UNI-T UT139A/B/C True RMS Digital Multimeter Operating Manual

I. Overview

The UT139A/B/C DMM is a compact, auto-ranging, hand-held multimeter featuring 3 1/2 to 3 5/6 bit True RMS measurement. It offers a complete range of functions, a novel structure, high reliability, and safety. Its large screen display makes it suitable for measuring AC/DC voltage and current, variable frequency voltage (V.F.C), resistance, diode, circuit on-off, capacitance, frequency ratio, and NCV (non-contact AC voltage sensing). It is an ideal portable maintenance instrument for users.

II. Accessory

Unpack and take out the instrument, please check the following attachments carefully for completeness or intactness. In case of any shortage or damage, please contact your supplier.

• An operating instruction manual
• A test pen (CAT III 600V) 10 A
• Point-type K (nickel chromium-nickel silicon) thermocouple (UT139C only)
• An optional current clamp (UT139C only)

III. Safety Information

The UT139A/B/C is designed in compliance with standards such as IEC/EN61010-1, pollution grade II, overvoltage (CATIII600V), and double insulation standards. Please comply with operation instruction specified in this manual; otherwise, the protection provided by the instrument may be affected.

CE: This Meter complies with the standards EN 61010-1, EN 61010-2-030: in pollution degree 2, overvoltage category (CAT III 600V), and double insulation.

ETL/CETL: CONFORMS TO UL STD 61010-1, 61010-2-030.

CERTIFIED TO CSA STD C22.2 No. 61010-1, 61010-2-030.

Important Safety Precautions:

• It is forbidden to use the product without the rear cap in place, as this will result in electric shock.
• Prior to use, inspect the insulation layer of the test pen for intactness, confirming no breakage or broken lines.
• When the LCD display shows the ? icon, it is required to replace the battery in time to ensure measurement accuracy.
• The range switch shall be set at the correct measurement position.
• In case of electric shock and damage to the instrument, signals being measured shall not exceed the rated limit value.
• To prevent any damage to the instrument, it is forbidden to change the gear of the range switch during measurement.
• After each measurement, disconnect the test pen and the circuit being measured. After current measurement, especially of large current, it is necessary to power off before disconnecting the test pen and the circuit.
• In case of electric shock, be cautious when measuring voltages higher than DC 60V or AC 30Vrms.
• Do not use the product in high-temperature or high-humidity environments, particularly in damp environments where the instrument's performance may be severely degraded.
• Refrain from changing the internal wiring in the clamp ammeter to prevent damage to the meter and danger.
• Clean the meter case with a damp cloth and mild detergent, rather than abrasive material or solvent.

IV. Electric Symbols

V. Comprehensive Specification

• Maximum voltage between input terminal and grounding: refer to instruction about each input terminal protection voltage.
• 10A terminal (CE) is equipped with: F 10A H 600V fast-acting fuse (Φ6x25) mm.
• mA/µA terminal (CE) is equipped with: UT139A-FF 0.2A H 600V fast-acting fuse (Φ6x32) mm; UT139B-FF 0.5A H 600V fast-acting fuse (Φ6x32) mm; UT139C-FF 0.6A H 600V fast-acting fuse (Φ6x32) mm.
• Maximum display: (UT139A): 1999; (UT139B): 4000; (UT139C): 6000. Refresh rate: 2~3 times per second. Displays "OL" in case of overrange.
• Capacitance and frequency (only applicable for UT139B/C): 9999 count.
• Duty ratio (only applicable for UT139B/C): 1~99.9%.
• Diode: about 2.1V (UT139A) and 3.2V (UT139B/C), displaying "OL" in case of overrange.
• Range: auto/manual.
• Polarity: auto.
• Working temperature: 0℃~40℃.
• Relative humidity: ≤75% when 0℃~30℃, and ≤50% when 30℃~40℃.
• Storage temperature: -10℃~50℃.
• Electromagnetic compatibility: In 1V/m radio frequency (RF) field: Overall accuracy=specified accuracy + 5% of range, and no specified index for RF over 1V/m.
• Operating ASL: 0~2000m.
• Internal battery: AA R6P 1.5V×2.
• Low electricity: LCD displays ?.
• Contour dimension: about (175×80×48.5) mm.
• Weight: about 350g (inclusive of battery).
• Safety Standard: IEC/EN 61010-1: CATIII600V; Pollution grade II.
• Accreditation: CE.

VI. Appearance Structure (See Figure 1)

Figure 1 is a diagram showing the multimeter's physical layout, illustrating the case, LCD display, buttons, range switch, and input terminals.

