaz

  • Contributor
  • Posts: 43
  • Country: br
Re: VK5JST's ESR Meter project
« Reply #30 on: April 11, 2021, 08:28:58 pm »

Your op-amp is not a TL082. Looks rather like a LM358.


That's your problem!

Have you any other op-amps on hand?

Jeez!!!

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. 



These are the best two I could take...[attachimg=1]


I SUPPOSEDLY now have a ua741 and a Lf356...

Which do you reckon would be better?

Thanks a lot!
 

Online SuzyC

  • Frequent Contributor
  • **
  • Posts: 604
Re: VK5JST's ESR Meter project
« Reply #31 on: April 11, 2021, 08:41:00 pm »
The LF356 would work fine, but  you get only a single op-amp per package, so you need two of 'em
 

Offline Gcsillaz

  • Contributor
  • Posts: 43
  • Country: br
Re: VK5JST's ESR Meter project
« Reply #32 on: April 11, 2021, 09:08:06 pm »
The LF356 would work fine, but  you get only a single op-amp per package, so you need two of 'em

I have exactly two of them  ^-^

I'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 :ß

Will be back in a sec!

Update:

Ok, I placed them but believe it or not, still there is something fishy! xD

When 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.

« Last Edit: April 11, 2021, 09:43:51 pm by Gcsillaz »
 

Online SuzyC

  • Frequent Contributor
  • **
  • Posts: 604
Re: VK5JST's ESR Meter project
« Reply #33 on: April 11, 2021, 10:07:55 pm »
more important, what is the output at pin 7 and input pin 3
 

Offline Gcsillaz

  • Contributor
  • Posts: 43
  • Country: br
Re: VK5JST's ESR Meter project
« Reply #34 on: April 11, 2021, 10:15:52 pm »
more important, what is the output at pin 7 and input pin 3

Sorry 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)
 

Online SuzyC

  • Frequent Contributor
  • **
  • Posts: 604
Re: VK5JST's ESR Meter project
« Reply #35 on: April 11, 2021, 10:30:47 pm »
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

« Last Edit: April 11, 2021, 10:34:43 pm by SuzyC »
 

Offline Gcsillaz

  • Contributor
  • Posts: 43
  • Country: br
Re: VK5JST's ESR Meter project
« Reply #36 on: April 11, 2021, 11:06:39 pm »
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



Yeeeey! 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!
 

Online SuzyC

  • Frequent Contributor
  • **
  • Posts: 604
Re: VK5JST's ESR Meter project
« Reply #37 on: April 11, 2021, 11:41:50 pm »
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's  going on now is 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.
« Last Edit: April 12, 2021, 03:46:47 am by SuzyC »
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf