DIY milliohm/volt meter kit

[iloveelectronics]

Not sure if this is the right place to post this but I thought although I didn't design it it's a project nonetheless for a novice hobbyist like myself

Inspired by another discussion about milliohm meters a few weeks ago, I went searching for one and found a Chinese designed DIY kit. Seeing that it's all 0805 and 0603 SMD components and quite a few IC's with small pin pitches I was initially a bit hesitant (as I failed miserably before with another DIY kit with a MCU in a LQFP package) but thought I would give it a go anyway.

The soldering job is a bit rubbish but at least this time no chips get killed and it works!

Besides the PCB and components the kit also comes with a lot of documentation (in Chinese), a pair of Kelvin clips and leads, an acrylic case, a lithium rechargeable battery, a programmer (for uploading the firmware), and even the source code of the firmware.

I took the attached pictures before I attempted to calibrate it but I was already quite impressed with its resolution and stability in the readings. It has 2 ohm ranges (0.2 and 20) and in the 0.2 ohm range it offers resolution all the way down to 1 micro ohm!

I have since done some not-so-precise calibration with the gear I have and compared the results against the HP3457A and this little device is tracking very well with the HP across the ranges. I will post more pictures in this thread later on as I'm trying to order some resistance standards for calibration.

The device also doubles as a 5.5 digit volt meter by changing some jumpers on the back side and I have played around with that a little bit today. Initial impression is that it has great accuracy but seems to drift a bit over time. I will also update on that later.

[daveshah]

Have you thought about selling these?

[iloveelectronics]

Have you thought about selling these?

Yes, I have. I have actually already got permission from the author to resell his kits but it will take me some time to translate the Chinese documentation into English.

[Thor-Arne]

Very interesting. 

[iloveelectronics]

I'm still waiting for some resistance standards to arrive but I went ahead and did some calibration using my 0.5%/2% tolerance (in the x1/x0.1 ohm ranges respectively) decade resistance box. I calibrated the device against my HP3457A on 18 ohms on the 20 ohm range and 0.1 ohms on the 0.2 ohm range.

Here are a few pictures of how the measurements taken by this little DIY device compare to the HP3457A. In the pictures they are measuring 10 ohms and 0.2 ohms. The HP measures 10.0140 ohms and 0.2102 ohms (with some jumping around on the least significant digit), while the milliohm meter measures 10013.90 mOhms and 210.213 mOhms (with actually less jumping on the LSD even though the resolution is higher than the HP).

I'm happy   

[BravoV]

Franky, I'm interested, as long the price is reasonable. Any discount for early bird ? .... j/k 

Btw, are you planning to sell it as kit or fully soldered ? No calibration needed right ?

Is there any schematic ? Or what voltage reference and precision resistor does it use in there ?
Appreciate if you can provide a high res photo of the board so we can see the component's label.

[george graves]

Interested!

[iloveelectronics]

Franky, I'm interested, as long the price is reasonable. Any discount for early bird ? .... j/k 

Btw, are you planning to sell it as kit or fully soldered ? No calibration needed right ?

Is there any schematic ? Or what voltage reference and precision resistor does it use in there ?
Appreciate if you can provide a high res photo of the board so we can see the component's label.

It will be sold as a kit. In terms of pricing, expect something in the UT61E price range for the full kit including the case and the programmer. If it's just the PCB and components obviously it will be a bit cheaper, but you will still need a programmer of some sort to upload the firmware. Having said all that, I think it will still take me a good while to translate the documentation into English so please don't hold your breath It works right after assembly and firmware upload but you will need to do some simple calibration to achieve the best accuracy.

There is a schematic, I will try to find the original post the author made in some Chinese forum later on, he talks about loads of design details (not included in the documentation that comes with the kit) but unfortunately it's all written in Chinese. In its current version it uses a 25R 0.05% 5ppm 0.5W precision resistor (no branding, looks like some Chinese made component) as a resistance reference and a REF5025A voltage reference. The MCU is a C8051F350. I'll try to take a high res picture of the board later on, need to get back to packing eBay orders right now 

[BravoV]

It will be sold as a kit. In terms of pricing, expect something in the UT61E price range for the full kit including the case and the programmer.

From the photos, I assume that it has no more space for battery right ? Btw, what voltage it needs ? 9 V battery ?

Btw, please if you had time, measure the current consumption too please, for sure if I'm going to purchase it, I will not purchase with the case cause I want it to be self powered and portable.

... but you will still need a programmer of some sort to upload the firmware.

I see you powered it from the USB at the photo, the word "programmer" here means a program that upload the firmware through that USB ? or we must "pre-program" a chip using external programmer before we solder it ?

Please clarify this, since this tiny step makes huge different at the practicality on assembling the kit, especially for user with limited tool.

... but you will need to do some simple calibration to achieve the best accuracy.

Such as ? What if the user doesn't have any precision reference or what so ever tool to do it at home ? Will it good enough without that extra cal ?

[iloveelectronics]

From the photos, I assume that it has no more space for battery right ? Btw, what voltage it needs ? 9 V battery ?

Btw, please if you had time, measure the current consumption too please, for sure if I'm going to purchase it, I will not purchase with the case cause I want it to be self powered and portable.
 