• 1. Case
• 2. LCD Display
• 3/4/7. Selection buttons
• 5. Range Switch
• 6. Measuring input terminal

VII. LCD Display (See Figure 2)

Figure 2 illustrates the LCD displays for the UT139A and UT139B/C models, showing various segments and indicators.

Symbols and Instructions:

VIII. Knobs and Keys for Range Selection

Figure 3 shows the multimeter's front panel with the range switch and various buttons.

Range Location Functions:

Button Functions:

• RANGE button: Used for selecting auto/manual range. Pressing cycles through gears. Pressing for >2s or switching range exits manual mode (for V/Ω/I).
• MAX/MIN button: Enters manual range mode, disables auto shutdown, displays max value. Pressing again displays min value, then cycles max/min. Pressing for >2s or switching range exits data recording mode (for V, Ω, I, °C/°F).
• REL button: Enters manual range mode, sets current display as reference, and shows difference. Pressing again exits relative measurement mode (for V, Ω, I, °C/°F, and ?).
• Backlight: Pressing any button for ≥2s illuminates backlight for ~15s. Pressing again for ≥2s turns it off. (Applicable for UT139A full range: REL/LIGHT button).
• NCV/mV button (Only applicable for UT139A): Switches between NCV and mV modes.
• Hz/% button (only applicable for UT139B/C): Selects Hz/% mode, applicable for frequency, AC voltage/current measurement.
• SELECT button: Selects range (for multi-range). Pressing ≥2s in AC mode enters V.F.C measurement mode. Pressing again exits V.F.C mode.
• HOLD button: Locks and holds the displayed value, showing "✋". Pressing again unlocks and returns to normal measurement.
• Backlight: Pressing for ≥2s illuminates backlight for ~15s. Pressing again for ≥2s turns it off. (Applicable for UT139A full range: HOLD/LIGHT button).

IX. Measurement Instruction

General Note: Check the built-in AA 1.5Vx2 battery; the ? symbol appears when power is low. Replace the battery promptly. Pay attention to the ⚠️ symbol near test pen sockets, indicating that testing voltage or current should not exceed specified values.

1. AC and DC Voltage Measurement (See Figure 3)

Connect the instrument with the load in parallel. Input impedance is about 10MΩ. In circuits with high impedance, the load may cause measurement error (typically <0.1% if circuit impedance is under 10kΩ).

Notes:

• Do not input voltage higher than 600Vrms, as it may damage the instrument.
• Avoid electric shock when measuring high voltage.

Figure 3 shows the connection for AC/DC voltage measurement, with the multimeter connected in parallel to the load. The diagram indicates the input terminals (COM, VΩHz%), the range selection knob, and the selection buttons.

2. Resistance Measurement (See Figure 4a)

Connect the instrument with the load in parallel.

Notes:

• "OL" is displayed when the measured resistance exceeds the maximum range or is open-circuit.
• Prior to measuring online resistance, switch off all power in the circuit and release residual charges.
• For low resistance measurements, a test pen error of about 0.1Ω~0.2Ω may occur. To ensure accuracy, short the test pen and use the REL mode.
• If resistance is ≥0.5Ω when the test pen is short-circuited, check the test pen for loosening or other issues.
• Reading stability for high resistance measurements may take several seconds, which is normal.
• The resistance function can be used to check the built-in fuse (see Figure 4b).
• Do not input higher than DC 60V or AC 30V.

Figure 4a shows the connection for resistance measurement, with the multimeter connected in parallel to the component under test. It indicates the input terminals and range selection. Figure 4b illustrates how to check the fuse using the resistance function.

3. Circuit on-off measurement (See Figure 5)

If resistance > 150Ω, it's a circuit break (no buzzer). If resistance ≤10Ω, it's good conductivity (buzzer sounds).

Notes:

• Before measuring, switch off all power and release residual charges.
• Do not input voltage higher than DC 60V or AC 30V.

Figure 5 illustrates the circuit on-off measurement setup, showing the test leads connected across the circuit points. It also indicates the resistance thresholds for buzzer activation.

4. Diode measurement (Figure 6)

"OL" is displayed for open circuit or reverse polarity. Normal value for Silicon PN junction is 500~800mV.

Notes:

• Before measuring, switch off all power and release residual charges.
• Test voltage for diode: 2.1V (UT139A), 3.2V (UT139B/C).
• Do not input voltage higher than DC 60V or AC 30V.

Figure 6 shows the diode measurement setup, with the test leads connected across the diode.

