NJTY T21 Series Digital Multimeter User Manual

1. Introduction

This digital multimeter is designed and manufactured in compliance with IEC-61010 safety requirements on electronic measuring instruments and handheld digital multimeters. It is compliant with IEC-61010 requirements pertaining to 600V CAT II and requirements on pollution degree of 2.

Please read carefully this user Manual and pay attention to the safety guidelines before operating this meter.

The meter is equipped with a backlight to facilitate users reading the measurement display in a dark place.

The NJTY T21 series offers a range of models (T21A, T21D, T21E, T21L) each tailored for specific needs, from basic household circuit testing to professional engineering applications, featuring high precision, fast response, and robust safety features.

2. Safety Information

2.1. Safety Instructions

• Before operating this meter, the operator must observe all standard safety procedures as below:
  • A: Safety procedures against electric shock
  • B: Safety procedures against unintended use
• To ensure your personal safety, please use the test lead that is compatible with the meter. Before operating this meter, ensure that the test lead is flawless.

2.2. Safety Notices

• When the meter is used around some equipment that produces strong electromagnetic interference, the readings on the meter may be unstable and even cause serious errors.
• Don't operate the meter or pen-shaped meter when the appearance is damaged.
• The safety function of the meter will become null if the meter is not properly operated.
• The meter must be operated with great care when working in the vicinity of an exposed conductor or bus line.
• The meter is prohibited from being used in the vicinity of any explosive gas, vapor or dust.
• The measurement must be made with correct input terminals and functions and within the allowable measuring range.
• To prevent the meter from being damaged, the value to be input shall not exceed the extremes permitted by each measuring range.
• When the meter has already been connected to the line being measured, the operator is forbidden to touch the input terminal not in service.
• When the measured voltage is higher than DC60V or AC36V RMS, you should be careful to avoid electric shock.
• When making measurements with a test lead, put your fingers behind its protective ring.
• When switching to another measuring range, be sure that the test lead has already been removed from the measured circuit.
• For all DC functions, to prevent potential electric shock that resulting to incorrect reading, please first use AC functions to check the absence of any AV voltage. Then, select DC voltage measuring range equivalent to or greater than that for AC voltage.
• Before any test on electric resistance, diode, capacitor or continuity, the operator must cut off the power to the circuit to be measured, and discharge all high-voltage capacitors contained in the circuit to be measured.
• The electric resistance measurement or continuity test cannot be carried out in any live electrical circuit.
• Before the current measurement, the operator must first examine the protective tube of the meter. Before connecting the meter to the circuit to be measured, the operator must first power off the aforesaid circuit.
• Before repairing TV sets or measuring power switching circuit, the operator must be careful enough to prevent Voltage impulse with high amplitude from damaging the meter.
• This meter uses 3x1.5V AAA batteries that must be correctly installed into the battery compartment.
• The [Battery Low] symbol is displayed on the screen when the battery power is insufficient, you should replace the battery in time to ensure accurate measurement.
• During measurement, the voltage shall not exceed 600V.

2.3. Safety Symbols

2.4. Safe Maintenance Habits

• Pull out the test lead before opening the multimeter case or removing the battery cover.
• Use appropriate replacement parts to repair the multimeter.
• Before turning on the meter, all relevant power must be disconnected.
• The calibration and maintenance of the instrument must be operated by professionals.
• If any abnormality of the instrument is observed, please stop using it immediately and send it for repair, and any use is forbidden until it is qualified.

3. Product Overview

3.1. Model Comparison (T21 Series)

The NJTY T21 series offers four distinct models, each with specific features tailored for different user needs. Refer to the table below to understand the differences:

Model Descriptions:

• T21A Basic Model: Features manual range control with a 2000 counts display. It covers core functions like AC/DC voltage, resistance, beep on/off, transistor testing, and NCV non-contact power check. Ideal for basic maintenance, circuit testing, and novice users.

