DMM EEVBlog 121GW

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This meter is partly specified by Dave from EEVBlog and made by UEI, it is packed with function, including some unique ones.

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The meter arrived in a white cardboard box with very little text on it.

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Inside was the pouch with the meter in it.

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The box included the meter, two probes, a thermocoupler, batteries, pouch and a Certificate of Conformity. There is no manual, it must be downloaded from the EEVBlog website.

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Probes are branded with B.T.C. and are rated for up to CAT IV 1000V. The meter is "only" CAT III 600V.

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The tip has 3 different configurations.

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The plug is fully shrouded and standard probe plug size.

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A standard thermocoupler with a standard dual banana connector.

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The meter is heavy and the tilting bale can hold it while the range switch is used and when the buttons is pressed carefully.

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The meter is rather thick.

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There is a micro SD card next to the fuse, it is used for firmware updating, for logging and the calibration can be stored on it.



Display

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The above picture shows all the segments on the display.

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Normal DC voltage with voltage, bargraph and temperature. The secondary display will often shows meter temperature.

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Battery voltage, it requires one or more presses on SETUP to display it.

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A configuration options (Auto Power Off), to change it: Hold SETUP down until it blinks, then use arrows to change it, hold SETUP to save value.

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Burden voltage, it is in mV, but the m is placed a bit far from the V.

The secondary display has a couple of functions:


Functions

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Buttons (Range selection and a few other are remembered): The two arrows above REL and 1msPEAK are used together with MEM and SETUP.
REL, MIN/MAX, 1msPEAK and MEM disables autorange.

Rotary switch: dBm uses 600ohm reference impedance.



Input

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Measurements 1uF

A look at the capacity measurement waveform.

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Frequency input resistance.

DMMVoltageSweepLowZ

The LowZ uses a MOV with about 1W power dissipation.

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Input impedance in DC+AC 5V range, the 11Mohm is DC, the 10Mohm is AC.

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High DC voltage can block for AC readings.
The 5Mohm range is unstable at 1Mohm and jumps a bit more than 1%, this is more than the specified 0.3%.
I had some trouble with autoranging in AC+DC and when showing burden voltage.
Runtime is without Bluetooth.



Logging on card

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The meter is supplied with a micro SD card mounted in the internal reader. To get it out the sleeve and two screws must be removed.
The logfile is in CSV format, but cannot be configured to support European CSV format.

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The top define the starting time and logging interval, the actual logging is the main display and sometimes auxiliary values. These auxiliary values are fixed and do not depend on what is shown in the secondary display.

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With VA the logging contains 3 values.

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AC+DC is marked as ACV with ACDC option. Here I selected fast logging (Interval 0), this logs 5 times each second.



Software

Android1s

The software will scan for 121GW meters and one (or more) can be selected.

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This is the standard readout in the software, it reflects the display on the meter. Some of the buttons on the meter is also accessible.
A bad connection will lock the display, the software do not reconnect automatic when in range again, instead SETTINGS and RESCAN must be used and a extra meter will show up.

Android3s

Turning the phone horizontal will increase the display size.

Android4s

And a tap on the display will change to the chart or back. It is possible to zoom in and out on the chart.
The SAVE button will make a logfile that can be saved to a lot of different Android software, I used email.

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The email log contains a time stamp and the value from the chart, no secondary values will be included.



Tear down

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I addition to the battery cover I had to remove four screws to get inside the meter.

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As usual the circuit board is shaped to fit the enclosure.

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Two more screws and I could get the circuit board out.

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Four more screws to remove the LCD display.

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The backlight panel is soldered to the circuit board, I had to unsolder it to see the microprocessor.

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The schematic can be downloaded from EEVBlog for this meter, but I will add some explanation anyway.
The current input has the usual 3 shunts, one wire and two resistors (R43, R33: 1ohm, 100ohm), but to get the low burden voltage there is a OpAmp (U8: MAX4238) with 10 times amplification next to the shunts, that is used for some current ranges. As usual the uAmA input is protected by a diode bridge (BD1) and due to the low burden voltage it do not need an extra diode. The detection of plugs in the current terminals goes to a dual OpAmp (U5: TL272C).
At the voltage input is a lot of PTC's, there are the two input paths (PTC, PTC4) with series resistors )R16 & R17: 1kOhm) and MOVS (MOV1, MOV2, MOV3) and then there is the LowZ path with two PTC's (PTC1 & PTC2) in series. The input resistor (R11: 10Mohm) is ceramic and there is a frequency compensation next to it (R9: 30kOhm, C13: 6pf) that can optionally be used. The meter has one transistor pair for protection (Q3 & Q5) and there is space for one more (Q1 & Q2), but there a transient diode (D13) is used instead)
Generally there are many multiplexers on the board (U9: HJ4053, U11: HC4053, U15:HC4052, U16:HEF4053). The 15V boost converter (U10: RT9271) is near the top of the circuit board,ear a SMD inductor (L1).
The multimeter chip (U3: HY3131) is close to the true RMS converter (U17: AD8436), there is also a reference in the area (ZD1: ADR3412). The bluetooth interface is a module (U6: BLE112). Near the buzzer is some hut melt glue, it secures a watch crystal for the RTC (U7: NJU6350R), this device is powered from the coin cell (CR1220) when the meter is off.

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This side has the current terminals detection input (R51, R52, R54, R55: 4x4.7Mohm) from the split terminals, it has the pads for the rotary switch, the buttons and the LCD and it has the microprocessor (U4: STM32L152ZDT6 ARM Cortex-M3, 384k flash, 48k ram, 12k EEPROM)

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Conclusion

It is an fairly advanced meter with some unusual functions like: 15V diode test, low burden voltage, VA measurement, SD card.
The meter has all the standard function for a good multimeter and also has all the common functions for a advance multimeter like: Bargraph, dual-display, average, peak, auto hold, logging, Bluetooth.
All this may sound like the perfect meter, but there are a couple of issues (There is a thread on EEVBlog), some of them will probably be fixed with software updates other is due to the design. The low burden voltage means a offset in some current ranges (Can be removed with REL). The meter is also a bit on the slow side.

For me the two most interesting functions are 15V diode test and low burden voltage, because I do not have them on other meters. The VA is fine for some DC measurement, but cannot measure AC power.


Notes

I got this meter from a Kickstarter campaign, as is often the case there was some extra delay, in this case about 4 months for second batch.
EEVBlog 121GW here is manuals, schematic and sale of the meter (when in stock).
The meter used firmware version 1.22 and during review I found a few issues, I reported some of them them to EEVBlog.

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