5. Capacitance measurement (Only applicable for UT139B/C) (See Figure 7)

The instrument displays a fixed reading (internal capacitance) when no input is present. This value should be subtracted for small range measurements. The REL function can be used for automatic subtraction.

Notes:

• "OL" is displayed if the capacitor is short-circuited or the value exceeds the range.
• Measuring high-capacity capacitors may take several seconds.
• Release all residual charges before testing, especially for high-voltage capacitors, to prevent damage and injury.

Figure 7 shows the capacitance measurement setup, indicating the input terminals and the use of the REL button.

6. Frequency/Duty Ratio Measurement (Only applicable for UT139B/C) (See Figure 8)

Press the Hz/% button at the frequency measurement gear to select frequency/duty ratio mode.

Notes:

• Do not input voltage higher than DC 60V or AC 30V.

Figure 8 illustrates the connection for frequency/duty ratio measurement, showing the input signal connection.

7. Temperature measurement (Only applicable for UT139C) (See Figure 9)

Uses K-type (Ni-Cr and Ni-Si) thermocouple. "OL" is displayed after startup. Celsius/Fahrenheit conversion: °F = 1.8°C + 32.

Notes:

• The K-type thermocouple is applicable for temperatures up to 230°C / 446°F.

Figure 9 shows the temperature measurement setup, with the thermocouple connected to the input terminals.

8. AC and DC current measurement (See Figure 10)

Connect the instrument in series with the load. AC measurement is True RMS.

Notes:

• Before connecting in series, switch off the power supply.
• Use the proper input terminal and function gear. Start with a high gear range if the current is unknown.
• Fuses are provided for 10A, mA/µA jacks. Do not connect test pins in parallel with any circuit, especially power supply terminals, to avoid damage and injury.
• For currents >5A, limit measurement time to <10s and allow at least 15min interval.
• When measuring AC current online, press Hz/% to display AC frequency/duty ratio.
• 60A AC and DC current clamp measurement (See Figure 11). Connect as shown with the attached clamp.

Figure 10 shows the connection for AC/DC current measurement, where the multimeter is placed in series with the load. It indicates the input terminals (mA, A, COM) and the range switch. Figure 11 shows the connection for the 60A current clamp.

9. NCV Non-contact AC voltage sensing (See Figure 12)

To detect AC voltage or electromagnetic field, bring the front end close to the object. The analog quantity of sensed AC voltage is indicated by "EF" (critical voltage) for UT139A, with a five-level buzzing sound. For UT139B/C, "- - - -" is displayed, with different buzzing sounds indicating voltage levels. Pressing NCV/mV~ (UT139A only) displays the analog quantity of sensed mV to indicate AC voltage difference.

Notes: Test pen is not required when the range knob is set to "NCV".

Figure 12 illustrates the NCV sensing function, showing the multimeter's proximity to a voltage source.

10. Battery voltage measurement (only applicable for UT139A) (See Figure 13)

The 1.5V range gear is for batteries <2V (load resistance ~5Ω). The 9V range gear is for batteries ≤15V (load resistance ~1kΩ).

Notes:

• Do not measure batteries or power supplies beyond the rated range to prevent fuse burnout.
• Keep measuring time short due to analog load power consumption, which may shorten battery life.

Figure 13 shows the battery voltage measurement setup, indicating the selection of 9V/1.5V range.

11. Other functions

• After 2s of full display at startup, the instrument enters normal measurement mode. "ErrE" is displayed for internal EEPROM faults.
• Auto Power-Off: The instrument powers off automatically after 15 minutes of no operation to save energy. Pressing SELECT (UT139A) or any button (UT139B/C) will "arouse" the instrument. Turning the knob to OFF restarts it. To cancel auto power-off, press SELECT and power on; the buzzer will sound five times. Restarting the instrument after power-off restores the function.
• Buzzer: A "Beep" sound (about 0.25s) indicates a valid function button press. For voltage/current measurements: AC/DC voltage > ~600V, mA gear AC/DC current > 190mA (UT139A), 390mA (UT139B), 590mA (UT139C). When A gear AC/DC current > 10A, the buzzer sounds continuously for over-range. The buzzer sounds 5 times about 1min before auto power-off, and then makes one long sound before power-off. If auto power-off is cancelled, 5 sounds occur every 15min.
• Low-voltage detection: Detects internal VDD. When voltage < 2.4V, the ? symbol is displayed, and normal operation is available. When voltage < 2.2V, no operation is allowed, and only the symbol is displayed after startup.
• When battery supply voltage drops to 2.6V, the LCD backlight may be weak or fail to start, but measurement functions remain usable.