  

• T21D All-round Advanced Model: Offers auto-ranging with 6000 counts high precision. Includes extended functions such as frequency, capacitance, and temperature measurement (-20°C~1000°C). Suitable for air conditioner/home appliance maintenance, car repairing, and industrial equipment maintenance requiring temperature monitoring.

  

• T21E Smart Electrician Model: Incorporates True RMS (TRMS) technology for accurate measurement of non-sinusoidal waveforms and complex waveforms. Features LIVE line identification for distinguishing between Neutral and Live lines, enhancing electrician safety. Recommended for power engineers and inverter equipment maintenance.

  

• T21L Professional Engineer Flagship Model: Exclusive support for inductance detection (0.01mH~60H), designed for electronics R&D and motor/transformer manufacturing. Combines temperature measurement (from T21D) and TRMS technology (from T21E) with 6000 counts auto-ranging for complex circuit analysis. Suitable for laboratories, electronic component production, and automation equipment debugging.

  

All models in the T21 series are equipped with a backlit screen, drop-proof sheath, intelligent hibernation, and a magnetic back with a support stand for enhanced user experience.

3.2. Multimeter Diagram

1. Flashlight
2. NCV detection area
3. LED display
4. Buzzer
5. Backlight/Flashlight button
6. "FUNC/HZ" function selection button
7. Rotary switch
8. VΩmA°C input socket
9. COM input socket
10. 10A input socket
11. Hold button
12. NCV indicator

3.3. Description of Display Unit Symbols

3.4. Description of Functional Keys

3.5. Description of Input Sockets

4. Operational Guidelines

4.1. Normal Operation

4.1.1. Hold Mode

In the hold mode, the reading can be maintained on the display unit. Changing the measurement function position or pressing the key Hold again to exit the hold mode.

To enter/exit Hold mode:

1. Press the "FUNC/Hz" key and the reading will be held and the symbol "H" will appear on the LCD screen.
2. Press the "FUNC/Hz" key again to reset the meter for normal measurement.

4.1.2. Backlight & Lighting

The meter is equipped with the functions of backlight and lighting so that the operator can read measurement results even in a darker place. The backlight function can be enabled or disabled by the steps below:

1. Short press ★/🔦 key to turn on backlight and short press again to exit. It turns off automatically with no operation for 20S.
2. Long press ★/🔦 key to turn on the illumination function and the backlight at the same time. Short press the key again to turn off the illumination function. It turns off automatically with no operation for 30S.

4.1.3. Auto Power Off

If no operations are made in 15 minutes following the initialization, the meter will sound to remind the operator to automatically cut off power supply and enter the state of dormancy.

4.2. Measurement Guidelines

4.2.1. Measurement of AC Voltage and DC Voltage

To avoid any electric shock and/or damage to the meter, do not attempt to conduct a voltage measurement if the voltage (valid value) is over 600V(DC/AC).

• DC voltage range: 600mV, 6V, 60V and 600V
• AC voltage range: 600mV, 6V, 60V and 600V

Steps for AC or DC voltage measurement:

1. Turn the rotary switch to the "∼mV" or "∼V" switch. Press "FUNC/Hz" to switch DC / AC voltage function.
2. Connect the black test lead to the COM input socket and the red test lead to the V input socket.
3. Use the other two ends of the test lead to measure the voltage of the circuit (in parallel connection with the circuit).
4. Read the measured voltage value on the LCD screen. When DC voltage measurement is attempted, the display unit will show the voltage polarity of the circuit connected to the pen-shaped meter in red.

Notes:

• Within the measuring range of DC voltage of 600mV or AC voltage of 6V, even if there is no input or no connection to the test lead, the meter will display some information. In this situation, press short circuit "V - Ω" and "COM" terminal to reset the meter to zero.
• Within the AC voltage function, press the "FUNC/Hz" key to measure the frequency of the AC voltage source.
• The value of the AC voltage measured with this meter is True RMS (root mean square). These measurements are accurate for sine wave and other waves (without DC offset), square wave, triangular wave and step wave.

