Problem with ESR Meter.

[Vancata]

Hey folks, I'm working on one ESR meter here. Before like 4months i made one by schematic via the net (the one with the 4op amps, and the Wheatstone bridge, but it was giving like 4.5v on the output leads so i decided to look for some new one so i can measure with out unsoldering the cap's.

I found the uploaded schematic in the net, and was satisfied with the ~200mV on the outputs (cant trigger any other component in most cases).

Anyway the problem is it kinda don't work... i made it on my own pcb (changed a bit the original one on EagleCAD to fit in my case for the meter) double, even triple checked the connections and everything looks fine.

I have used M74HC14B1 instead 74hc14N Texas Ins. one and i used 400v 0.68uF (Polyester Capacitor MKS4) for C5, 24kR instead 25kR for R10 and bc547 in to92 instead 2n2222 the other components are kinda the same as in the schematic
Here is the problem: I have nothing to my transistor, no ~mV180 when i short the leads, so i have nothing to trigger my transistor. I have like 110khz between the buffers and the 4x680R (IC 4,6,8,10,12 points) Measured with my multimeter in HZ mode (don't have scope). But on in the (R2,R3,R4,R5,R6,C4,R7,R18,TP1 points) nothing at all, only some ~180mV DC voltage with out any frequency.

Could this be from the M74HC14B1, i did not really found any major difference between the M74HC14B1 and 74HC14N. Both Cmos, 25mA output, 2-6V inputs.
Any tips? I have removed a lot of components, tested and so on, but forgot to type the results on a paper.. and I'm kinda frustrated now. If anyone made this Meter with this schematic would be nice to give me some measurements and hints.

PS (Btw the dude had made little mistake there with the oscillator. f = 1/CR so it's not 156khz, more like 100khz in the perfect case).

[Simon]

you really need to use a scope to check the actual output of the oscillator the fact that it outputs 110KHz instead of 153 KHz is not good.

[Vancata]

you really need to use a scope to check the actual output of the oscillator the fact that it outputs 110KHz instead of 153 KHz is not good.

Kinda edited my first post several times (sorry for that, always forgot to read my post before i post it)

Anyway this was my first major hit so i checked the oscillator and found that it is okey, because the schematic have in build mistake there on the freq. Measured my C1 cap, found its like 11nF (in the 10% spec) so i have like 109kHz freq. So i concluded the oscillator works fine.

P.S. I have one didigtal-PC scope, but believe me I really don't want to even see it. Still hunting ebay for some cheap analog scope

edit: Guess i need to figure out how to test this low pass filter with the 5x R and the C4... Seems it eat my freq.. lol
edit2: With few calculations found the Cut off frequency of that low pass filter is: fc = 117025.69345hz, so 117khz way upper then 110khz, so it shouldn't cut my freq.
agh I caulculated this with 10nF... with 47nF it seems like 25kHz cut off.. low. Any suggestions here? Seems I'm making some wrong calculations or...
edit3: Ofc I'm lazy guy on thinking, ofc it should be 25khz cuz its Low pass, don't know sometimes I really don't think.. So its fine agein it cuts everything udner 25kHz as nosie and so on from the i 5x inverting buffers and the oscillator.

[Simon]

P.S. I have one didigtal-PC scope, but believe me I really don't want to even see it. Still hunting ebay for some cheap analog scope

Well just try it, see what the waveform is

[Vancata]

P.S. I have one didigtal-PC scope, but believe me I really don't want to even see it. Still hunting ebay for some cheap analog scope

Well just try it, see what the waveform is

Well will have to plug in that Hantek crap. I really don't trust that "scope" but anyway will have to dig a bit here to find it first.

[Simon]

it's far better than you crap meter, you have no idea what the meter uses as a reference, getting some glimpse of the waveform will be better than nothing, then move around the circuit.

[Vancata]

Huston we found them!

Will dig some more in this thing. Last time when i used this crappy scope it was far away from taking actual probes of the freq. Anyway on low freq like this seems its okey.

edit: I'm ashamed. I have the 180v p-p on 105kHz freq on the output, so seems my problem is somewhere in the amplifier logic. Will dig in, just started to be interesting again!

