Yet another ESR meter project

[RiverTown]

Hi,
I wasn't sure where to put this post- here or in the Project section of the forum. But I'll have some questions, and will need some advices, so I have put it here.
If mods think that Project section is better place for this topic, they can move it freely.

Main requirements/ specifications:
- Analog scale. Cheap ebay analog multimeter is used as a case and galvanometer (1000 Ohms, 130uA)
- Scale from 0 to 20 Ohms (Impedance at 100kHz). True ESR measurement isn't necessary
- Basic protection from the charged capacitors
- Incircuit measuring capability (test signal about 100mVpp)- Yes, yes, I know for all the problems related to incircuit measurement
- Powered from the single cell Li-Ion battery, or from the wall adapter/USB. Integrated charging and power switching circuit.
- Low Batt indication
- Mainly SMD design (0805, SOIC and similar. I'll try to avoid pain in the arse packages. MCP73871 is an exception.)


As you can guess from the specs, I'm building ESR meter with the main purpose to quickly detect bad caps while repairing electronic gear. Under the analog scale in Ohms I plan to draw GOOD-BAD scales for most common capacitors capacitance and voltage ratings. Something like scale on this picture.
[PICTURE TAKEN FROM THE ELEKTRONIKA.BA FORUM]

For precision measurement I plan to buy decent LCR meter.


I know that a lot of people hate block schematics, so I'll try to describe planned design in a few sentences.
MCP73871 or similar IC (I'm open for suggestions) will be main part of the power supply section. His task is charging, switching (between batt and wall adapter), and low batt indication.

I definitely need split power supply, so...
My first idea was to use voltage divider to create a virtual ground, but battery voltage can go as low as 3- 2,8V and I'm afraid that voltage will be too low for the measuring circuit.
Now I'm planning to use 2,5V LDO and some filtering after output of an MCP73871 for the positive rail.
For the negative rail I would use an voltage inverter IC on the MCP73871 output, than some heavy filtering and another LDO regulator.
What you think about this idea?
I'm little scared because of noise and ripple.
Can you suggest some low ripple, low noise, 20mA output voltage inverter ICs?
Some suggestions on which LDO/LDOs to choose are also welcome...

Measuring circuit is an classic design for ESR/Impedance meters.
100kHz square wave will be generated via opamp oscillator/ square wave generator. Then signal will be attenuated so that peak to peak voltage is 100mV.
DUT is connected to the test signal via simple protection circuit. Current is measured on the 10 Ohm shunt resistor.
Inverting amplifier and diode bridge rectifier with schottky diodes will be used for the amplification and rectification of the signal.
Other purpose of the diodes will be creation of the nonlinear scale (due to voltage characteristic of the diode) so that small ESR values can be displayed.

I still have some questions and I'll need your advices on the schematic design. I'm not very experienced, and this is one of my firsts "from start to finish project". So I'll probably have a lot of questions.
This is all for now, I'll put schematic diagram, simulation results, and some questions in the next post, this post is already too long.

[RiverTown]

In this post I'll write some details about schematic. I'll need your advices about some design decisions, and components selection.

At the beginning of the design I've selected LTC6244 as a main opamp. It has great specs for the price (low noise, low offset voltage, high slew rate, and it is rail to rail opamp).
Later on I find out that TME.eu don't have them, and I planned to get all the parts from them.
I haven't decided yet will I use another similar opamp or will I order LTC6244 from different supplier.
To be honest I've had hard time finding opamp with similar specs and price from a different manufacturer.
Can you list or recommed some similar opamps?
What you think should change the opamp?

Ok, enough about opams. Let's talk about schematic.
This is first version:


U1 is configured as an 100 kHz square wave generator, some minor values adjustment are possible after prototyping to achieve oscillation frequency closer to the 100 kHz mark.
C2 is here to block any unwanted DC component to enter the transformer.
My first idea was to use switching transformer for attenuating voltage from 5 Vpp to 100 mVpp, as you can see in schematic.

Later on I realized that it wasn't very good idea. Transformer is big, expensive, and there is great possibility that transformer with needed turns ratio won't be avaliable. And I realy don't want to wind my own transformer.
The only other easy way to reduce voltage which I was able to think of is using a voltage divider and opamp as a buffer.
I'm not sure if this is an good idea or if there is some better way to do this but simulation works fine.

This is second version of the schematic:


As you can see almost everything is the same.
ESR and DUT capacitors are the parts of the capacitor under test.
R6 acts as a bleeder resistor.
C3 is here to provide DC isolation, his voltage rating must be higher than the highest voltage capacitor being tested.
Anti-parallel connected diodes (D1 and D2) are the "last line of defence".
R7 is the shunt resistor, and R10 is the galvanometer.

