Sonsivri

Hi everyone,

I would like to share a project I made recently : I adapted a shorty device and added a display.
it becomes a 0.1mΩ meter with surprising stability.
the original shorty project is here : https://hackaday.io/project/3635-shorty-short-circuit-finder
I did one showed here (http://kripton2035.free.fr/Projects/proj-shorty.html) and was pleased with its performances, but it lack a display.
so I decided to add one, and here is the result.

here is a video showing the measuring of some small values resistors :
https://youtu.be/4F6QNSpWTos


here another video showing continuous resistance measuring along a soldier roller :
https://youtu.be/8mYyr9uKpFU


and last another video showing how to find a shorted capacitor on a 820-00165 macbook air motherboard.
fast and easy.
https://youtu.be/L4V3BWReZLY


I'm cleaning the schematic and sources, and will share it when it's ready.
I'm building a pcb right now, and will test it before.

Nice work!

This project reminds me of a Toneohm 580 which we used many years ago to find shorts in pcb's.  It had an inductive sensor, the same company also made a resistance one like your unit.

Could you tell me what frequency and duty cycle the tester is feeding to the PNP transistor?

Thanks for sharing!

I agree it's a nice project, Seems to work very well like the video's

Looking forward to see the project grow, I will definitely be making one looks like a really useful tool

it is DC +5v. it is powered at the beginning of the arduino sketch and never powered off.
it is a DC milli ohm meter.
I suppose it acts as a sort of constant current source.

Posted on: February 14, 2021, 07:54:31 19:54 - Automerged

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actual state of the pcb I'm building.

Looking nice.
What is the current source 100Ma ?

Good to that your using the 4 wire method for the probes

around 60mA

Excellent work. Just one question and not related to the instrument. Which was the capacitance value of the piece you had to replace on the macbook? Just curiosity.
Regards.

all these capacitors that may fail on the 3 banks are 62uF/11V. I've fixed 3 of them by now and it was never the same capacitor.

Did you get it to work? Or (I'm guessing) it starts working and then another cap goes to short and so? Maybe they blow up because another reason... is that the power source stage?

yes the macbook works fine after capacitor replacement. these capacitors are crap they fail after 4-5 years of use.
I did not bother to replace them all, but it would be a good practice in fact.

Nice! I´ll wait for your schematics. Seems to be a great tool to build and add to the lab.
Greetings.

it's almost mandatory since you're dealing with milli-ohms, even less.

Looking forward to more details and schematics!  

I built up a couple of "half-ohm" adapters for multimeters, they have been extremely useful for finding shorted traces on home-made PCBs and shorted components.  
http://jaanus.tech-thing.org/category/half-ohm/

I've always wanted a "shorty" but now that you've added a display it is even better!  Thank you. 

@kripton2035

I would be interested in buying a bare pcb from you

I've published the first draft of schematic and source code of the project. enjoy !
http://kripton2035.free.fr/Projects/shorty-display.html

very nice project... is this able to use a milliohm range?

I can read stable result down to 10µΩ and up to 3.5Ω
but as there is no precision resistor in the device, you don't have a great accuracy in the displayed value. (but I'm working on it  ::)
but you can make relative measurments in the 10µΩ range for a low cost.

take a look at this video for the measuring of some low values resistors :
https://www.youtube.com/watch?v=4F6QNSpWTos

prototype pcb have been ordered. now wait 2 weeks to get them and test them.

Excellent project and a one stop solution for the headache of finding the exact culprit on board. Can you please clarify regarding the probe connections, as the connector shows 5 connections and the red probe picture shows two wires shorted at the soldering point.

In addition to the present concept, (Just for a reference) here in this video he uses an additional protection for accidentally frying it with 230v. He uses a Transient Blocking unit (TBU) in series with the probes. His project doesn't display the resistance but simple using an op-amp.
https://www.youtube.com/watch?v=N2M-p-OGvPg

well at some point, the closest possible of the device to test, you must soldier the wires...
here I did it at the end of the metal part of the probes, the closest I could do with these probes.
so there are 4 wires from the pcb to the probes, and they are soldered at the top end of the probes.