There is actually enough room for a small 3.7 lithium polymer battery (which is what I use now), which came with the kit I bought, but will not be included when I sell due to the shipping restrictions. The board includes charging circuit for the battery through USB too. You can see in the HP3457A comparison pics that it is self powered by battery, no USB cable. It is perfectly portable. I will try to measure current consumption later.

I see you powered it from the USB at the photo, the word "programmer" here means a program that upload the firmware through that USB ? or we must "pre-program" a chip using external programmer before we solder it ?

By “programmer” I mean a small board (already assembled) for interfacing between the device and your computer through some header pins on the PCB. The software is a free download from the MCU manufacturer. There's no need to program the chip before soldering, you can program it on board, very easy to update the firmware in the future too. BTW, the device also spits out serial data at the same rate the screen updates (10 times a second in fast mode or 1 time per second in slow mode). Potentially you can write your own software to do data logging etc.

Such as ? What if the user doesn't have any precision reference or what so ever tool to do it at home ? Will it good enough without that extra cal ?

Basically all you need to do is measure a known resistance (or voltage in volt meter mode) and adjust the reading to that known value, and you repeat that in each range. The meter “out of the box” would measure reasonably well  (as I showed in the first post) but you will need some references to achieve better precision. Without some precision references you will simply have to calibrate it against any best standards you have (a trusted DMM etc)

[BravoV]

Now we need you to take a photo on the back where the battery is installed with the cover opened, this at least to give us the idea the size of the battery that the user needs to find, for li-ion cell only, right ?

Also with the current consumption you gathered later, this will be useful at least for me to decide whether its enough just to use that case, or should I just skip it if the runing time is too short with the given battery size.

Great to know that the "programming" part is not potential problem for user with limited tool (and probably limited skill as well). 

What is the specification anyway ? Max range for voltage and resistance measurements ? Only DC volt ? Resolution and etc.

[dannyf]

Here is the link, with full documentation:

http://bbs.38hot.net/thread-45486-1-1.html

It utilizes the wonderful f350's internal 24bit adc + external reference.

Good for 5.5 digits.

Light years ahead of what has been discussed on this forum.

btw, take a look at the layout.

You will notice another design mentioned in that very thread - far more advanced than most amaturers could do.

[dannyf]

The prices: seem to be ~170RMB (30 usd) for the kit (lacking a few parts) + ~300 rmb ($50) for the fully assembled meter + postage (~10 usd?).

[iloveelectronics]

Yes, that's the link to the Chinese forum where he talks about many design details, which is far beyond what I can understand

[iloveelectronics]

Now we need you to take a photo on the back where the battery is installed with the cover opened, this at least to give us the idea the size of the battery that the user needs to find, for li-ion cell only, right ?

I've attached a picture which shows where I put the battery. I just use some double sided tape to keep it in place.

Also with the current consumption you gathered later, this will be useful at least for me to decide whether its enough just to use that case, or should I just skip it if the runing time is too short with the given battery size.

I measured the current consumption without the backlight to be around 16-17mA, and with the backlight on it's about 56-57mA. It doesn't seem to make a different in either the milliohm meter or volt meter mode. The battery I use is labelled 1.9Wh, and at 3.7V it should be equivalent to about 500mAh. If my calculation is correct, on a full charge it should give me just under 30 hours of use without the backlight.

What is the specification anyway ? Max range for voltage and resistance measurements ? Only DC volt ? Resolution and etc.

The author claims 0.1% + 40 counts in the milliohm mode and 0.01% + 4 counts on the volt meter mode, and he says he's being conservative. It has 2 ranges for resistance measurement (0.2 and 20 ohms with resoution down to 1 micro ohm and 10 micro ohm respectively), and 4 ranges (0.1, 1, 10, 100V with 150000 maximum counts resolution) for DC voltage measurement.

I've also attached a high res photo of the board (after opening it right click and choose "view image" to see it in full size). Please don't pay too much attention to the soldering and part alignment. It's not the best in the world, but I tried

[dannyf]

which is far beyond what I can understand

Not impossible to follow with google translate, - the schematic is fully transparent.

[iloveelectronics]

which is far beyond what I can understand

Not impossible to follow with google translate, - the schematic is fully transparent.

It's not the language. I can read Chinese no problem. It's my lack of knowledge in basic electronics, design and theories

[Wytnucls]

What's the latest with the voltage drift you noticed earlier, Franki?
Calibrating that instrument on voltage is going to be a bitch for someone who hasn't got access to something with an order of magnitude more accuracy.

[iloveelectronics]

What's the latest with the voltage drift you noticed earlier, Franki?
Calibrating that instrument on voltage is going to be a bitch for someone who hasn't got access to something with an order of magnitude more accuracy.

I haven't done more testing on the volt meter part yet. I never seem to have enough time in the lab... Will try again tomorrow.

[BravoV]

It utilizes the wonderful f350's internal 24bit adc + external reference.

Good for 5.5 digits.

What is so special about this 24bit ADC compared to others ?

Light years ahead of what has been discussed on this forum.

Which thread is that ? Mind pointing it out ?

[BravoV]

Franky, thanks for the replies & the hi-res photos. 

Please don't pay too much attention to the soldering and part alignment. It's not the best in the world, but I tried

To my amateur eyes, that looks perfect to me. But you missed a tiny spot, since its working fine as its soldered at the other side securely, I guess its not a big deal. 

[don.r]

Did this go anywhere? I am actualy even more interested in his other design in the Chinese thread for the $150 meter that goes deep into the uOhm range.