X. Technical Index

Accuracy: ± (a% reading +b word number), one year of guarantee period. Ambient temperature: 23℃±5℃ (73.4°F ±9°F). Relative humidity: <75%.

1. DC voltage measurement

10MΩ: There will be unstable digital display in case of "*/**" range open-circuit; after connecting with the load, it can be controlled ≤ ±1.

• Range: 20mV range for UT139A, 40mV range for UT139B, 60mV range for UT139C.
• Range: 200mV range for UT139A, 400mV range for UT139B, 600mV range for UT139C.

Maximum input voltage: ±600V.

2. AC voltage measurement

Input impedance: about 10MΩ.

Display the true RMS. Frequency response: UT139A 45~400Hz, UT139B/C 45~1kHz. (VFC: 45~400Hz). Assurance of accuracy: 5~100% range, an allowance of <10 words of residual reading for short-circuit. It will be up to 3.0 when AC crest factor reaches full value (with except for 600V range, which is up to 1.5 when the range reaches the full value). Maximum input voltage: 600Vrms.

3. Resistance measurement

4. )) Circuit on-off, ? diode measurement

Overload protection: 600V-PTC.

5. Capacitance Measurement (Only applicable for UT139B/C)

Overload protection: 600V-PTC.

For capacitor < 1µF, it is recommended to adopt REL measurement mode to ensure measurement accuracy.

6. Frequency/Duty Ratio measurement (only applicable for UT139B/C)

Over-load protection: 600V-PTC.

Input range a: (DC level is zero)

• <100kHz: 100mVrms<a<20Vrms
• >100kHz~1MHz: 200mVrms ≤a≤20Vrms
• >1MHz: 500mVrms ≤a≤20Vrms
• 5MHz~10MHz: 900mVrms<a≤20Vrms

Duty ratio %: only applicable for measurement ≤100kHz

Remark:

• During measurement of AC voltage or AC current, if need to read online frequency value or duty ratio, following input should be met: frequency response: ≤1kHz;
• AC voltage: mV range input ≥100mV; V range input > range ×6%
• AC current: input range a
• 4000/6000μA, 400/600mA, 10A range: a range ×6%
• 400/600μA, 40/60mA, 4/6A range: a range ×60%

7. Temperature measurement (only applicable for UT139C)

Over-load protection: 600V-PTC.

Remark: The point type K (Ni-Cr and Ni- Si) thermocouple is only applicable for the measurement of temperature under 230°C/446°F.

8. DC current measurement

Over-load protection:

• μA mA range: F1 fuse: (φ6×32)mm FF0.2A H 600V (CE) UT139A FF0.5A H 600V (CE) UT139B FF0.6A H 600V (CE) UT139C
• 10 A range: F2 fuse: (φ6×25)mm F 10A H 600V (CE)

9. AC current measurement

XI. Upkeep and Maintenance

Warning: Power shall be switched off before opening the rear cover of the instrument; and the test pen shall be away from the input terminal and circuit to be measured.

1. Conventional upkeep and maintenance

• For upkeep and maintenance, wet cloth and mild cleanser rather than abrasive or solution shall be used to clean the meter housing.
• Please stop using and send for maintenance in case of any abnormal condition about the instrument.
• The inspection or maintenance for instrument, if necessary, shall be performed by the qualified professional maintenance personnel or designated maintenance department.

2. Battery or fuse tube replacement (See Figure 14)

Built-in battery shall be replaced in time when LCD displays the under-voltage prompt "?", otherwise measurement accuracy may be affected.

Battery specification: AA 1.5Vx2.

Operating steps:

1. Set the power switch at "Off", take the test pen away from the input jack and remove the protective sleeve.
2. Screw off one screw securing battery rear cover with screwdriver, remove the cover and replace:

The under-voltage used battery and burnout fuse:

• F1 fuse: (φ6×32) mm FF0.2A H 600V (CE)(UT139A)
• FF0.5A H 600V (CE) (UT139B)
• FF0.6A H 600V (CE) (UT139C)

3. Screw off the second screw securing the rear cover with screwdriver and remove the cover to replace the burnout F2 fuse (φ6×25) mm F 10A H 600V (CE).

Figure 14 is a diagram illustrating the process of replacing the battery and fuses, showing the location of screws and covers.

Manufacturer Information

UNI-TREND TECHNOLOGY (CHINA) CO., LTD.

No6, Gong Ye Bei 1st Road, Songshan Lake National High-Tech Industrial Development Zone, Dongguan City, Guangdong Province, China

Tel: (86-769) 8572 3888

Website: http://www.uni-trend.com