Ok i understood that the 4 wires take an average reading to the measuring tip to calibrate at least resistance at the tip. So can you please shed some light over the centre pin used shown in the schematic which is a ground. I guess it is for the shielding of the 2 probe wires?

yes I used an audio shielded cable. left and right goes to the probe, and ground for shielding to the center pin of the din plug.

Please, share the results.
Regards,
kreutz

Hi there,
I've got the pcb and tested them.
but I have some spurious oscillation at the output of the opamp
and I'm struggling to find the reason for some times now...
as I have changed some parts of the design, it may come from that, but I have to investigate.
more to come soon, thanks for your patience.

If you don't want to build your own you can already buy such a milliohmmeter  with 4 wire probes, Alu case and LiIon battery for 60$ here:
https://www.aliexpress.com/item/1005002597400113.html

Did you want some help troubleshooting this issue? Post a pic of the spurious oscillations and more details..

the schematic looka actually like this, with a more stable current source.
I have oscillations at the output of the aop, around 10-20Hz that forbid the precise measure
I've put decoupling capacitors 1-10µF on each IC with no luck.

If initially there was no such problem, then most likely this is due to the layout of the components on the printed circuit board.

is there a LC or RC filter into the opamp  +ve supply line ?

it will need decoupling from the rest of the circuit.

suggest something like a ferrite bead or a 10R resistor, and 1UF or 10uF capacitor,

It will probably benefit if you use some kind of EMI filtering at the input of the opamp. See http://ww1.microchip.com/downloads/en/appnotes/00001767a.pdf (http://ww1.microchip.com/downloads/en/appnotes/00001767a.pdf) (Solutions for Radio Frequency Electromagnetic Interference in Amplifier Circuits by Microchip) for ideas. Or better yet [url]https://www.analog.com/media/en/training-seminars/tutorials/MT-070.pdf/url] by Analog Devices.

there is no LC or RC or ferrite bead on the +V line, but as the power supply is a 9v battery is it really necessary ?

no the initial schematic was a little different, there was no zener look at the preivious page of this thread, or the schematic on my web page for the project
the new current source is less dependant of the supply voltage, so it should have a better reproductibility of the measures.

Zener diodes tend to produce white noise, it may be worth analyzing in this direction.

Did it ever work correctly with the new current source?
What is the amplitude of the oscillations?
Can you show the scope display capture?

Use an oscilloscope to measure your oscillations and post the pic.

or better he used temperature compensated voltage reference like LM336DZ 3.6 volt   

kripton2035  wrote

there is no LC or RC or ferrite bead on the +V line, but as the power supply is a 9v battery is it really necessary ?

no the initial schematic was a little different, there was no zener look at the preivious page of this thread, or the schematic on my web page for the project
the new current source is less dependant of the supply voltage, so it should have a better reproductibility of the measures.


I suggest you carefully lift the AD +ve supply pin, and add the RC

Then you can see if it fixes the problem

10 - 20Hz is a strange frequency. Too low to be a loop oscillation directly or mains. But it might be an alias of a much higher frequency aliasing with your scope or whatever it is you are using to measure with.

If you have supply oscillations, you should be able to see them

1) measure your power supply pins. Do they also oscillate?

Do you have a common mode oscillation because the whole thing is floating, sitting on your desk or ESD mat ?

2) Measure your ground, is it also oscillating?

I used a LM317 as an adjustable current source. After tweaking the correction factor in code and current adjust, I get repeatable readings.