These are some of the simulation results:

Voltage on the output of the oscillator:


Voltage on the output of the buffer:


Current through the shunt resistor:


Voltage on the output of the amplifier:


Current through the galvanometer:


Scale deflection (Current through the galvanometer) VS ESR graph:


Some minor changes and adjustments will probably happen.

What you think about this design, what would you change?
Is my idea of using voltage divider for reducing test voltage ok, can you suggest me some alternatives?
Any comments are welcome.

Regards

[RiverTown]

No one has an advice, or can help me with my doubts/ questions ?
I know that posts are long, and probably boring for lot of forum members but I really need a little push to continue  with design .

[trueblue2k2]

I was a bench tech for many years (now retired), and always wanted an ESR tester but never could find one at my price.  Creative Electronics had a nice analog one using a TL084 quad op amp back in the 1980's, but wanted $99 for it.  I had the schematic and parts, but never got around to it.  Since then I've watched for an even simpler analog circuit.  Most LCD repairs involve electrolytic caps so it's even more needed.

Most newer designs have the un-needed accuracy, complexity, and $$ of digital displays.  Another disadvantage is that analog displays tell you more at a glance from the meter's go - no go scale markings.  It is the ideal type of circuit for using such older technology.  Most users are quite happy with their metered testers. 

Several years back I found a great circuit using fewer and less critical components.  Revisiting the web recently I see many others who agree, and are building that design.  A similar protection circuit to yours has been added .  The designers name is Manfred Mornhinweg and his original project is here:  http://www.ludens.cl/Electron/esr/esr.html 

Here are some more links to recent builders: https://duckduckgo.com/?q=manfred+Mornhinweg+%2Besr

Best Regards
William Bourne
Edmonton, Alberta, Canada

[trueblue2k2]

A couple of things I should mention.  Manfred is an electronics engineer.  You should know that this simple, free ESR meter circuit is designed by someone who can avoid possible errors a lay person might make.  His other projects are as simple and yet elegant in function as is this one as he explains them step by step to the reader.   

To better appreciate Manfred's ability,  you can read his explanation in the link below how he unwittingly bought a new, cheaply built Chinese 3.5kw power inverter that didn't work.  He went through it from one end to the other with a commentary about its serious deficiencies and explaining the knowledge and advanced principles only an engineer would know to correct each one in turn.  http://web.archive.org/web/20100929082239/http://ludens.cl/Electron/chinverter/chinverter.html

He got it working reliably even though it was junk, and entirely under-designed at the factory.  They should have paid him to proof this and their other products as well!

[KD0CAC John]

2K2 , broken link
Here are some more links to recent builders: https://duckduckgo.com/?q=manfred+Mornhinweg+%2Besr
 
Thanks guys

[RiverTown]

First of all I want to thank for your comments, and want to apologize for late reply.

Most newer designs have the un-needed accuracy, complexity, and $$ of digital displays.  Another disadvantage is that analog displays tell you more at a glance from the meter's go - no go scale markings.

Yes, quick reading of approximate value is why I decided to use analog scale.
I want to build meter with reasonable accuracy, I know that that is not necessary, but I'm building this meter for myself so cost reduction isn't one of main parameters. Few bucks extra is not the big deal.
LI-Ion battery and charging circuit have already increased cost.

Several years back I found a great circuit using fewer and less critical components.  Revisiting the web recently I see many others who agree, and are building that design.  A similar protection circuit to yours has been added .  The designers name is Manfred Mornhinweg and his original project is here:  http://www.ludens.cl/Electron/esr/esr.html 

I've already seen that schematic and heard that excellent. But there is still problem of the transformer which is used for reduction of the test signal. I really want avoid that transformer.
Other thing is that I want to design my own circuit, I know that it will probably be similar with other ESR meter designs, but I still want my schematic.

Wow, I don't know what to say about that "inverter". It's shocking how bad it is. I believe it managed to failed on the field of ALL safety and EMC regulations.

[kripton2035]

on my dedicated esr web page you can find lots of analog or digital esr meters you can build yourself, or get inspired to make your own
http://kripton2035.free.fr/esr-repository.html

[RiverTown]

I've already been on your site, and it was very helpful.
As you can see from the first two post, I've designed my schematic for meter.
Now I need comments about design, and some advices related to the power supply, and usage of the voltage divider and voltage follower for reducing voltage.

[RiverTown]

Power supply section of the meter is finished. Final version of a PSU is in attachment.
Schematic is tested and everything works great. There is less than 5 mV ripple on the output with 20mA load on each rail. I'm still waiting for battery to arrive so that I can test charging circuit, but signal leds work as expected without battery so everything should be fine.
R8 is a zero ohm resistor, so it can be left out. I've put it there so that in case of high ripple I can put resistor or inductor there to create RC/LC filter.
All capacitors are ceramic.