DMM UNI-T UT139S

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Uni-T has a large selection of DMM in their program, this model exist in A, B, C, E and S versions, In this review I will look at the S version that is the most advanced model (Together with E) and has a inverted LCD display.

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The meter arrived in a red box with Chinese writing and the dealer had added a pouch for the meter.

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The box contained the meter, probes, thermosensor and a manual in Chinese (I could download a English version).

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The probes are branded UNI-T and rated for 10A and 1000V. As usual the CAT rating depends on mounted or unmounted tip cover.

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The plug is fully shrouded and the shroud is slightly shorter than standard probes.

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The thermocoupler has a single connector to the multimeter end, that is considerable better than the two loose banana plugs.

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The tilting bale is just about stable enough to change range and use switches with one hand. The tilting bale is not locked in either position.

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The 600mA fuse is accessible from the battery compartment.

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The battery lid holds the screw.

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Display

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The above picture shows all the segments on the display. Due to the way the display works some of the segments can have a fixed color.

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Typical display during usage, it will show the number and what measurement is selected.
The circle with arrow shows that auto power off is enabled, hold down SELECT when turning on to disable this.

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The electric field detection (NCV) shows from 1 to 4 bar depending on field strength and a red led will turn on together with the buzzer.



Functions

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Buttons: Rotary switch:
Input

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Measurements
1uF

A look at the capacitance measurement waveform.

DMMInputVoltageSweepHz

Frequency input impedance depends on voltage.

DMMInputVoltageSweepmVDC

Meter specify 3Mohm input impedance, it is around that at 0.6V

DMMVoltageSweepLowZ

The LowZ uses a PTC and regulates to about 0.5W load.

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Meter is not very stable or precise at very high ohm values (50Mohm+)
Runtime is based on a Duracell Pro Power discharged at 20mA


Tear down


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3 screws (One was for the battery cover) and the back could be removed.

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The circuit board was mounted with 3 screws and 4 screws for the input terminals.

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Current input uses the fuses, as usual there is a wire shunt for 10A range and some resistors for uA (R19: 99ohm), mA is a bit larger (R20: 1ohm). Both uses some diodes (D3, D4, D5, D6, D7) for protection. A nice detail on the 10A shunt is that there are sense wires connection where the shunt is soldered to the circuit board.
The voltage input has a couple of PTC's and MOV's. There is two input paths each through a PTC and with a MOV after the PTC, the 3th MOV is common. Path 1: PTC1 and SG4, after the PTC it splits into a 10Mohm (R1..R4: 4x2.5Mohm) path and a 900kOhm (R42, R43, R44: 3x300kohm) path. Path 2: PTC2 and SG2, this path also has a LowZ PTC (PTC3) and is used for ohms output. The last MOV (SG3) is common for the two other MOV's.
On path 2 there is two transistor pair (Q4 & Q5 and after optional 1kOhm (R29) Q2 & 3) the is used for protection in temperature and ohm ranges. The 1kOhm resistor (R29) cna be disabled with a analog mux (IC5: SGM4157).
Some of the diodes (D9, D10, D11, D15) are probably related to range switch encoding, the other diodes for protection (D16, D17, D18). It looks like there is a boost converter (IC3: marked SC9HE) for the background light.
The meter uses a 6000 count multimeter chip (IC1: DTA0660) with a EEPROM (IC2: 24C02), this chip has help from a processor (IC4: HY11P14: 8K ROM, 512 byte data). This processor has a ADC that have enough resolution for a multimeter, but it lacks the multimeter support circuit.

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To remove the cover around the display I had to remove two screws.

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This side only has the pads for the switches and for the LCD, there is also a few LEDs.

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Two of the LEDs are the background light, this module is soldered directly to the circuit board.

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A look under the backlight module, there is no parts here.

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At the top of the circuit board is the NCV led and antenna.


Conclusion

This meter has all the common ranges and functions for a good multimeter, but do not have the more advanced functions. The reverse display is a novelty (Get UT139E for normal display and longer battery life) and means considerable higher battery consumption
With large ceramic fuses, MOV's, PTC's and multiple resistors in series the protection looks fairly good.

I will call it a good basic meter, both for low volt and mains usage.


Notes

UNI-T sell rebranded versions of their meters, i.e. you can probably get this meter with another name on it.

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