[iloveelectronics]

Did this go anywhere? I am actualy even more interested in his other design in the Chinese thread for the $150 meter that goes deep into the uOhm range.

Unfortunately between taking care of the existing eBay items and orders, other work and family duties there has been little time left so this han't gone anywhere yet I have a bunch of these already sitting on the shelves so I really need to make time for thi ASAP...

Which other design are you referring to?

[Tinkerer]

I would certainly be interested in buying something like this.
I have a UNI-T 61E and while pretty good, obviously not one for measuring such low resistances.

[don.r]

Did this go anywhere? I am actualy even more interested in his other design in the Chinese thread for the $150 meter that goes deep into the uOhm range.

Unfortunately between taking care of the existing eBay items and orders, other work and family duties there has been little time left so this han't gone anywhere yet I have a bunch of these already sitting on the shelves so I really need to make time for thi ASAP...

Which other design are you referring to?

Sorry, it was the same meter but following another thread linked to from this one. No worries about making time for this but there is plenty of interest on this thread so when you are ready, it looks there will be a few orders for you.

[jaxbird]

Very interested, please let us know when/if this becomes available in your store.

I think you can try sell a few with just the original Chinese instructions as long as you make it clear the instructions are in Chinese.

And you should probably cover yourself by stating this is a kit, skills and proper equipment required, and no guarantees, once bought the buyers are on their own

An already assembled and pre-calibrated version would also be interesting.

[HeyTom]

OK, you guys have inspired me to make my own milliohm meter for fun.  I wanted to have:

• two buttons, one to take a reading, the other to zero set.
• Runs from a 9v battery
• Critical parts are 0.1 % or better
• Reads to 4 decimal places
• component cost around $20

Photo is the breadboard prototype reading 0.9994 ohms on a 1 ohm 1% resistor.
I'm going to send out to have a few boards made soon.

I can share more if anyone is interested or wants to critique the design.

[dannyf]

Interesting.

I published a digital milliohm meter design a while back - it is based on a high value resistor (=ccs) + dut, and utilized the differential adc on an avr. Minimal part count.

But it requires calibration.

A few other approaches:

1) dut + known value resistor: this requires two instrumentation opamp;
2) an unknown ccs + dut + known value resistor: doable with just a mcu.

[HeyTom]

This one doesn't require calibration because it uses a fixed voltage reference.
It works by dumping 100mA through the test resistor just long enough to get a reading on the ADC.
You can use the "null" button to compensate for any resistance in your probe wires.
Here's a pencil cad diagram.  The "control on/off" is a couple of transistors used to start/stop the 100mA flow.
The only real precision you need is the 25ohm resistor and the 2.5 voltage ref.  The ADC is a MCP3425.  It's cheap, has an 8x gain, I2C and 0.05% built in reference.

[dannyf]

This one doesn't require calibration

It needs a precision Vref and a precision resistor.

You could have easily eliminated one of the two, or both.

[pelud]

Hi, I'm new here, and I like the project that you are working on. 

Here are my suggestions:

1. If you switch to a ADC with two inputs (maybe MCP3422?), then you can use one input to measure the voltage across the 25 ohm resistor and another to measure the device under test.  That way the only critical parameters (that I know of) become the various offset and linearity errors of the ADC and the tolerance of the 25 ohm resistor when measuring resistance.  You can even generate the 2.5V reference using a resistor voltage divider instead of another expensive Vref, as you can compensate for any variations using the reading from the 25 ohm resistor.

2. Don't forget that there will be 2.5W of power across the 25 ohm resistor, which can throw off the tolerance of precision resistors if they are not designed to handle surges, even if the time is brief.  You might want to lower the current to 10mA to stay within spec for a 1/4W resistor.

[don.r]

2. Don't forget that there will be 2.5W of power across the 25 ohm resistor, which can throw off the tolerance of precision resistors if they are not designed to handle surges, even if the time is brief.  You might want to lower the current to 10mA to stay within spec for a 1/4W resistor.

I get 0.25W across the 25 ohm resistor. (0.1^2 * 25). A 1/2W or 1W should have no problem with that, particularly if it is pulsed.

[pelud]

Oops, my bad.  I mistakenly applied P=VI instead of P=I^2R. Thanks for noticing. 

2. Don't forget that there will be 2.5W of power across the 25 ohm resistor, which can throw off the tolerance of precision resistors if they are not designed to handle surges, even if the time is brief.  You might want to lower the current to 10mA to stay within spec for a 1/4W resistor.

I get 0.25W across the 25 ohm resistor. (0.1^2 * 25). A 1/2W or 1W should have no problem with that, particularly if it is pulsed.

[pelud]

After thinking about this some more, you could also generate the Vref for the current source from a voltage divider from one of the I/O pins of the micro.  That way you can also control the power going to the resistor under test by setting the output to high or low.

[HeyTom]

The boards came back from the fabricator.  Here's a picture of a completed version.
For the 25 ohm resistor, I used 4 x 100 ohm (1/4 watt) in parallel.  Yeah, I could have gone with two 50s or three 75s but 100 ohm 0.1% are really easy to get. 

Initially I was just going to make this an adapter where you used you own meter to read the value.  The better the meter, the better the reading, but if you have a 6.5 digit multimeter you probably wouldn't need one of these and it's pretty inconvenient to have to hook up adapters.