[Vancata]

So some conclusions:
--Condition: leads shorted--

24kOhm R vs 25kOhm R (R10). Found out that it only changes the voltage divider ratio really low, only 40mV or 0.04V. So instead of 1v at the base of the transistor i have 0.9677V.
Anyway some weird things happens there after that C2. (Point C2,R9,R10,Base BC547) No stable freq at all, just hitting randomly from 110khz to 50hz et.c  On the base of the transistor like nothing, but i guess its the same randomly stuffs, just hard to clip my probe there.
A) On the Emitter 105khz / 144mv p-p crappy curved signal, close to flat draining to ground.
B) On the collector flat DC 94mV with out any freq, instead 0 to 2Vpp

BC547C vs 2N2222 or? Any other guesses? Was thinking that BC547C would fit as general purpose NPN transistor instead 2n2222 (we don't have them here).

[w2aew]

So some conclusions:
--Condition: leads shorted--

24kOhm R vs 25kOhm R (R10). Found out that it only changes the voltage divider ratio really low, only 40mV or 0.04V. So instead of 1v at the base of the transistor i have 0.9677V.
Anyway some weird things happens there after that C2. (Point C2,R9,R10,Base BC547) No stable freq at all, just hitting randomly from 110khz to 50hz et.c  On the base of the transistor like nothing, but i guess its the same randomly stuffs, just hard to clip my probe there.
A) On the Emitter 105khz / 144mv p-p crappy curved signal, close to flat draining to ground.
B) On the collector flat DC 94mV with out any freq, instead 0 to 2Vpp

BC547C vs 2N2222 or? Any other guesses? Was thinking that BC547C would fit as general purpose NPN transistor instead 2n2222 (we don't have them here).

Oh, I think the BC547C is one of those wacko devices whose pinout in the TO-92 is reverse from "normal".  Collector and Emitter are swapped.

[Vancata]

Oh, I think the BC547C is one of those wacko devices whose pinout in the TO-92 is reverse from "normal".  Collector and Emitter are swapped.

Yeah they are. Kinda noticed that when I was installing it in the date sheet: Collector, Base, Emitter when you look it with the flat side fronting you. My other guess is that it is kinda low on collector current, maximum 0.1A, but i calculated I=5/2200R ~2mA. So its rated to like 100mA, guess it cant handle it... or don't really know, i can try with some KSP42 or KSP13 that i have here.

[Simon]

Do you want me to send you a 2N2222, if it says to use one you should use it unless you know what your doing, this is a measuring instrument, you can't just change parts willy nilly.

[Vancata]

Well found PN2222A which is 2n2222 in TO92 box in local dealer. After the Easter holiday will go grab some.

[Simon]

yes they are the same, it's just the case changes from metal to plastic and it can handle slightly less current (amps)

[Vancata]

Found one  PN2222A from old project of one friend. Now everything seems to work fine.

Anyway have one more question. I have analog bar graphed 100uA meter. I removed the 10k R17 so i can regulate the output from like 30uA to 180uA, but my analog meter don't want to show more then 60uA (tested it with two digital uA meters), no matter how much current I'm pushing. It have no problem when i tested it with 100kOhm and liner power supply, tested it 100uA, 90uA, 80uA no problems at all.  So when i measure the uA with digital meter no problems at all, i mean its something with that analog meter i guess... AC current on that freq make it buggy... or im not sure.
(Pic of a meter that kinda looks like mine: http://thumbs1.ebaystatic.com/d/l225/m/mCOcq4RUQpybxe7fNC1v9aQ.jpg )

The output freq is 105khz 117mv p-p 10uS. Will attach a photo, but the meter cant show more then 60uA and seems its madly incorrect.
What is the accuracy level of this kind of meter on 100kHz freq?

Uploading my output singal with the meter and shorted probes x10 on the probes.

[w2aew]

What's the meter's coil resistance?  It could be that you're not generating enough voltage out of your circuit and the peak detector to put enough current through the meter coil? Measure the voltage you're getting across the meter leads and at the output of your peak detector when the leads are shorted. 

[Vancata]

What's the meter's coil resistance?  It could be that you're not generating enough voltage out of your circuit and the peak detector to put enough current through the meter coil? Measure the voltage you're getting across the meter leads and at the output of your peak detector when the leads are shorted.