****corrected schematic attached

Looking at the used Opamp DS, the slew rate is rather low (the AD8628 https://www.analog.com/media/en/technical-documentation/data-sheets/AD8628_8629_8630.pdf (https://www.analog.com/media/en/technical-documentation/data-sheets/AD8628_8629_8630.pdf) opamp being an instrumentation opamp that's not surprising) which should make it rather insensitive, but it might help if you add some small capacitors in parallel with the negative reaction resistor (33k by value): a 10pF to 30pF might help but you can try with a higher value, into the nF range. Like @optikon said, the oscillation you see might be just an envelope of a higher frequency oscillation.
Other than that, given that you have placed the decoupling 0.1uF capacitor between VCC/VSS, the only thing that may be the cause is to have some capacitance connected where is not supposed to :)

If I were in your place I would disconnect the 1k resistor found on the negative input of the opamp and check if there are still oscillations. This way you can determine where the oscillations are caused. You can also remove the current source transistor (or whatever you have in the new schematic). Bottom line is that you have to isolate the exact location where those oscillations are generated. After that it is much easier to fix the problem by adding some capacitors.

Here is a cheap modification for the current source that is worth a try maybe.

Replacing the zener with a classic TL431 may provide improved regulation with consuming about 50% of the current required for the existing regulator.

Below shows, the diagram and the result of imposing a 2 Vpp ripple on the  5 VDC for testing purposes. The new current source (AM1) shows a variation of only 0.02 mA while the old one (AM3) varies by a few mA's. The variation of the old source shows some non-linearity while the new one appears more symmetric.

These are just simulation results. I have not tested it in hardware.  

would be good to understand why the osc is occurring. If you want to change the current source, just do it with an opamp and transistor. You can crush supply LF supply noise by nearly 120 dB. it will have superior performance in all regards. That is, if it was the problem to begin with-- which hasn't been established yet.

I totally agree that using an opamp based CS will be superior. However, that needs a complete pcb modification. I was thinking along the lines of minimal modification if there are already pcb's produced. The TL431 can be implemented without much trouble in place of the zener.

It will be useful to see some waveforms and to establish if the design performed correctly after the current source was modified the first time round.

thanks guys for all these inspiring ideas.
will try them
but I have a work project that has to be closed in the next weeks and I won't have much time for the shorty project now.
Hope to have more time for this in july. stay tuned.

why not use a linear adjustable regulator as current source which will also safe some space in circuit see attach milliohm metter, in Elektor electronic a nice design was published with op-amp based current source I will search it and share

i have found this
http://cappels.org/dproj/dlmom/dlmom.html
lock-in amplifier  can be a good solution

no, because it tests AC ohms, I want to test DC ohms.
as there are capacitors on supply rails, if you use such an ac ohm meter, it will always tells you there is zero ohms, the capacitor impedence.
using dc ohms meter, you can find shorted capacitors.

Ahhh Yes you have a good point about making it fairly easy to modify an existing PCB. The various flavors of 431 are one of my favorite parts.. You can get a lot of performance from them for very little cost. Nice little parts they are.

Yes we're still waiting for OP to post some waveforms. 10-20Hz has piqued my curiosity because its quite odd to me. Guess Kripton is busy.. I guess there is no hurry.

Simple yes or no will do.

I made the prototype on a perfboard, with the original "current source" (in fact it is not one...)
I made a pcb and during that time I thought I would make this new true current source (at least better)
I modify the pcb to be able to have both current sources on the same pcb if anything went wrong I could still use the original one.
I received the pcb, tried the new current source and it never worked dur to these oscillations.

So Finally Found it Elektor Milliohm used two topology PWM pulses and constant current share for reference and an idea

sadman

At least we know now that the issue is caused by using the new CS.  :) maybe?

next thing I will try is to resoldier the original current source, and see if these oscillations are still here. it's not a big work to do. they are very similar.
then I will try to capture these oscillations if still there.
the original "current source" is not one in fact. each time you power off-on the device, there are small changes in the voltage supply
and the displayed value is not the same. I find it annoying, and tried to have a more stable current source for that purpose.

What about keeping the original circuit and stabilizing the voltage? What is important is to treat the circuit as a "short finder" and not as a "precision micro/milli -ohm meter".