4.2.2. Measurement of Electrical Resistance

To avoid the meter or the measured equipment from damage, do not attempt to conduct a resistance measurement unless the operator has already cut off all power sources for the circuit to be measured and fully discharged all high-voltage capacitors.

Measuring ranges: 600Ω, 6kΩ, 60kΩ, 600kΩ, 6MΩ, 60MΩ.

Steps for electric resistance measurement:

1. Turn the rotary switch to the "Ω" position. Press "FUNC/HZ" to choose "Ω" range.
2. Connect the black test lead to the "COM" input socket and the red test lead to the "V/Ω" input socket.
3. Read the measured electric resistance value on the LCD screen.

Notes:

• The measured value of the electric resistance of the circuit differs a bit from the rated value of the electric resistance (Effect of parallel resistance).
• To ensure measurement accuracy, in attempting a low resistance measurement, first put two pen-shaped meters in short circuit and capture the resistance reading of these short circuits. Then subtract the aforesaid reading from the measured resistance.
• At 60MΩ position, you have to wait a few seconds before the reading grows stable. This is quite normal for a high resistance measurement.
• The display shows "OL" indicates the measured value is over the measuring range.
• When the meter is in open circuit, the display unit will show "OL".

4.2.3. Diode Test

To avoid the meter or the measured equipment from damage, do not attempt to conduct a diode test unless the operator has already cut off all power sources for the circuit to be measured and fully discharged all high-voltage capacitors.

Steps for diode test (outside the circuit):

1. Turn the rotary switch to the "→|•" position. Press "FUNC/Hz" to select "→|•" function.
2. Connect the black test lead to the COM input socket and the red test lead to the "V / Ω" input socket.
3. Connect the black test lead to the negative pole and the red test lead to the positive pole of the diode to be tested.
4. The meter displays the forward bias value of the diode to be tested. If the polarity of the test lead is reversed, the meter will display "OL".

A normal diode still produces a forward voltage drop of 0.5V to 0.8V; the reverse bias voltage reading depends on the variation in electric resistance of other channels between two pen-shaped meters.

4.2.4. Beep Continuity Test

To avoid the meter or the measured equipment from damage, do not attempt to conduct continuity test unless the operator has already cut off all power sources for the circuit to be measured and fully discharged all high-voltage capacitors.

Steps for a continuity test:

1. Turn the rotary switch to the "•))" position. Press "FUNC/HZ" to select "•))" function.
2. Connect the black test lead to the "COM" input socket and the red test lead to the "V / Ω" input socket.
3. Use the other two ends of the test lead to measure the resistance of the circuit.

If the measured distance is no more than 50Ω, the beeper will sound continuously.

4.2.5. Capacitance Measurement

To avoid the multimeter or the measured equipment from damage, do not attempt to conduct a capacitance measurement unless the operator has already cut off all power sources for the circuit to be measured and fully discharged all high-voltage capacitors. Use the DC voltage position to determine that all capacitors have been discharged.

Measuring ranges: 6nF, 60nF, 600nF, 6µF, 60µF, 600µF, 6mF and 100mF.

Steps for capacitance measurement:

1. Turn the knob to the "→|•" position.
2. Connect the black test lead to the "COM" input socket and the red test lead to the "→|•" input socket.
3. Use the other two ends of the test lead to measure the capacitance of the capacitor and capture the measured value on the LCD screen.

Notes:

• The measurement of a large capacitance requires a given period of stabilization of reading.
• To avoid damage to the meter, the measurement of a capacitor with polarities requires much attention to its polarity.

4.2.6. Frequency Measurement

To avoid any electric shock and/or damage to the meter, do not attempt to conduct a frequency measurement if the voltage is over 250V (DC/AC).

Steps for frequency measurement:

1. Turn the rotary switch to the "HZ%" position.
2. Connect the black test lead to the "COM" input socket and the red test lead to the "Hz" input socket.
3. Use the other two ends of the test lead to measure the frequency of the circuit and read the measured frequency on the LCD screen.
4. Press "FUNC/Hz" button to display the frequency and duty ratio.

