YMC01 Micro-ohm Meter - Anyone got a 20 or 200 ohm version?

[mbenn42]

I just bought a YMC01 micro-ohm meter off Aliexpress (cheap and cheerful, obviously ;-)) It is available with a 2, 20 and 200 ohm maximum range.  I bought the 2 ohm one to help me find PCB shorts, etc.,  but in true Dave fashion the first thing I did was to "tear it apart".  the PCB has solder links which can be used to select between 200R, 20R and 2R ranges, so I was wondering if I can just add a single pole triple throw switch in there to get all three options, or if I'd need to change anything else as well?  Does anyone have one of these in either the 20R or 200R range who can do a comparison with mine to see if anything else has changed?

The resistor / capacitor values on mine are as follows:

Resistors
R1 331
R2 1R0
R3 331
R4 203
R5 2001
R6 4702
R8 331
R9 1003
R10 203
R11 Not populated
R12 Wire link
R13 1R00
R14 Not populated
R15 680
R16 36

Capacitors, measured in-circuit (measured with Aneng 8009, so take with a pinch of salt!)
C1 102nf
C2 97nF
C3 Measures open?
C4 231nF
C5 95nF
C6 7.8uF
C7 100nF
C8 Not populated
C9 50nF
C10 0.1nF
C11 7.5uF
C12 7.5uF

Also, there is an unpopulated SW footprint, which seems to be for a push switch to hold the value, as shown on some online images / videos.  Anyone know how I can implement this please?  I've tried just shorting it, but that doesn't work :-)

Many thanks in advance for any assistance.

MB

[Overspeed]

Hello

Following the specs that a current level problem , following the specs that far to be a metrology instrument , better to post in '' project ''

Can you post pictures of the PCB

That can be easier to built a simple current source by using a TL431 as voltage ref and a transistor general purpose to generate 1 mA and use a multimeter to read the Voltage value

see TI slyt768 in pdf
or
ApplicationReport
 SNOAA46–June2020 in pdf
or the nice Scullhom video on LT3098 a very nice IC and easy to use



Regards
OS

[Kleinstein]

I makes perfect sense to have the current source and voltage reading integrated, as they should use the same voltage reference.
For the voltage reading one anyway want good resolution (like 1 µV range), which most of the shelf simple DMMs don't offer.

[mbenn42]

Thanks for that :-)

I did consider making my own milli / micro-ohm meter, or doing something like the Scullcom unit, but I've got loads of other projects on the go, so thought I'd just add the YMC01 to an in-progress order from Ali :-)

I've added images of the PCB of the YMC01, as requested.  I thought I'd done it on the original post, but I must have screwed that up :-S

Cheers

MB

[Overspeed]

Hello

OK but .......

a complete project with schematic and gerber available on PCBWAY

https://www.pcbway.com/project/shareproject/Miliohmmeter_accurate_instrument_up_to10_mohms_8844aa44.html

video



Most consuming time in a project is generate the schematic ''working'' and the PCB files , after that quite fast if the BOM is not a 3 pages BOM with a lot of adjustments and calibration to do . 

this solution use a TL431 with an adjustment of the voltage to drive an OP07 which is interesting OP AMP regarding performance ( there is alternative with even better performance  )

Try to hack / modify an existing circuit is sometime more time consuming than start with a well documented set of files , not included the possible ?? result in precision which is directly linked to the voltage ref and the OP AMP performances but also the resistors quality .

Regards
OS

[Kean]

Those three jumpers seem to connect to the 7-segment LED displays.  I suspect all they do is move the fixed decimal place position, and other components change the actual reading range.

[Overspeed]

Those three jumpers seem to connect to the 7-segment LED displays.  I suspect all they do is move the fixed decimal place position, and other components change the actual reading range.

Hello

Following the specs in the screen print : 2 Ohm use a 100 mA current and 20 Ohm a 10 mA current so 10 factor that perhaps just resistor to change

Regards
OS

[Overspeed]

Hello

Too bad as this subject looks dead , a picture of the PCB back copper without the battery glued on it , was a good point to have a better view of the schematic used .

Regards
OS

[mbenn42]

Thanks to all who have responded, and apologies for the delay in replying :-)

It was a pain getting the battery off, then a bigger pain getting the black gunk off the board!  Anyway, here are a few pictures of the back of the PCB without the battery.

Cheers

MB

[Overspeed]

Hello

Thanks , no problems but that demonstrate that modification / hacking is not so easy / fast

As you can see you have a 2R / 20 R / 200 R soldering pads , look to be a soldered jumper so as Kean says that the display management as
0.000
00.00
0.000

the DH 7107 is a display management pinout is

Pinout
DH7107GP Pin Configuration

Detailed Pin Description:

Pin 1 (A): Segment A Control Input.

Pin 2 (B): Segment B Control Input.

Pin 3 (C): Segment C Control Input.

Pin 4 (D): Segment D Control Input.

Pin 5 (E): Segment E Control Input.

Pin 6 (F): Segment F Control Input.