1.8V p-p on the uA output pins of the PCB with shorted leads on the cap. tester
400mV p-p across the uA meter connected to the PCB with shorted lleads on the cap. tester
6.5kOhm resistance measured between the two inputs of the uA meter. It is model from the ~1980s i think.

[fable]

HI.First of all sorry for my bad english.
 I build this meter long time ago and it serves me well.Il try to help u.For meter of 100uA u need to to replace 3 resistors : R17 (10k) to 1k,R11 (2k2) to 6k5 and R12 (100) to 47ohm.Instead of R1 put a 1k trimer so u can adjust frequency.

[Vancata]

HI.First of all sorry for my bad english.
 I build this meter long time ago and it serves me well.Il try to help u.For meter of 100uA u need to to replace 3 resistors : R17 (10k) to 1k,R11 (2k2) to 6k5 and R12 (100) to 47ohm.Instead of R1 put a 1k trimer so u can adjust frequency.

Hey there.
First thanks to everyone for trying helping me, really appreciate that!
No problem for your English mine is not even close to good one too.

1) Why should i adjust the frequency? Isn't it most of the meters work on like ~100-200khz FIXED  range?
2) I have already removed the R17 so i can get nice output current there. But no problems can place one 1k there.
3) R11 - we are lowering the output current on the collector of the transistor? Why?
4) R12 sounds reasonable if we want to output more current to rise the R12 so less current will draw to ground.

And my main question: Whats goign with this analog uA meter.. 6.5k Ohm internal resistance... the 50uA have like 6.5kOhms... lol Just checked in the net, most of the 100uA have like 1 or 2k Ohms internal resistance of the coil...

[w2aew]

HI.First of all sorry for my bad english.
 I build this meter long time ago and it serves me well.Il try to help u.For meter of 100uA u need to to replace 3 resistors : R17 (10k) to 1k,R11 (2k2) to 6k5 and R12 (100) to 47ohm.Instead of R1 put a 1k trimer so u can adjust frequency.

Hey there.
First thanks to everyone for trying helping me, really appreciate that!
No problem for your English mine is not even close to good one too.

1) Why should i adjust the frequency? Isn't it most of the meters work on like ~100-200khz FIXED  range?
2) I have already removed the R17 so i can get nice output current there. But no problems can place one 1k there.
3) R11 - we are lowering the output current on the collector of the transistor? Why?
4) R12 sounds reasonable if we want to output more current to rise the R12 so less current will draw to ground.

And my main question: Whats goign with this analog uA meter.. 6.5k Ohm internal resistance... the 50uA have like 6.5kOhms... lol Just checked in the net, most of the 100uA have like 1 or 2k Ohms internal resistance of the coil...

Looks like that's your problem.  If the meter resistance is 6500ohms, you'll need to have 650mV across it in order for 100uA to flow.  If you're only getting 400mV, you can't get 100uA to flow, so no full scale reading.  You'll have to increase the voltage across it.  Looks like "fable" has some suggested resistor changes for you to be able to use a 100ua meter.  There are other design changes you could make, but simple resistor changes to increase the available voltage (reducing the resistance in series with the meter, increasing the gain of the transistor amplifier, etc.) are the simplest way to go.

Decreasing R17 makes more current available to the meter.
Decreasing R12 and increasing R11 result in increasing the gain of the amplifier.

[Vancata]

Got your point w2aew
0.650mV/6500Ohms = 100uA (full scale on the meter).

Will try playing with the resistors and the transistor or will go just grab, a new one and measure the internal resistance before buying it. Seems 4kOhms will be my limit for internal resistance of 100uA or any uA meter.

[Jay_Diddy_B]

Hi Group,

I have done a little simulation on the circuit. Here is a modified version of the amplifier and meter part of the circuit:



The modified circuit will drive 100uA into the meter, if the total resistance R6 + R7 + R8 is approx. 10.7 k Ohms and the drive signal is 180mV peak to peak.

These are the simulation results:



R4 and C1 allow the gain of the amplifier stage to be set (almost) independently from the bias point.

I have attached the LTspice model.(zip file)

Regards,

Jay_Diddy_B

[Vancata]

Hey. Thanks for the simulation and the efforts trying of helping me!

