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.