Milliohm Meter Kit from Ascel Electronic Review

[Martin72]

Hi,

Last month I´ve ordered this kit here:

AE20218.

I´ve choosen all order options, so it comes with the suitable housing, a usb interface, kelvin clamps and power supply.
All smd devices are soldered before they claimed - well, this means exactly 3 components...
The rest is to mounting and soldering by yourself, by following the instruction guide (component placement).
This takes about an hour(pic1, pic2).
The correct mounting of the 4mm banana jacks was a time waste…
You have to mount them in the frontpanel FIRST, then solder it...Should be remarked in the manual but it doesn´t.
Fun fact :
Everything is diy, even the powersupply- you have to solder the 3.5mm plug on the chord...

Soon I´m finished it (pic3) and try some measurements(pic4, pic5), to see if it´s working.
The final adjustments according to the manual I´ve done at work - You have to trim the current to 100mA as precisely as possible at 24 range, 10mA at the 240 range.
This I had done with our yearly calibrated keysight benchmultimeter.
After this, after warming it up, I´ve done several measures, it´s really precisely.
I think, for it´s money it´s a really good meter.
Further measurings (long time and under different ambient temperatures) will follow.
Conclusion:

Pro:

Cheap because it´s a kit.
Precise measuring (so far).
USB connection, software included for logging/remote operation, works proper.
Con:

Components are not specified by position, you have to look at the value and then to the component placement ( Example 10K would not be marked as R10, R11 on the bag, even the value is not marked on it, you have to use the colour code for identify).
Except the measuring resistors (0.1%) all other resistor are carbon type.
Grid dimensions for the diodes are really small…
Kit is therefore not for beginners


Martin

edit 07.10. : Tolerance of the measure resistor corrected to 0.1%

[HKJ]

A milliohm meter is many different things, I made one for checking mains and other cables:

https://lygte-info.dk/project/CableLowResistance%20UK.html

It is designed to be practical (i.e. show if a cable is within specs or not), not to have a lot of significant digits.

[Martin72]

Not bad 

A thing I´ve forgotten is this:

One of the screws for the lcd-display is close to another circuitpath, this could cause a short circuit when the screw is tighten too hard…

[Electro Fan]

Nice project and product - Nice work and Thanks for sharing.

[Martin72]

Thank you, I thought it was worth a thread meanwhile some cheap ready-to-use units are on the market like this.

[nctnico]

Did you try measuring 20mOhm resistors with it? There is no accuracy spec and it seems it uses a rather simple ADC (can't read the part number from your pictures) to measure the voltage across the resistor while assuming the current is adjusted to precisely 100mA using a single turn pot. This doesn't instill much confidence in me.  For about the same price you can buy an XJW01 LCR meter which can also measure capacitance and inductance with good accuracy (resistance with 0.1 milli Ohm resolution at the lowest range). I have the Ascel LC meter (if I haven't thrown it away yet) but I have to conclude it was a waste of money; the measurement principle is just too simple and from the looks of it this resistance meter has the same problem.

[Martin72]

Hi,

Did you try measuring 20mOhm resistors with it

50mOhm Shunt on heat sink, but I couldn´t find out it´s tolerance.
Will repeat it in the next week, hopefully I find the specs for it.
First measuring was 52mOhm +/- 100µOhm, between 20min duration time.

[nctnico]

Hi,

Did you try measuring 20mOhm resistors with it

50mOhm Shunt on heat sink, but I couldn´t find out it´s tolerance.
Will repeat it in the next week, hopefully I find the specs for it.
First measuring was 52mOhm +/- 100µOhm, between 20min duration time.

A 1% current shunt (and I'd assume a current shunt is 1% or better) should measure between 49.5 and 50.5 milli-Ohm.
My XJW01 measures a 10 milli-Ohm 1% resistor as 10.2 milli-Ohm (where .2 is the last digit).

[Martin72]

My XJW01 measures a 10 milli-Ohm 1% resistor as 10.2 milli-Ohm

Fine. 

it uses a rather simple ADC

Yepp, a 22bit one, MCP3551.

[Kleinstein]

It is odd to see 2 trim-pots instead of calibration in software.
I see no CMOS switches or similar to switch the inputs / reference resistors on a regular basis. So I would assume they use just a simple divider from the ADCs reference voltage with a range setting resistors and the DUT. So the wire- and relay- resistance is only partially compensated.
With large resistance there is the obvious problem that the diodes used for protection can start to conduct. One may be able to fix this if for one of the diodes 2 in series are used.