Also, since the microcontroller only polls the ADC, handles the "null" function, and turns the current on and off, it would be pretty easy to change it to some kind of arduino or atmel if you wanted to do that.  The software is not complex. 

I also didn't want to have to fiddle with calibration.  I soldered in the parts, programmed the pic through icsp and it ran immediately.  So that's why I did the resistor/reference design.

I had the boards done at oshpark. I got the LCDs from buydisplay dot com.  They don't offer the orange ones anymore, and in my order I got one completely bad one, and one with a bad backlight, so maybe that was the end of the batch.

I'll attache the schematic and board files also.  If you want the software I can post that too.  Let me know.
In my eagle project folder I have a bunch of files names milliohm.b#1 2 3 ...  anyone know what those are for?

[sachleen]

In my eagle project folder I have a bunch of files names milliohm.b#1 2 3 ...  anyone know what those are for?

Those are backup files Eagle creates.

See page 20 on the Eagle Tutorial

[quarks]

Have you thought about selling these?

Yes, I have. I have actually already got permission from the author to resell his kits but it will take me some time to translate the Chinese documentation into English.

Looks interseting, please let us know where it can be ordered.

thanks
quarks

[iloveelectronics]

Yes, I have. I have actually already got permission from the author to resell his kits but it will take me some time to translate the Chinese documentation into English.

Looks interseting, please let us know where it can be ordered.

thanks
quarks

OK, I think the only way to force myself to hurry up with the translation of the document is to actually start taking pre-orders. PM me if you're interested. I am setting the pricing for the pre-orders at $62 per kit, including the PCB, the parts, the acrylic case and a programmer for uploading firmware, shipped worldwide with a tracking number. The LiPo battery will NOT be included. The external voltage reference REF5025A (for the volt meter part) will also not be included in that price, the internal reference will still work quite fine. I suppose I can stock some of these references too in case people want them, let me know when you PM me.

[bronson]

Hi Frankie, does the kit comes with source code?  With an appropriate toolchain, would I be able to change the behavior of the meter?

Also, what's the update rate?  If better than 2 readings/sec or so, I'm definitely interested.

Great find!

[iloveelectronics]

Hi Frankie, does the kit comes with source code?  With an appropriate toolchain, would I be able to change the behavior of the meter?

Yes, I believe it does, but I will need to double check.

Also, what's the update rate?  If better than 2 readings/sec or so, I'm definitely interested.

Great find!

There are 2 modes, fast and slow. The slow mode is perhaps about 2 1 update per sec, but the fast mode is fast, at least 6-7 times I reckon. It's just a button push to switch between the 2.

EDIT: I just had another look and the slow mode update rate should be closer to 1 update per second.

[kripton2035]

any video to see it in action ?

[zapta]

Franky, have you considered carrying a smd tweazers like this one http://r.ebay.com/zOiBYY ? Dave had a video about a more expensive Canadian product and ebay had a bunch of cheap ones but it will be useful to have it pre qualified and sold by a reputable seller.

[iloveelectronics]

any video to see it in action ?

I'll be away on a short trip for the next couple of days but will try to shoot a short video to demo the device when I return.

[iloveelectronics]

Franky, have you considered carrying a smd tweazers like this one http://r.ebay.com/zOiBYY ? Dave had a video about a more expensive Canadian product and ebay had a bunch of cheap ones but it will be useful to have it pre qualified and sold by a reputable seller.

Thanks for the suggestion. I'll add that to my to-do list

[kripton2035]

Here is the link, with full documentation:

http://bbs.38hot.net/thread-45486-1-1.html

It utilizes the wonderful f350's internal 24bit adc + external reference.
Good for 5.5 digits.
Light years ahead of what has been discussed on this forum.
btw, take a look at the layout.
You will notice another design mentioned in that very thread - far more advanced than most amaturers could do.

can you please upload the files here on eevblog ? I dont understand chinese and google is not a good help on this web site
and I dont want to register on this site to download !
thanks.

[iloveelectronics]

can you please upload the files here on eevblog ? I dont understand chinese and google is not a good help on this web site
and I dont want to register on this site to download !
thanks.

That's the author's post on the design decisions and theory on the volt meter part. He has another post on the milliohm meter section. I am not sure I have the technical expertise to translate these posts properly. As for the documents (mainly assembly instructions and calibrtion procedure etc), I will include those with the kit. If I get the author's permission to post them here I will do that too.

[kripton2035]

the message was at first for "dannyf" who gave the chinese web site link !
anyway thanks for answering.

[dannyf]

The  milliohm meter is quite simple and unremarkable (in a good way). It basically uses a resistor string consisting of a known value precision resistor and your dut. The voltage drop over the precision resistor goes to the (differential) Vref on the chip; and the voltage drop over the dut to the (differential) adc on the chip. A voltage is applied to the resistor string: the magnitude or even the stability of this voltage is unimportant. Once you know the adc result (and the adc maximum resolution + value of the precision resistor), you know the value of your dut.

There are still a few tricks in the software however.

The archilles' heel is in the millivolt meter, particularly its (innovative) use of analog switches to adjust range. The Ron and variation of Ron can be quite difficult to predict and in my view would require calibration for the meter to be a true 5.5 digit millivolt meter.