Pin 7 (G): Segment G Control Input.

Pin 8 (DP): Decimal Point Control Input.

Pin 9 (NC): No Connection.

Pin 10 (LE): Latch Enable Input.

Pin 11 (BI): Blanking Input (Active Low).

Pin 12 (LT): Lamp Test Input (Active Low).

Pin 13 (RBI): Ripple Blanking Input.

Pin 14 (RCO): Ripple Carry Output.

Pin 15 (CLK): Clock Input.

Pin 16 (VCC): Positive Power Supply Pin.

Pin 17 (GND): Ground Pin



Regards
OS

[Kean]

As I suspected, you can see that the 3 jumpers just connect to different decimal point segments on the LED displays.  So selecting 200R will put the fixed decimal between 3rd and 4th digit.

I am pretty sure U1 is just a battery charging IC.
The main chip would be a SMD clone of a ICL7107 which is a 3.5 digit A/D converter that directly drives a 4 digit 7-seg display.
I believe the big orange caps are the sampling and reference capacitors for the ADC.
Not sure about the DH7107G clone, but the original ICL7107 needs +/-5V so the circuit to the left of the big IC could be a switching regulator to create those voltages (explains D3 & D4).

That doesn't leave a lot to reverse engineer - so it isn't a particularly fancy circuit.  I imagine it is just a (poorly regulated) current source and basic volt meter.

[Overspeed]

As I suspected, you can see that the 3 jumpers just connect to different decimal point segments on the LED displays.  So selecting 200R will put the fixed decimal between 3rd and 4th digit.

I am pretty sure U1 is just a battery charging IC.
The main chip would be a SMD clone of a ICL7107 which is a 3.5 digit A/D converter that directly drives a 4 digit 7-seg display.
I believe the big orange caps are the sampling and reference capacitors for the ADC.
Not sure about the DH7107G clone, but the original ICL7107 needs +/-5V so the circuit to the left of the big IC could be a switching regulator to create those voltages (explains D3 & D4).

That doesn't leave a lot to reverse engineer - so it isn't a particularly fancy circuit.  I imagine it is just a (poorly regulated) current source and basic volt meter.

Hello

on the poorly regulated current source a simple Tl431 , a LM358 with a small Mosfet can provide better result ,  I have never built a voltemter as a basic multimeter is in the 20 Euro range and I use mostly my UNI-T 61 as swiss knife

Regards
OS

[kripton2035]

Please be aware that you need 0.1mΩ resolution to precisely detect short circuits on pcb. 1mΩ is not enough.
build yourself a shorty-with-display (see my signature below) to get more than 1mΩ resolution (I did not say accuracy...)

[schmitt trigger]

Very clever approach the use of the relative and probe compensation routines.
How often does it need to be “re-compensated”?

[kripton2035]

I usualy calibrate it every time I check a new pcb for short, which does not happen a lot !
as you most often need some relative low ohm value in an copper area, if I can't find a lower ohm in a place, I re-calibrate.

[Overspeed]

Please be aware that you need 0.1mΩ resolution to precisely detect short circuits on pcb. 1mΩ is not enough.
build yourself a shorty-with-display (see my signature below) to get more than 1mΩ resolution (I did not say accuracy...)

Hello

Interesting tool , PCB will help a lot as that time consuming to redraw the schematic and generate the Gerber set

Accuracy can be improved by resistor selection for R1 R2 R3 and matching R4 and R5 ?

MCP3421 performance looks quite good for a portable instrument

main concern can be to locate accurate resistor for test / calibration

Regards
OS

[kripton2035]

in case you haven't seen this topic : https://www.eevblog.com/forum/testgear/finding-short-on-motherboards-with-a-shorty-(with-display)/

[Overspeed]

in case you haven't seen this topic : https://www.eevblog.com/forum/testgear/finding-short-on-motherboards-with-a-shorty-(with-display)/

Hello

Thanks for the link

That solution to add a LDO 3.3V and remove the transistor sound interesting ( EVI solution page 8 )

Regards
OS

[Doctorandus_P]

For occasional measurements, you can use a standard lab power supply and set it's current limit to 1A (or 100mA, etc), connect it to a resistor, and then measure the voltage over the resistor with a standard DMM. This has inherent 4-wire measurement and is easy to setup and you can measure micro ohms with it, providing your device can handle the 1A test current. This is good enough for me, and I never bothered to buy or make more specialized equipment for this.

Concerning the YMC01. The PCB is simple enough to reverse engineer it. Is that a standard 7107 in the circuit?

[kripton2035]

For occasional measurements, you can use a standard lab power supply and set it's current limit to 1A (or 100mA, etc), connect it to a resistor, and then measure the voltage over the resistor with a standard DMM. This has inherent 4-wire measurement and is easy to setup and you can measure micro ohms with it, providing your device can handle the 1A test current. This is good enough for me, and I never bothered to buy or make more specialized equipment for this.

the shorty device above is portable, uses 4 wires measurement, sends only less than 40mA into the device, and displays 0.1mΩ resolution.