Not so perfect 4 wire measurement would be noticeable in the low range with a relatively large resistor. Some extra resistance in the drive wires would than change the result, in proportion to the reference resistance. With an assumed  200 Ohms as a reference values 1 ohms wire / contact resistance would result in 0.5% of error - noticeable, but not dramatic, given the limited accuracy.

At the 0.1 mOhms resolution and maybe up to 10 mA of test current, it would need to resolve some 1 µV at the DUT. At this level, there can be thermal EMF causing problems. For a much higher test current the reference resistors look like rather low power rating.

Does the software alow to use the sense input as a low voltage voltmeter, e.g. to measure thermocouples ? Chances are the Hardware could allow this.

[Martin72]

Hi,

Looks like not, but maybe it could be implemented in the next update.

It is odd to see 2 trim-pots instead of calibration in software.

The current source is independed from "everything", the µC switches the relay k1 between the 2 values, so you have to trim it with an external currentmeter for each range.
The incoming voltage will be converted by the 22bit ADC and serial data goes to the µC.
With the given hardware, a calibration in software would not be possible, I guess.

[Kleinstein]

The resistance values is calculated from the ADC reading and calibration could work with the factors used in the software instead of adjusting the resistors. So quite sure the same HW could support software calibration, just a question of the software.

It could still be beginners project - especially if the software or at least some documentation is available so one could write ones own software for it.

With an independent current source (worst case even using something like an LM317 / LM 1117 regulator would avoid a problem with the wire resistance, but have the ratio of the 2 voltage reference and thus not so good long time and temperature stability.

Anyway the hardware side looks rather crude compared to the  XJW01 RCL meter and resistance resolution seems to be not much better, though measured in a different mode.

[Martin72]

Current source is realized by LT3092 and LT1634, see pic below.

They recommend to trim the current appx every 6 months or by using a reference resistor.

The XJW01 was interesting too, for L and C I need a meter.

[Kleinstein]

The test current is quite high - so quite some heat to the current setting resistors. This could lead to some drift after connecting the DUT.

If there is no extra capacitance, there is some danger to see oscillation if the DUT is quite inductive.

The trimmers also have quite some effect. Normally the trimmer should have less effect as trimmers tend to drift over time and have a high TC.

With the given current source and the diodes for protection it is not a surprise that it does not work well with more than some 100 Ohms. 

[Martin72]

With the given current source and the diodes for protection it is not a surprise that it does not work well with more than some 100 Ohms.

Therefore it´s limited to 240ohms, I guess.
I´ll test some different resistors in the next week under several conditions, comparing to the keysight.

[ogden]

They recommend to trim the current appx every 6 months or by using a reference resistor.

Precision current source? - It's brain surgery through **shole  There is ratiometric and bridge resistance measurements that requires just shunt with good TC (already there) and differential ADC with external reference voltage input (there as well, MCP3551).

[Martin72]

Here too.

[HighVoltage]

I have a client that needs to read 1 Ohm values at 100mA, this might be a solution. So I will order one and see how stable it is.
Thanks for sharing

[ogden]

Here too.

What you are talking about? Yes, THIS is where I was pointing to. - That "Milliohm Meter Kit" contains all the parts needed for ratiometric resistance measurement that does not require precision current, precision voltage sources that would require trim every 6 months.

[Martin72]

Yes, THIS is where I was pointing to

So I must have misunderstood it, my failure.

[Kleinstein]

It is an odd construction. With less parts (skip the references and trimmers) they could have reached better performance. For better long time stability they might still need better reference resistors. So spend the money saved on the references on a better resistor.

Maybe wait for a better version and see this one as a chance to learn from mistakes.

[Martin72]

We will see how bad it is, I´ve ordered some measuring shunts from isabellenhuette (100,50,10mOhm 0.5%) and will compare the measurings with a calibrated keysight meter.

[Martin72]

Arrived, tomorrow or friday I´ll make the measurements

[Martin72]

Not bad so far….

[ogden]

Not bad so far….

Good to hear that. Inefficient or overengineered instrument does not necessarily mean bad. It still may measure well and be fit for purpose