[babysitter]

What is wrong with the good old voltage divider ? Promote one of your resistors to "house standard", connect it in series to the DUT, power it up and do two voltage measurements with your trusted voltmeter giving you the ratio of your house standard to DUT ?
(Simplified by some software showing the Ohms reading if you can get the measurement data out of your dvm ?)

Alternative: Wheatstone bridge of reference+DUT in one arm, potentiometer in other arm, Voltmeter for nulling, measuring pot resistance ratio which is more comfortable for your meter ?

[jaxbird]

I have been interested in this kit. It looks like it's a well designed, useful and fun little project, plus a good excuse to have a closer look at the Silabs C8051F350 mcu. 

So I asked Franky if possible to buy one.

Sure it was, but understandably, no guarantees or support as it's a kit. Sold, as is, with a warning that all the documentation is in Chinese.

Just want to share my experience with this kit. I have no affiliation with Franky, and no discounts or anything.

All the parts included:





Yes, it includes a programmer for the mcu and even a pair of kelvin clips (not very good ones though).


The PCB is good quality with reasonable accuracy, tented vias etc. It will withstand rework if you mess up.




Soldering is not difficult for anyone with at least some basic SMD experience. Smallest components are 0603s and SOT23s. Finest pitch ICs are 0.65mm for the USB/UART controller, the rest are 0.8mm and 1.27mm. All resistors are 0805. Most capacitors 0603. No need for microscope, I use a cheap x10 loupe to check joints when needed.





The only real issue I had during the soldering was that I managed to lose one of the little 0603 LEDs (oops). I was checking the polarity, then noticed my tweezers were a bit sticky, and while cleaning tweezers the LED escaped. 

Luckily I have a good selection of 0603 LEDs, so no problem.

The assembly instructions are sufficient, just a BOM in Excel and the PCB with labels in silkscreen. But it's quite time consuming to locate all the components one at a time. It would make assembly much faster if it had included a nice large image of the silkscreen with e.g. color coding based on component type or even small images indicating the location of each item type in the BOM.

One slightly odd detail is that all diodes are marked with a + sign to indicate polarity, opposite of what I'm used to.

Also some of the holes for through hole parts are a bit too small for the solder to flow properly, it's only a minor detail though.

[jaxbird]

Here a quick test before final assembly:



Almost everything worked when I first powered it on, no excess current draw, programming the mcu was fine, usb worked, battery charging was fine etc. But I did have a problem with the milliohms measurements, took a bit of investigation, source current was ok, and it turned out to be a 0603 capacitor on the sense inputs that was shorted Easy to find as the kit comes with a complete schematic. 

Just moved it around a bit with the iron and everything was fine.

Then a good flux cleaning and ready to put it in the box.

There doesn't seem to be any support for the display, other than it will just sit on the board. I used a couple of small strips of thick double sided tape to secure it at a good height to fit in the box close to the top plate.

Here the complete device:



Small detail, the included screws for the top plate were too short, but I had some other ones that would fit, so no big deal.

Edit: Note the kit does, of course, not come with a battery, so you'll need to find one yourself. I found the battery in my old Sony/Ericsson phone still worked fine, so I used that.

[Vgkid]

Thanks for the mini review, it looks pretty neat. I guess i will need to start messing atound with smd components soon.

[jaxbird]

Thanks for the mini review, it looks pretty neat. I guess i will need to start messing atound with smd components soon.

Thanks, yeah, it's a useful little device. But I'd recommend at least some experience with SMD before having a go at this kit.

For milliohms, I used to use a power supply in CC mode adjusted as close as possible to e.g. 100mA or 1A using a reasonably accurate meter, then with a pair of Kelvin clips measure the voltage over the resistor.

Now I don't need to do that anymore Plus it doubles as a nice logging DC voltmeter.

From what I've worked out these are the main features:

MilliOhm Meter:

Measurement source current is fixed at around 100mA at max 3V.

There are 2 ranges: 0.2R and 20R.

In the 0.2R range, the resolution goes down to 1 micro ohm.

At the 20R range it's 10 micro ohm.

Overrange is > 50%, as in, you can measure a 30 Ohm resistor, on the 20 Ohm range.


Voltage Meter:

150,000+ counts, 4 ranges: 0.1V, 1V, 10V and 100V

Again with > 50% over range. As in, 10V range will measure +/- 15V

Inputs are protected by PTCs and ADC/mcu protected by resistors and diodes.

Common for both Ohms and Voltage are 2 speeds, Slow and Fast. Slow is approximately 1 update per second and fast is updating something like 14-15 times per second.

The display has got a nice uniform backlight.

USB for charging and logging. Output is very simple, just the value currently being displayed and at the selected speed range through an USB/UART 115200 baud 8N1 (not isolated).

It's also got battery voltage monitoring with a visible indicator.

Auto off after 10 minutes (can be disabled).

Power consumption is about 15mA in Voltage Meter mode. And ~110mA as Ohm Meter. Add about 35mA for backlight.

Not sure what the accuracy is, but all ranges can be calibrated against a reference. (positive, negative and zero for voltage and zero plus value for ohms)

The standard reference is a MAX6192A, ADC is 24bit in the C8051F350 and the current resistor is a low tempco type.

It appears to have good accuracy.

[kripton2035]

nice review, how much did it cost to you at end ?

