DIY milliohm/volt meter kit

[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.