4.2.7. Current Measurement

Attention: Do not attempt a measurement on the current in a circuit when the voltage between the open-circuit voltage and the ground is over 250V. If the fuse is blown at the moment of measurement, you are likely to damage the meter or get yourself hurt. To avoid any damage to the meter or equipment to be measured, do not attempt a current measurement unless you have examined the meter's protective tube. In attempting a measurement, you should use the correct input sockets, function positions and measuring ranges. When a test lead is plugged into the current input socket, do not put the other end of the test lead connected to any circuit in parallel.

The meter provides measuring ranges as follows:

• DC current measuring ranges: 60mA, 600mA and 10A
• AC current measuring ranges: 60mA, 600mA and 10A

Steps for current measurement:

1. Turn the rotary switch to the appropriate position. Press "FUNC/Hz" to switch DC/AC current function.
2. Connect the black test lead to the COM input socket. Connect the red test lead to the mA input socket when the measured current is less than 600mA; connect the red test lead to a 10A input socket when the measured current is 600mA ~ 10A.
3. Connect the black test lead to the negative pole of the load, and the red test lead to the positive pole of the load.
4. Connect the power to the circuit and capture the displayed reading. If the display unit only shows "OL", it means the input is over the selected measuring range. At this moment, turn the rotary switch to a higher measuring range.
5. Within the AC current function, press the "FUNC/Hz" key to measure the frequency of the AC current source.

4.2.8. NCV Test (Non-Contact Voltage Detection)

Turn the rotary switch to NCV position, and place the top of the meter (NCV detection area) near the conductor. If the meter detects the AC voltage, the indicators for signal density (high, medium and low) will be on in accordance with the detected density, while the beeper will sound alarms at different frequencies.

Note: Voltage may remain in the absence of any indication. The operator shall not rely on non-contact voltage detector to check the presence of voltage. The detection operation may be affected by various factors, including socket design, insulation thickness and type. When the voltage is fed into the meter's input terminal, the voltage sensor LED may be on as a result of induced voltage. External sources of interference (like flashlight and motor) may trigger non-contact voltage detection.

4.2.9. Temperature Measurement

Steps for temperature measurement:

1. Put the range switch at the gear of "°C / °F". The instrument displays the temperature of the surrounding environment.
2. Insert the red plug of the thermocouple into the "VΩmA" terminal and the black plug of the thermocouple into the "COM" socket. Directly read the temperature value from the display screen after the reading is stable.

Note: The maximum measuring temperature for the K-type thermocouple dispatched at random is 25°C, and its instant measuring value can reach 300°C.

5. Technical Specifications

5.1. General Specifications

• Operating environment: 600V CAT II, Pollution level: 2, Altitude < 2000 m.
• Working temperature & humidity: 0~40°C (<80%RH)
• Storage temperature & humidity: -10~60°C (batteries shall be removed when RH is below 70%).
• Ambient temperature for testing or calibration: 20°C ± 2°C
• Allowable max voltage between terminal to be measured and ground: AC600V
• Protection of protective tube: mA position: protective tube F 600mA / 250V; "A" position protective tube F10A / 250V.
• Rotation rate: approximately 3 revolutions/S.
• Display unit: 6000 counts displayed on LCD screen. Automatically display the symbol for unit in accordance with measurement function position.
• Outrange indication: the LCD screen will display "OL".
• Battery Low indication: ☑ will appear when the battery's voltage is below the normal working voltage.
• Input polarity indication: "-" will automatically appear.
• Power: 3 x 1.5V AAA batteries
• Dimensions: 147mm(L)×71mm(W)×45mm(H).
• Weight: approximately 220g (exclusive of batteries).