[sorin]

schematic
http://www.filedropper.com/ohmmeterf350v21


on taobao is sold as a kit, and cost ¥170  around $28
http://item.taobao.com/item.htm?id=17790993945

[kripton2035]

on taobao is sold as a kit, and cost ¥170  around $28
http://item.taobao.com/item.htm?id=17790993945

no one sell it with an english page and accepting paypal

[iloveelectronics]

schematic
http://www.filedropper.com/ohmmeterf350v21


on taobao is sold as a kit, and cost ¥170  around $28
http://item.taobao.com/item.htm?id=17790993945

In defence of my pricing that Taobao price doesn't include the programmer and the acrylic case, nor shipping (within China)

[sorin]

In defence of my pricing that Taobao price doesn't include the programmer and the acrylic case, nor shipping (within China)

What is your price (if you want to make it public)?

[sorin]

on taobao is sold as a kit, and cost ¥170  around $28
http://item.taobao.com/item.htm?id=17790993945

no one sell it with an english page and accepting paypal

Use a taobao agent which accept paypal
You need to add 2-3$ for shipping inside china, ~5$ for international shipping and +10% commission for the agent, +0.3% paypal fee.
Total around 40$

[jaxbird]

nice review, how much did it cost to you at end ?

Hi Kripton2035,

Thanks, I think I paid $62 including shipping and tracking.

I really don't think there would be much saved from buying on Taobao, the complete kit there is something like 250 Yuan, then plus fees and shipping, it would be close. I've only had good experience with buying from Franky, fast shipping good communication etc.

Whether it's worth $60 something, is up to the individual. I'm pretty happy with it, as I didn't have any meters that would do milliohms with good resolution, and it does work very well.

A nice detail is that the kit comes with full source code, so very easy to customize it to specific needs. I had a quick play and created a modified version, that adds an extra digit of resolution in slow Voltmeter mode.

[iloveelectronics]

In defence of my pricing that Taobao price doesn't include the programmer and the acrylic case, nor shipping (within China)

What is your price (if you want to make it public)?

I think I did make it public earlier in this thread. I'm taking pre-orders at $62 shipped worldwide while finishing the translation (half done there). Jaxbird didn't mind the Chinese-only documentation so I sent him the first unit and he managed well with Google translate

[KD0CAC John]

I may have missed it , but was there some info on ranges of measurement ?
I picked up couple used / questionable HV test probes , in 30,000 volt max on one , and none of my gear will measure the resistors in the probes .

[iloveelectronics]

I may have missed it , but was there some info on ranges of measurement ?
I picked up couple used / questionable HV test probes , in 30,000 volt max on one , and none of my gear will measure the resistors in the probes .

For resistance there are 2 ranges, 0.2 and 20 ohms.

[dannyf]

0.2 and 20 ohms.

I would take a look at the design / implementation of the 0.2ohm range. If a separate precision resistor is used, I would like to see the pcb design for that. If it is done via the pga, I would think the pga's gain error would render the 24-bit adc largely ineffective.

[jaxbird]

0.2 and 20 ohms.

I would take a look at the design / implementation of the 0.2ohm range. If a separate precision resistor is used, I would like to see the pcb design for that. If it is done via the pga, I would think the pga's gain error would render the 24-bit adc largely ineffective.

It does use the full 128x gain on the 0.2R range, but it also limits the counts to just ~300,000 compared to the 20R range using 1x gain and ~3,000,000 count resolution.

Both ranges are stable on the last digit.

[KD0CAC John]

I may have missed it , but was there some info on ranges of measurement ?
I picked up couple used / questionable HV test probes , in 30,000 volt max on one , and none of my gear will measure the resistors in the probes .

For resistance there are 2 ranges, 0.2 and 20 ohms.

I guess that's a good example of short-term-memory loss , as soon as I saw your reply , I thought to myself , ya saw that at least a couple times
Old-Farter

[robrenz]

Does it do offset compensation to eliminate thermal EMF issues?

[jaxbird]

Does it do offset compensation to eliminate thermal EMF issues?

No, it doesn't, the source current is on continuously.

I guess it would need to do a measurement without the source current and then use that as compensation to eliminate thermal EMF.

[robrenz]

Does it do offset compensation to eliminate thermal EMF issues?

No, it doesn't, the source current is on continuously.

I guess it would need to do a measurement without the source current and then use that as compensation to eliminate thermal EMF.

Correct, micro Ohm measurements without some type of offset compensation is meaningless. That would be my first programming change (if I were a programmer)

[dannyf]

I guess it would need to do a measurement without the source current and then use that as compensation to eliminate thermal EMF.

The chip can do the calibration (both offset and / gain). Not sure if the software takes advantage of that.

[jaxbird]

Correct, micro Ohm measurements without some type of offset compensation is meaningless. That would be my first programming change (if I were a programmer)

It would also require some hardware modifications to fully implement thermal EMF error compensation.

I did a quick experiment, simply disconnecting the current source leads to get some idea of the error. In this case, measuring a 3W 0.1R resistor, the error is about 25 micro Ohm.

I guess using this procedure one could manually zero out the error.

[jaxbird]

The chip can do the calibration (both offset and / gain). Not sure if the software takes advantage of that.

The 8051F350 ADC does have a nice set of features:

https://www.silabs.com/Support%20Documents/TechnicalDocs/an217.pdf

I haven't checked but I assume it does an internal gain/offset calibration during initialization, but no continuous re-calibration. 