I decided to go for other analog uA meter, because I'm really frustrated fighting with this schematic and kinda ruin already my PCB and I'm too lazy to etch, drill, coat another one.

I have few question for the schismatic and the simulation, that might sound totally noobish.
3.2V sounds good from the output of the amplifire, but why did you removed the full bridge rectifier and placed half one? Going to measure only the positive period and its given current? And for R6, my meter got like 6500Ohms if i understand right your simulation is for 2000ohms one?

[Jay_Diddy_B]

Hi Vancata,

The voltages shown on the amplifier are the bias voltages. I positioned the collector of the transistor at 3.2V to maximize the peak-peak output.

The  procedure was like this:

1) I positioned the Base voltage at 1.5V to get 0.85V on the emitter. This gives the ratio of R1 and R5.

2) I selected R3 for an Emitter current of 0.85 / 560 = 1.5mA

3) I selected R2 to get a collector voltage of 3.2V.

     (5 - 3.2) /1.5mA = 1.2 k Ohms

4) I selected R4 for a gain approximately = 20. The gain = R2 / (R4 in parallel with R3)

R4 // R3 =

560 x 68 / 560 + 68 =  60.6 Ohms

R2 / (R4 // R3) = 1.2K / 60 = 20

5) I selected C1 so that Xc at 100 kHz was much less than 60 ohms

C1(min) = 1/ ( 2 x Pi x F x Xc)

= 1 / (2 x Pi x 100K x 6)

= 265 nF

use 0.47 uF ceramic or film capacitor.


I chose a doubler configuration over the bridge rectifier to reduce the diodes from 4 to 2.

The circuit requires a total resistance of about 10.7K. This is the meter, the zeroing potentiometer and the fixed resistor. If your meter has 6.5 k Ohms of resistance, use a 2.2K fixed resistor and a 10 K potentiometer.

With the exception of C1 and R4, you should be able to build the circuit or your existing circuit board.


Have a look at this thread: https://www.eevblog.com/forum/projects/5-transistor-esr-meter-design/msg171364/#msg171364

This is a simple meter, similar to the one you are building that measures the impedance of the capacitor and uses an analog meter.

or this thread:

https://www.eevblog.com/forum/projects/esr-meter-adapter-design-and-construction/msg341177/#msg341177

This ESR meter uses a DMM for the display, and is very well protected from charged capacitors and applied voltage.

Good luck !!

Regards,

Jay_Diddy_B

[Vancata]

Thanks a lot again, now I kinda get in to the "business" and the concept of your idea or i guess so. Will check out what spare junk parts left in my box for the resistors and try it out tomorrow.

And your project with the 3npn and 2pnp transistors looks interesting too, might learn something new when I try it and make some fails as usual, no matter how simple it is always making dumb mistakes  . I might make it just for fun trying my smd skills... I'm going to move to smd pretty soon because I'm kinda tired from this DIP projects and all the drilling. Just need to buy one fat pack of x20 resistors each from ebay.

I have uploaded the actual board that I'm working on right now 

[Vancata]

After buying second smaller Analog scale meter and founding that the resistance was 4kOhms of the coil again and it wouldn't work for me I decided to try Jay_Diddy_B way. Recalculated the common Emitter and Collector that he posted and kinda messed up the things, so i used online calculators lol... It can be really hard few simple math calculations when you always forgot to predict something and was hard to match the resistors, so the calculator showed that the theory was right. Bought the resistors that i was missing, took the 0.47uF cap from my junkbox and everything runs smooth and nice now. Both of the meters, the 6.5kOhm scale and the small one work to 100uA with out problems(tested them on few good and bad electr. caps). Preferred the small one so i can snack everything in one case/box that i already had, but needed few repairs on holes that were made for one ex. project.  Placed all inside with one 9v battery and one liner 7805 + led indicator for "power on".
I'm only sad that i couldn't find trim pot with nob, only the screw driver one, so i need falt screwdriver always when i need to adjust it 

Kinda proud of my self, no matter that i preused someone design :/ But normally my projects wont work at the end or they are working in a strange way....

Thanks for the help folks! Really glad on the guys in this forum, for letting help to beginners hobbysts.

Will upload few photos to the final look..