5.2. Precision Indicators

5.2.1. DC Voltage

Input resistance: 10MΩ; Maximum input voltage: DC600V

5.2.2. AC Voltage (True RMS)

Input resistance: 10MΩ; Maximum input voltage: AC600V; Frequency response: 40HZ - 1KHz TRMS

5.2.3. Frequency

Input voltage range: 200mV-10V AC RMS; Overload protection: 250V DC/AC

5.2.4. Electrical Resistance

Overload protection: 250V DC/AC; Open-circuit voltage: 2.4V

5.2.5. Diode and Buzzer

Overload protection: 250V DC/AC

5.2.6. Capacitor

Overload protection: 250V

5.2.7. DC Current

Overload protection: mA range with F600mA/250V; 10A range fuse with F10A/250V. When the current is more than 5A, the test time should be less than 10 seconds and one minute should be given to stop testing after such measurement.

5.2.8. AC Current

Overload protection: mA range with F600mA/250V; 10A range: F10A/250V. Maximum input current: mA range: 600mA AC TRMS; 10A range: 10A AC TRMS. When the current is more than 5A, the test time should be less than 10 seconds and one minute should be given to stop testing after such measurement. Frequency response: 10Hz-1KHz, TRMS.

5.2.9. Temperature

6. Maintenance

This section provides the basic information on maintenance, including the descriptions about replacement of protective tubes and batteries. Do not attempt the meter maintenance unless you are experienced in maintenance and have read the information on calibration, performance test and maintenance.

6.1. Battery Replacement

Follow these steps to replace the battery:

1. Turn off the power of the multimeter.
2. Unplug all test leads from the sockets.
3. Unscrew the screw on the back cover of the multimeter.
4. Open the back cover.
5. Remove the old battery.
6. Replace with new 3x1.5V AAA batteries.
7. Close the back cover and tighten the screws.

6.2. Fuse Replacement

Follow these steps to replace the fuse:

1. Turn off the power to the multimeter.
2. Unplug all test leads from the sockets.
3. Remove the rubber cover with your hands.
4. Unscrew the rear screws with a screwdriver.
5. Gently push the fuse back in, then replace it with a new one. (Usually it needs to replace the 0.6A fuse)

7. Troubleshooting

• "OL" Display: If the display shows "OL" during a measurement, it indicates that the measured value is outside the selected range. Try switching to a higher range or ensure the circuit is not open.
• Unstable Readings: Strong electromagnetic interference can cause unstable or erroneous readings. Move away from such equipment if possible.
• No Display/Meter Not Turning On: Check if the batteries are correctly installed and have sufficient charge. Replace if necessary. Ensure the rotary switch is not in the "OFF" position.
• Incorrect Readings: Verify that the correct function and range are selected for the measurement. Ensure test leads are properly connected and not damaged. For resistance, diode, or capacitance measurements, ensure the circuit is de-energized and capacitors are discharged.
• Backlight/Flashlight Not Working: Check battery level. Short press the ★/🔦 button for backlight, long press for flashlight.

8. User Tips

• Always Start with Higher Range: When measuring an unknown voltage or current, always start with the highest range setting and then gradually decrease it until you get a stable reading. This prevents overloading the meter.
• Check Test Leads: Before every use, inspect your test leads for any signs of damage (cracks, frayed wires). Damaged leads can be a safety hazard and lead to inaccurate readings.
• Magnetic Back Feature: Utilize the magnetic back feature (available on some models) to attach the multimeter to metal surfaces, freeing up your hands for easier and safer measurements.
• Auto-Ranging vs. Manual-Ranging: If your model has auto-ranging, it simplifies operation. For manual-ranging models (like T21A), understanding the appropriate range for your measurement is crucial for accuracy.
• PDF Manual Reference: For detailed diagrams and specific instructions not covered here, refer to the comprehensive user manual PDF available at NJTY T21 Series User Manual (PDF).

9. Certifications

The NJTY T21 series digital multimeters are certified to meet various international safety and quality standards, ensuring reliable and safe operation.

• Certifications: CE, FCC, RoHS, UL

10. Packing List

• 1x Multimeter
• 1x Instruction Book
• 1x Test Leads
• 1x Fuse