[robrenz]

IMO you don't want recalibration. You calibrate once to get correct gain and offset of the voltage readings. Then when doing a resistance measurement you take a reading with no current applied and store that value. Now take a reading with the current on. algebraically subtract the "no current" reading from the "with current" reading and display that value. This should repeat at least once per second.

[dannyf]

The 8051F350 ADC does have a nice set of features:

The whole 35x family is amazing.

You calibrate once ...

Read the datasheet and see how your implementation could be simpler than recalibration.

[jaxbird]

IMO you don't want recalibration. You calibrate once to get correct gain and offset of the voltage readings. Then when doing a resistance measurement you take a reading with no current applied and store that value. Now take a reading with the current on. algebraically subtract the "no current" reading from the "with current" reading and display that value. This should repeat at least once per second.

It is more a recalibration of the ADC's internal gain and offset. It will drift slightly with temperature.

But, the part that requires a bit of thought is not how to do the compensation in software, whether subtraction or just updating the offset, it's more how to implement the changes in hardware.

A simplified schematic of the circuit looks something like this:





It would need to switch the source current from the dut to a dummy load for the reference +/- voltages to remain approximately the same.

[robrenz]

I don't know squat but this is what I see:
You have the ability to turn the 3V on and off since the unit can measure voltage alone. The unit measures ratiometrically comparing the voltage drop of the R17 reference resistor to the voltage drop of the DUT.

The offset compensation procedure would be: turn off V3 for 200mSec measure and store sense connection voltage. Turn on V3 just long enough to get a stable reading maybe 200 mSec. measure and store the value. Algebraically subtract the voltage off value from the voltage on value. Display the calculated value and repeat the procedure continuously.

[jaxbird]

I don't know squat but this is what I see:
You have the ability to turn the 3V on and off since the unit can measure voltage alone. The unit measures ratiometrically comparing the voltage drop of the R17 reference resistor to the voltage drop of the DUT.
...

It is good thinking 

But unfortunately, switching between the two modes (Ohm and Volts) is a very manual process that requires moving 5 jumpers around on the back. My bad, I didn't cover that or post any pictures of the back side 

I was thinking a couple of tiny MOSFETs (P and N) would do the job together with a few resistors, but looking again, I don't think that is the case. Also, I also wonder if it's really worth the trouble as a ~10C change in temperature will make the meter drift about 7-10 micro Ohms (I slowly heated it up over about 20 minutes). That could of course also be compensated as the Silabs 8051 mcu contains two complementary diodes that can be internally connected to the ADC for fairly accurate die temperature measurements

[dannyf]

switching between the two modes (Ohm and Volts) is a very manual process

Yeah. Those two modes of operations are mutually exclusive - I think I might have mentioned it earlier in this thread.

[robrenz]

IMO you would only need to add a mosfet to turn the 3V on and off while the jumpers are in the resistance mode. The unit will be measuring the sense voltage in resistance mode. You just need access to the sense voltage value itself to get the two readings and be able to turn the mosfet on and off.

[jaxbird]

IMO you would only need to add a mosfet to turn the 3V on and off while the jumpers are in the resistance mode. The unit will be measuring the sense voltage in resistance mode. You just need access to the sense voltage value itself to get the two readings and be able to turn the mosfet on and off.

It would still need something to keep the reference part of the ratio approximately the same.

I was thinking something like this, with a couple of SOT23 MOSFETs and a few resistors, but I'm not convinced it would work:

[jaxbird]

Yeah. Those two modes of operations are mutually exclusive - I think I might have mentioned it earlier in this thread.

Yeah it's 2 completely different parts only sharing the mcu, display etc. it would have been a lot simpler (cheaper) without the volt meter part. No need for precision reference, chopper amp for buffering, amp for virtual ground, switch for resistor ratio etc.

But then again, the Voltmeter part is actually very nice.

[robrenz]

It would still need something to keep the reference part of the ratio approximately the same.

I don't think so. you just need to have it read the sense voltage with NO 3V then read it again with the 3V and do the math. Now that calculated sense voltage needs to go into the ratiometric calculation that is using the ref voltage that was present when the 3V was on.

[jaxbird]

It would still need something to keep the reference part of the ratio approximately the same.

I don't think so. you just need to have it read the sense voltage with NO 3V then read it again with the 3V and do the math. Now that calculated sense voltage needs to go into the ratiometric calculation that is using the ref voltage that was present when the 3V was on.

It's using the ref+/- (from my simplified schematic) as the reference for the ADC to easily get the ratio between the reference resistor and the sense+/- over DUT. So the ref+/- must be kept at approximately the same level for reasonably accurate offset readings while the source current is removed from dut, that is the task of the NMOS + 2.7R resistor.

[robrenz]

Ok, I understand now

[jaxbird]

Ok, I understand now

Nice, yeah, I wasn't providing the complete picture, too much implicit info.

But I do think sometimes when getting down to these more extreme measurements I just want to swallow the blissful ignorance pill and ignore all the potential errors, being happy with no worries

On the other hand, I do find an unhealthy attraction towards the nearly impossible. 

[dannyf]

So the ref+/- must be kept at approximately the same level for reasonably accurate offset readings while the source current is removed from dut,

Yeah - you want to maintain rougly the same common mode voltage levels.

that is the task of the NMOS + 2.7R resistor.

Those two mosfets can be just two io pins.

[jaxbird]

...
Those two mosfets can be just two io pins.

I haven't checked but I don't think the IO pins will source/sink ~100mA.

[dannyf]

yeah.it wouldn't work with high current.

[jaxbird]

yeah.it wouldn't work with high current.

Understand .. yeah, you could do an extremely simply ohm meter like that. Even with a number of reference resistors for different ranges.

[iloveelectronics]

I am finally done with the English translation of the documentation that comes with this kit. For those who have sent me PM's for pre-ordering, I have just sent you a PM, please check. If you haven't got one, please send me a PM to remind me. Thanks a lot for all your patience!

[AKM]

When will it be available to the rest of us?

[iloveelectronics]

When will it be available to the rest of us?

It is available now. Please PM me for details. I will make a post in the "Buy and Sell" section soon.

[jaxbird]

Been playing around a bit more with an implementation of thermal EMF compensation, as robrenz pointed out, is needed for more valid measurements in the micro ohm range.

I'm going to attempt a slightly modified version of my previous schematic (using 2 NMOS and a transistor instead of both P and N MOSFETs)



It should be possibly to test it, without having to permanently modify the PCB (no need to cut traces or anything, just use existing pads)

[jaxbird]

Test modification soldered in:




A bit fiddly with the little sot23 and resistors  but no cut traces.

I'll make it a bit more permanent with some glue if it works.

[dannyf]

I'm going to attempt a slightly modified version of my previous schematic

What if you wire the lower mosfet's gate to the upper mosfet's drain? you can save the npn + resistors.

[jaxbird]

I'm going to attempt a slightly modified version of my previous schematic

What if you wire the lower mosfet's gate to the upper mosfet's drain? you can save the npn + resistors.

It's a good idea, but I think the voltage will be too low on the upper mosfet's drain, down to a max of ~300mV, depending on dut.

Anyway, it works with the transistor. Only I got some headaches from the mcu pin I assumed available, as it isn't connected to anything.

It's the only "free" IO pin, the other unused pins are analog only.

I'm not experienced in silabs 8051, so it took some datasheet reading to discover why it seemed to have weak pull up and not reacting to being set, when it was configured as push pull output.

Explanation found when I stared looking at the hardware peripheral assignments. Turns out it's assigned as MISO for the hardware SPI used to write to the display 

So I had to switch it to a soft SPI to actually free up the pin. Not a big deal as it was running just 1 MHz SPI and waiting around for each byte to complete anyway.

It appears to be working correctly, but still needs a bit of software fine tuning.

[jaxbird]

Just a short update on the thermal EMF compensation modification experiment.

The compensation works very well now, display updates every ~1.5 seconds in slow mode with some averaging.

It's easy to test the effect when e.g. measuring a resistor. Without compensation, when gently heating one side of the resistor leads, breathing on it is enough, the measurement will change by something like 30 micro Ohm, + or - depending on side.

With the compensation enabled there is little or no change.

It was a bit tricky to implement in software as the code structure is build around running the ADC in continuous mode. It's doing 19.2 ksps internally with an output rate of 10Hz from the SINC3 filter for best performance. So when switching between offset and ohm measurements the first few samples are inaccurate and must be discarded.

A nice side effect is that it auto zeroes the measurement. I haven't added relative measurements with thermal EMF compensation, maybe some other time.

[jaxbird]

In case anyone want to try it out, here are a couple of detail pictures of the modification:








I used AO3400 for the MOSFETs and BC847 for the NPN, as that is what I had. Shouldn't cost more than $1 or so in components.

PM me for a copy of the modified firmware hex file.

[jaxbird]

Decided to print a small foot for it so it will sit on the bench at a nice angle:



Just a quick design and print, nothing too fancy:



Not too tall, good angle for easy readability:




OK, now I think I'm done with pimping this little meter kit

Overall pretty happy with it and it also served my secondary goal of getting to know the Silabs F350 better. It's actually a good kit to experiment and get familiar with the features and performance of the mcu and some of it's main peripherals.

[jaxbird]

I'm done with pimping this little meter kit

I lied  , just one more little modification, my battery is a bit too thick, about 0.5 - 0.75mm, but it sits partly on top of the reference, adding a bit of pressure.

So I routed a small 1.5mm deep pocket in the back plate for a much better fit.



Old phone battery, wrapped in kapton tape, as it's in an aluminium package.

[dannyf]

Nicely done.

[robrenz]

Yes, very nice work on all of the project.   


What about calibration for the reference resistor error?

[jaxbird]

Nicely done.

Yes, very nice work on all of the project.   

Thanks

What about calibration for the reference resistor error?

Yes, it does need to be calibrated, not sure what the non calibrated accuracy is. Calibration is a simple procedure as long as you have a know resistance that matches the range being calibrated.

As I don't have any resistance standards, or meters with equal resolution. I measured a reasonably low tempco, high wattage, wire wound resistor using a 5A current source and a volt meter and then used that as my reference.

[scopeman]

Does anyone have the documentation for this project (hopefully in English but I will take what I can get)?

Sam

[iloveelectronics]

Does anyone have the documentation for this project (hopefully in English but I will take what I can get)?

Sam

Let me know your email address via PM and I'll send you the documentation.

[croma641]

I apologize for reopening this old thread

I'm looking for the firmware modified by Jaxbird. Can someone help me ? Thank you !